Knowledge journal / Edition 1 / 2021


Research with a view to practical application

This is the twelfth edition of Water Matters, the knowledge magazine of H2O. You will find twelve articles on diverse subjects, written by water professionals based on thorough research.

When assessing the articles, the editorial board made a selection, looking for a clear relationship with daily practice in the water sector, which is the purpose of Water Matters. Research, results and findings must be new and generate articles that provide new knowledge, insights and techniques with a view to practical application.
This edition covers a wide range of topics, including current affairs such as drought (two contributions from KNMI researchers), estimating flood probabilities (five major improvements), the need for nature-friendly banks, the use of Swiss and German key figures for removing micro-pollutants (there is still a lot to learn) and the climate scan of large waters. Furthermore: data driven asset management, waste stream characteristics for bioplastic production, the 'back bank' concept, the role of bird manure in the nutrient balance, continuous monitoring of ecological targets and the usefulness of wet cultivation in stream valleys.
Water Matters is, just like the magazine H2O, an initiative of the Royal Dutch Water Network (KNW), the independent knowledge network for and by Dutch water professionals. Members of KNW receive Water Matters twice a year as a free supplement to their magazine H2O.
The publication of Water Matters is made possible by leading players in the Dutch water sector. These Founding Partners are ARCADIS, Deltares, KWR Watercycle Research Institute, Royal HaskoningDHV and Foundation for Applied Water Research STOWA. With the publication of Water Matters the participating institutions want to make new, applicable water knowledge accessible.

Enjoy reading this edition. Would you like to respond? Please let us know via

Monique Bekkenutte Publisher (H2O Foundation)
Huib de Vriend Chairman editorial board of Water Matters


Knowledge journal / Edition 1 / 2021

Is the 2018 drought due to climate change?

In the summer of 2018, the Netherlands was hit by extreme drought. The damage to farmers, shipping and water authorities, for example, is estimated at between 450 and 2080 million Euro. The drought was so exceptional that the question soon arose whether climate change was the cause.

In 2018, the entire six-month summer period (April-September) was very warm, with many more hours of sunshine than usual. In May, June, July and September, there was very little rain. This combination of low rainfall and high evaporation due to high temperatures and high solar radiation led to drought conditions. The problems in the south and east (the inland region) were much greater than in the west and north (coastal region). This was partly because the sensitivity to lack of rain is higher inland, especially on the high sandy soils. Supplementing water deprived areas with river water is not possible there, whereas it is possible in the west and around the IJsselmeer lake.
The central question of this study is whether we see trends in the occurrence of droughts in inland or coastal areas in the Netherlands and, if so, whether we can relate those trends to climate change.

Potential precipitation deficit

In the Netherlands, a commonly used measure of drought is the potential precipitation deficit during the growing season (April to September). This is the cumulative difference between daily potential evaporation and precipitation, with negative values being set to zero. Potential evaporation is calculated using Makkink's formula, which depends mainly on temperature and direct and indirect solar radiation ('global radiation'). In 2018, the potential precipitation deficit at the end of the growing season, on 30 September, was greatest in the south and east of the country (Figure 1).

Aspects of drought

We know from previous research that precipitation trends differ between the coastal and inland areas (Lenderink et al., 2009; van Haren et al., 2012). Here we investigate whether, and if so how, this difference affects trends in drought. To this end, we define the coastal area as a strip of width 50 kilometres along the North Sea coast and at least 30 kilometres along the Wadden Sea – amounting to about 45 percent of the Netherlands. We refer to the remaining land as the inland area.
In this study, we consider four aspects of drought. The simplest is drought as a reduction in rainfall (meteorological drought). Secondly, we analyse trends in temperature. Thirdly, we look at the potential evaporation according to Makkink. Finally, we consider soil moisture drought (agricultural drought).

Figure 1. Spatial distribution of the potential precipitation deficit on 30 September 2018. Source: KNMI.

Observed trends

Figure 2 shows the trends in precipitation, temperature, solar radiation and Makkink potential evaporation. For soil moisture, no trend analysis is possible due to the lack of sufficiently long observational series. In order to determine the effect of climate change, the trends have been plotted against the global mean surface temperature (GMST). The time series of GMST has been smoothed by applying a 4-year running average, to filter out fluctuations caused by, for example, El Niño.
The GMST is now about one degree higher than it was around 1950, so the changes in the studied variables since then are about the same as the trends per degree of global warming in the figures.

Warmer everywhere, more precipitation on the coast
The temperature increase in the Netherlands is more or less uniform. Precipitation, on the other hand, shows a spatial pattern, with a clear increase along the coast. On average, this is significant at p<10% - meaning that there is a 90% chance that this trend does indeed exist (usually we look at the 95% level). Precipitation inland shows no clear trend. The difference between inland and coastal precipitation, however, is statistically significant. This is because the weather fluctuations in both series are very similar, so taking the difference between the two results in a better signal-to-noise ratio.
For the coastal area, the discharge from the river Rhine is also relevant. It shows a slight decrease in the six-month summer period since 1901. From 1950 onwards, the decrease is about 9 percent (significant at p<10%). Precipitation in the upstream part of the catchment area has also decreased significantly. We will not discuss this further in this study.

Figure 2. Trends for the period April-September from 1950, presented as the regression against smoothed global mean temperature (solar radiation and evaporation have shorter time series). Not all trends shown are statistically significant. Data: KNMI; each dot represents a KNMI station, not all variables are measured at each station.

More solar radiation and evaporation inland
The observations since about 1970 show that the increase in global radiation is stronger inland than near the coast (Figure 2). This is confirmed by re-analysis datasets such as the ERA5 weather reconstruction from the European Centre for Medium-Range Weather Forecasts.
This trend has two components. Due to increasing air pollution with aerosols, the amount of solar radiation reaching the ground decreased until around 1985. Afterwards, this effect disappeared thanks to measures against air pollution. In addition, there is a trend towards more hours of sunshine in the summer months over the whole period (Van Oldenborgh et al., 2009).
Due to relatively more solar radiation inland, Makkink potential evaporation has also increased more there than near the coast. In combination with air pollution, we see a decrease until about 1985 and a stronger increase until about 2005.

Is climate change behind these trends?

In order to attribute these trends to climate change, we need climate models. They allow us to study the difference between a climate with increasing amounts of greenhouse gases and a simulated climate without such an increase. Of course, we first checked whether the climate models describe the real climate realistically enough. We also analysed the output of hydrological models based on these climate models. We evaluated the following models and, if sufficiently realistic, used them for attribution:
• ISIMIP, 16 runs of four global climate models with four hydrological models;
• EC-Earth / PCR-GLOBWB ensemble (Van der Wiel et al., 2019);
• RACMO ensemble of a regional climate model.

Precipitation and temperature
For the coastal area, the models do not show the observed higher precipitation. Therefore, we cannot make any statements about the influence of climate change on drought in the coastal zone. For precipitation inland, the climate models function better. Like the observations, they show no change in summer precipitation. We can conclude that climate change has so far not caused a precipitation trend in the inland area.
With regard to temperature, the situation is more complex. The observed trend in the six-month summer period since 1950 is 1.9 degrees per degree of average global warming, with a 95% uncertainty margin of 1.3 to 2.4 degrees per degree. The models, on the other hand, show a warming of around 0.9 times the global mean warming (Van Oldenborgh et al., 2009). We can thus attribute about half of the warming to emitted greenhouse gases. The other half either has other causes or the climate models underestimate the effect of warming on hot summers.

Potential evaporation
As expected, the climate models show an increase in potential evaporation inland. These models use formulas for potential evaporation that are more common internationally: Priestley-Taylor, Penman-Monteith, Hamon and Bulk. However, the differences between the models turn out to be greater than the differences due to the different formulas. In other words, in this case, it is not so important which particular calculation method is used.
The increase in potential evaporation is smaller in the climate models than calculated for the real world based on observations or reanalyses. This is consistent with the lower trends in temperature in the models.

Soil moisture
The climate models show no change in the amount of soil moisture inland for the Netherlands. However, in the Netherlands, irrigation plays a major role in summer soil moisture but is not realistically modelled in these models. Therefore, in this study we consider the precipitation deficit, potential evaporation minus precipitation, to be a more relevant measure of drought than the modelled soil moisture. Further research with models that can also realistically simulate irrigation is needed to properly model trends in soil moisture and groundwater.


This study presents a more nuanced view compared to the analyses of drought trends for the Netherlands that have been published so far. Along the coast, there is an increase in the amount of rainfall in the six-month summer period, but not inland. Inland, the amount of solar radiation increases more than on the coast, which, together with the higher temperatures, leads to more potential evaporation.
We therefore conclude that warming has increased the likelihood of inland droughts, due to higher temperatures and sunshine leading to higher potential evaporation, combined with no trend in precipitation. It is difficult to say by how much the likelihood has increased because climate models underestimate the warming of the Netherlands. We cannot make any statements for the coastal area, because the climate models do not reproduce the observed increase in precipitation there well enough.
The data and insights gained from this study allow for a better assessment of the impact of climate change on, for example, agriculture, nature, shipping and urban water management. This can contribute to improvements in climate adaptation plans.

Sjoukje Y. Philip
Sarah F. Kew
Karin van der Wiel
Niko Wanders
(Utrecht University)
Geert Jan van Oldenborgh


The very dry summer of 2018 raised the question of whether droughts are occurring more frequently in the Netherlands and, if so, whether that is due to climate change. The answer is nuanced and differs for the coastal strip and inland. For the inland area, the risk of drought has increased due to warming. It is difficult to say by how much the risk has increased because climate models underestimate the warming in the Netherlands. We cannot make any reliable statements for the coastal area, as the models are not reliable enough to make any hard conclusions for this area.


G. Lenderink et al., 2009. Intense coastal rainfall in the Netherlands in response to high sea surface temperatures: analysis of the event of August 2006 from the perspective of a changing climate. Clim. Dyn., 32, 19-33, doi:10.1007/s00382-008-0366-x

K. van der Wiel, Wanders, N., Selten, F.M., Bierkens, M.F.P., Added Value of Large Ensemble Simulations for Assessing Extreme River Discharge in a 2°C Warmer World, Geophysical Research Letters, doi:10.1029/2019GL081967

R. van Haren et al., 2012. SST and circulation trend biases cause an underestimation of European precipitation trends. Clim. Dyn., 40, 1-20. doi:10.1007/s00382-012-1401-5

G. J. van Oldenborgh et al., 2009. Western Europe is warming much faster than expected. Clim. Past, 5, 1-12, doi:10.5194/cp-5-1-2009.

^ Back to start


Due to climate change?

Knowledge journal / Edition 1 / 2021

More drought in the Netherlands?

In this article, we will use two indicators that are used internationally to answer the question of whether there has been more drought in the Netherlands recently. To this end, we examine trends in the period 1965-2020. The answer is twofold. Throughout the year, precipitation increases and drought decreases, but this is not true for every season. In fact, spring has become drier.


We use two well-known indicators for drought and wet periods: the Standardized Precipitation Index (SPI; McKee et al. 1993) and the Standardized Precipitation-Evapotranspiration Index (SPEI; Vicente-Serrano et al. 2010). Both are based on the deviation from long-term averages. The main difference is that SPI is based exclusively on precipitation data, whereas in SPEI the potential evapotranspiration (as formulated by Makkink) is subtracted from the precipitation. This substraction is the same as is used for the potential precipitation deficit that is commonly used in the Netherlands.
In a country like the Netherlands, SPI is suitable in autumn and winter, when evaporation does not play a major role. SPEI can however be used all year round and has added value in spring and summer. SPI has been in use since the 1990s and is mainly used for areas with low data availability. SPEI was introduced about 10 years ago and is preferred in well monitored areas, such as the Netherlands, where evaporation data is also available.
A positive SPI or SPEI value means 'wetter than normal' and a negative value means 'drier than normal'. Because SPI and SPEI can be calculated for different time scales, they can provide information on different types of drought. The time scale is represented by a figure for the number of months, e.g. SPEI-12 stands for the past 12 months. A negative SPI or SPEI with a short time scale (less than 6 months) indicates meteorological drought and during the growing season agricultural drought. A negative SPI or SPEI with a longer time scale is an indication of hydrological drought, with shortages of ground and surface water. A negative SPI or SPEI with a time scale of 24 months or more implies multi-year drought.


In the calculation of both SPI and SPEI, a standardization is included. This allows for comparison between areas with different precipitation characteristics and between seasons. A reference period is also needed, in this article the full period for which precipitation and evaporation data is available (1965-2020) has been used. For Figure 1, the values of SPI and SPEI have been calculated using monthly averages from 13 KNMI stations. For Figure 2, SPEI is determined per grid cell based on countrywide grid maps of precipitation and potential evaporation available on the KNMI Data Platform. These maps are made by interpolating from 5 (in 1965) to 35 (in 2020) stations for evaporation and over 300 stations for precipitation.
Like SPI and SPEI themselves, their trends are dimensionless and therefore difficult to interpret. Therefore, in this article we focus on the sign and possible statistical significance of the trend, rather than the size of the trend figures. The trends and their significance were calculated using the Mann-Kendall test.

More precipitation in the Netherlands

SPI and SPEI, like precipitation, have a large year-to-year variability, which means that trends are often not significant. Figure 1 shows that SPI has a significant trend more often than SPEI, especially for longer windows. This is caused by the known long-term increase in precipitation. In SPEI-12, the increase in precipitation is largely compensated by the increase in evaporation and the trend is close to 0, but almost always positive. Over a whole year, the increase in precipitation thus appears to be slightly stronger than the increase in (potential) evaporation. In addition, because the potential evaporation during dry periods is somewhat higher than the actual evaporation, we can conclude that the Netherlands has become wetter year-round in the period 1965-2020. In this analysis, however, it is unclear whether the trend is evenly distributed over the year or is determined by one or more seasons. To see this, we need to look at smaller time scales of SPI/SPEI.

Figure 1. Trend per calendar month in the period 1965-2020 for SPI (black) and SPEI (blue) for periods of 12, 6 and 3 months. The figures are based on the average precipitation at 13 KNMI stations across the Netherlands, and on potential evaporation based on radiation and temperature data from nearby stations. Trends and significance were calculated using the Mann-Kendall test, the 95% reliability intervals are shown in grey and pale blue.

More precipitation and more drought

Looking at smaller windows, we can see that in addition to the long-term increase in precipitation, there is also an increase in drought. For example, Figure 1 for SPEI-6, related to agricultural drought, shows five calendar months with negative trends. These trends are not significant, but do indicate an increase in drought. The relatively small positive trends for SPI-6 during the summer months indicate that precipitation increased the least during the first half of the year. The negative trends for SPEI-6 between May and September indicate that on average potential evaporation increased more than precipitation from December up to and including September. The increase in evaporation is due to a combination of higher temperatures and more sunshine (hours).

Dryer from March

SPEI-3, the shortest time scale considered here, is the only indicator that shows statistically significant negative trends. SPEI-3 can be used as an indicator for meteorological drought and during the growing season for agricultural drought. The growing season traditionally runs from April to September. However, global warming is increasing the length of the growing season and arable farmers could sow earlier. However, this brings risks because it is also the first half of the growing season that experiences an increase in drought. The trend in SPEI-3 shows a significant decrease in May and June. Given the 3-month window, this trend covers the months of March, April, May and June. This implies that ‘spring drought’ (months of March to June) has increased in the period 1965-2020.

Spring drier almost everywhere

Figure 2 shows the trend in SPEI-3 in May, August, November and February, providing information about the four seasons. Spring trends show an increase in drought that is significant almost everywhere. Summer trends show an interesting pattern with more drought in the south-eastern half and less drought in the north-western half of the country, but the trends are not significant. In autumn, the well-known increase in coastal precipitation can be observed in the pattern. Other than that, the trends in autumn are rather diffuse and not significant. Winter trends show some parts of the country have gotten significantly wetter.

Figure 2. Trend in SPEI-3 in spring (March to May), summer (June to August), autumn (September to November) and winter (December to February) over the period 1965-2020 based on the Mann-Kendall test. Areas with a significant trend (p < 0.05) are shaded.

Challenges for water management

The fact that on average there is more precipitation throughout the year today than there was 70 years ago has consequences for the water balance. This mainly concerns flooding in cities, run-off into surface water and flooding in winter. In spring and summer, however, there is a trend towards short-term more extreme droughts, which can lead to salinization, subsidence and low river levels. This may affect nature and agriculture, the supply of drinking water and the inland waterway transport sector and citizens and companies depending on these sectors. Coping with both the long-term increase in precipitation and more short-term droughts is a challenge for water management in the Netherlands. Taking these trends (and future projections) into account can lead to better adjustments in relevant sectors and a better understanding of risks.

Emma Daniels
Jules Beersma
Gerard van der Schrier


Unlike the long-established Standard Precipitation Index SPI, which only covers precipitation, the SPEI indicator also includes evapotranspiration. Using SPEI, drought can be captured with one indicator that can be applied to whole years but also specifically to months and seasons. If we look at whole years with this indicator, it appears that long-term drought in the Netherlands may have decreased slightly in the period 1965-2020. In contrast, the probability of short-term drought in spring has increased. There are indications that summer droughts have increased especially in the higher parts of the Netherlands. Here, river water is often not easily available, and more drought can have major consequences.


McKee, T. B., N. J. Doesken, and J. Kleist (1993). The relationship of drought frequency and duration to time scales. Eighth Conference on Applied Climatology. California, p. 6.

Vicente-Serrano S.M., Santiago Beguería, and Juan I. López-Moreno (2010). A Multi-scalar drought index sensitive to global warming: The Standardized Precipitation Evapotranspiration Index - SPEI. Journal of Climate 23: 1696-1718.

^ Back to start


The answer is twofold

Knowledge journal / Edition 1 / 2021

Lowest societal costs through advanced data-driven asset management

Waterleiding Maatschappij Limburg (WML) drinking water company replaces 1.25% of its distribution network annually. This is a higher percentage than what is average in the Dutch water sector. Will it be necessary to maintain this percentage in the next decade? WML, together with consultant Spatial Insight, investigated whether data-driven asset management could responsibly reduce replacement investments.

In 2019, WML received the ISO-55001 certificate, belonging to the international standard for systematic asset management. WML's question in this study is whether it will be necessary to continue to replace 1.25% of the distribution network annually over the next decade.

Objective decision on optimal replacement rate

The – often unintended – influence of personal preferences of staff on asset management decisions can be limited by relying upon objective data, and by defining calculations in such a way that they focus on the strategic business objectives. This will result in consistent, optimal and reproducible decisions. The strategic goal is to achieve the lowest societal costs while minimising inconvenience to citizens. In addition, WML is looking for a way to easily adjust asset management in the future.

In order to find an objective and reproducible balance between performance, costs and risks of the distribution network, both internal and external failure and asset data are used.

The performance of the distribution network is defined as the extent to which the network is able to meet the water supply requirements. This is enforced by law, demanding a pressure of 15 MWC (metres of water column) at a flow rate of 1 m3/hr at each customer's water meter. In addition, WML sets conditions with regard to sensitivity to pressure fluctuations, undesired resistance and hydrant flows.
The replacement rate of the network is used as a measure of cost. The number of leaks has been chosen as parameter for the risk in this study.

Integrated data

Uniform and integrated data are required for modelling. In this study, we have two types of (spatial) data: failure data and data about the pipeline network.
WML uses SAP's ERP (enterprise resource planning) software to register failure data. Data of the second type, about the location of pipes and their characteristics (such as material, year of construction and diameter) are recorded in a database of Smallworld.
Overarching, data warehouse ADI (asset data integrator) has been installed, which unifies both types of spatial data, relates them to each other and makes them accessible. ADI combines even more data, such as the activities of other network operators, KLIC (cable and pipeline activities information) reports and, in the near future, process data such as the flow rate of the treatment plants and pressures. In the daily data refresh, the quality of new entries is automatically checked and discrepancies are identified. For example, if a repair to a PVC pipe is reported, but the database shows a steel pipe at the same location, then either something went wrong with the registration, or the original data was incorrect. When it has been assessed what the correct information is, the database is updated accordingly.

Modelling with spatial data

Using a regression analysis on WML's own data and data from USTORE (the joint failure database of the Dutch drinking water companies), the failure characteristics for various materials and ages of pipes were recalculated. A sensitivity analysis was also carried out using the TransparantNL knowledge model, which showed that some parameters have a dominant influence on the calculated replacement year. These included parameters such as the proximity of a shopping mall and high pressure in combination with PVC pipes near pumping stations. Other parameters, including the number of historic leaks, had a lower effect than expected.

Machine learning

Machine learning is a method of deriving relationships between parameters from training data and creating a predictive model. The training data is separated from the historic dataset, the remaining part is used to validate the algorithm. We applied machine learning to groups of pipes that together contribute on average to the largest number of leaks, by means of a cluster analysis. The identified clusters appeared to be the same as the new problem areas identified by engineers of WML. In practice, the water supply consultant now proactively informs the pipeline operators about vulnerable areas and plant renovation based on the results of the cluster analysis (example in Figure 1).

Figure 1. Larger and smaller projects as determined by the cluster analysis
Yellow, orange and red lines: the replacement priority of pipes. Dotted lines: pipes that have been removed. Red diamonds: places where leaks have actually occurred. Most leaks occur at spots identified in the cluster analysis.

Finally, for various replacement scenarios, the number of leakages to be expected and the corresponding development of the average age of the network were calculated. Figure 2 shows the effect of a higher or lower replacement rate, on the number of leakages in the coming decades. The reference is the continuation of the replacement strategy from the period 2016-2020, being a fixed replacement rate of 1.25% per year.

Figure 2. The effect of the annual replacement rate on the expected number of additional failures
Reference is the '1.25% per year scenario'. ‘Proactive' is the scenario in which, on the basis of cluster analyses, more than 1.25% is replaced per year for the first 10 years and less later.


Based on the modelling results, WML has decided to maintain the replacement rate of 1.25% per year for the time being, but to proactively replace more pipelines in the calculated clusters. This is expected to reduce the number of leakages per year. Leak-based replacement will be reduced, creating more opportunities to execute renovation works with third parties or to solve other bottlenecks in the pipeline network.

Coordination with other network operators

In order to achieve the lowest societal costs and limit inconvenience to the environment, WML aligns its pipe replacement activities with other parties, such as network operators, municipalities and the province. WML, Enexis and a number of Limburg municipalities use the SynergieNL tool for this goal. The network operators upload their planned work in a standardised manner and discuss it periodically. Before doing so, WML has already applied a prioritisation tool to determine which activities are eligible for cooperation. In the coordination meeting, it is determined for all overlapping projects whether it makes sense for one or more parties to adjust their own planning and 'follow' the other(s). Finally, a quarterly review with WML’s project execution department shows whether the planned projects have actually been executed.

The described long-term planning and coordination with other parties, have decreased the ratio of updates and changes in the planning of WML's executive department from around 40% in 2016 to less than 10% in 2020. This has been achieved by, together with Enexis, engaging with all municipalities and the province, by executing plans as scheduled ('agreed is agreed') and by continuous monitoring of the projects in the prioritisation tool. As a result, all parties can rely upon their planning thus reducing frustrations and unnecessary costs for preparations and disinvestments. An additional advantage of the project coordination tool is that Enexis and WML always work with the same up-to-date data and that staff can be replaced during a consultation while having access to the available data needed to make optimal decisions.

After this coordination, the selected projects go through the implementation process of hydraulic design, implementation and execution, including registration in the GIS system. During all these phases, data is enriched or added to the project area and used again afterwards for analysis purposes. For example, some analyses are automatically performed on completed projects, such as checking 'selected pipes replaced', 'sectioning' or 'dead ends'.

If a gas pipe has recently been replaced within two metres of an existing drinking water pipe, the failure frequency turns out to be significantly higher – up to four times higher than for pipes in which proximity no gas pipe has been replaced. Leaving the former water pipes in place therefore increases the risk of failure. This example underlines the urgency of good coordination and cooperation with other parties. In the near future, research into the influence of third-party activities should result in a predictive model.

Implementation in operations and policy

The complete replacement process has been assigned to the chairman of WML's renovation team. In his role as linking pin, he ensures cooperation with the various internal departments and with Enexis, the municipalities and the Province. In addition, various tools were developed to control and monitor the processes and the projects.
In the short term, automated checks can be carried out at project level. In the medium and long term, the effects of the replacement strategy will become measurable. This will give the asset manager the opportunity to alter the strategy and thus impact the anticipated effects.

The annual plans for 2018, 2019 and 2020 have already increasingly been aligned with the results from the knowledge model. WML is now so convinced of the additional quality insights from the cluster analysis that the impact of the strategy of 1.25% replacement per year will be increased by integrating the results of from the cluster analysis in the coming years. By doing so, WML is one of the first Dutch drinking water companies to implement all the elements of data-driven asset management needed to close the Deming circle (plan-do-check-act) from the ISO-55001 standard, including data registration and integration, modelling, decision-making and coordination of activities.

WML considers the cluster analysis to be a very powerful and valuable tool to generate short-term replacement plans. The TransparentNL knowledge model is more suitable to generate medium- and long-term replacement plans. By combining these two approaches, an optimum working method will be applied in the coming years. WML does however emphasise that these knowledge models will always be decision support models - and not 'crystal balls'.

Richard Peerboom
Ignaz Worm
(Spatial Insight)
Jurjen den Besten
(Spatial Insight)
Geert Linssen


With this study, the Limburg drinking water company WML and consultant Spatial Insight close the circle of generating, processing and using data in the management of a drinking water network (plan-do-check-act). Combined asset and failure data are leveraged to determine the optimal ratio between performance, costs and risks of the distribution network. In other words, data-driven asset management that makes use of integrated spatial data and data-science techniques such as machine learning and regression analysis. The optimized replacement strategy reduces the inconvenience for citizens as well as the societal costs of managing underground pressurised infrastructure.

^ Back to start

Data driven – low societal costs

Knowledge journal / Edition 1 / 2021

Five major improvements for estimating flood probabilities

Since 2017, in the Netherlands the government has been working with new legal standards for the primary water defences: river dikes, sea dikes and dunes. The transition phase from the old to the new system is still ongoing. However, the estimated failure probabilities sometimes turn out to be clearly too high, and plans for dike heightening unnecessarily drastic. In this article, we look for ways to allow for more realistic flood probabilities. This allows for better design of flood defences.

The new system for the periodic assessment and reinforcement of primary flood defences is based on a risk approach. Scenarios are used to estimate the probability and consequences of several potential floods. Uncertainty is essential: both the strength and load of flood defences and the consequences of flooding are uncertain. The legislator has decided not to incorporate the risk method in its entirety into the Water Act, but to include maximum permissible flood probabilities for primary flood defences. However, this is based on the risk approach: where the consequences are significant, the requirements are also more stringent. An levee assessment instrument (in Dutch: ‘Wettelijk Beoordelingsinstrumentarium, WBI’) is available to support the watermanagers who maintain these levees. However, the content and application of this toolkit still contains many challenges (NWO Domain Science (ENW), 2020).
The central question in this article is how the estimation of flood probabilities can be improved, and what this means in practice for the assessment and design of flood defences. We focus on 5 selected themes: ‘the BIG5’. Based on a rough list of possible improvements, experts from various disciplines (risk methodology, hydraulics and geotechnics) determined the BIG5 topics: application of fragility curves, tightening of the failure definition for piping, time dependency for revetments and macro-stability, model uncertainties and the length effect.
The selection of BIG5-topics is based on experiences with the WBI and expertise about the underlying data and models. In the development of concrete cases, we have worked closely with water boards. A full overview of the study design can be found at

Figure 1. Fragility Curves describe the failure probability of a flood defence as a function of water level. The probability contribution of different classes of water levels can be compared with measurements, knowledge and experience and adjusted accordingly.

Case study 'Fragility Curves’

The current WBI toolbox is largely based on a semi-probabilistic approach. This means that assessments and designs are based on one carefully chosen combination of water level and wave height. The idea is to make the approach more simple but this has two disadvantages. Firstly, ENW (2019) noted that this approach does not provide a handle for the manager if the outcome is unrealistic. Secondly, the manager has no insight into the behaviour of the flood defence at different water levels, while that is essential for crisis management, for example. Working with fragility curves offers a solution for both disadvantages, especially when the calculated failure probabilities are relatively high.
Fragility Curves describe the probability of failure of a flood defence as a function of water level (Figure 1). It is relatively easy to create and adjust Fragility Curves (Kanning and Schweckendieck, 2017). The result is a better understanding of the failure probability and a practical tool to contribute managerial knowledge and verify model results in an unambiguous manner. This may lead to a sharpening (and understanding) of the schematization of flood defences and the judgement. In time, other loads besides water levels may be included in a multidimensional Fragility Curve.

Case study 'Failure definition for piping'

In the current system, the calculated failure probability for a failure mechanism is not equal to the flood probability. There is therefore ‘gap' between the applied definition of failure (on which the failure probability is calculated) and the occurrence of a breach in a levee which starts a flood, this is called 'residual strength'. In the case study 'Failure definition for piping' it was investigated how this residual strength can be interpreted and included.
By adjusting the WBI definition of failure in this respect, the calculated failure probability becomes more realistic. With probabilistic analyses, the various sub-mechanisms of piping (bursting, heave, recurrent erosion) can be described in conjunction, which does more justice to the physical processes. The failure probability can then be determined at different load times, by taking into account the response and pipe growth. Thus, the ‘educated guess’ that piping is less relevant with short loads can be taken into account as well.
The recommendation for dike managers is therefore to extend the current WBI failure definition of piping to include time dependency, and to make an informed choice as to which sub-mechanisms are to be quantified. Much is already known about this. This advice also applies Rijkswaterstaat (RWS, the national agency for public works and water management), which plays an important role in disseminating the existing knowledge.

Figure 2. Illustration of the current failure definition for an existing pipe based on the Sellmeijer formula (left) and a grown pipe according to the new failure definition developed in the BIG5 (right).

Case study 'Time dependency for revetments and macro-stability’

Macro-stability concerns the danger of shearing off large parts of a dike along shear planes. Unrealistically high failure probabilities for macro-stability are the result of conservative choices in the geohydrological schematization. When determining the water pressures in and under the dike, a constant water level against the dike is now assumed. This does not take into account that these water pressures need time to build up and that a high water only stands against the dike for a limited time.
In our BIG5 study, we developed a time-dependent approach. Based on a schematic representation of a water level development, we analyse how the water pressures in and under the dike react to this. This provides a more realistic picture of the water pressures.
A time-dependent approach leads in a probabilistic calculation to a lower failure probability than in the standard approach (the so-called 'quasi-stationary analysis'). The water pressures in and under the dike are lower. However, these time-dependent analyses are still laborious and time-consuming.
The failure probabilities resulting from the assessment of grass revetment on the outer slope (failure mechanism erosion of outer slope (GEBU)) are often relatively high and not in line with the experience of the dike managers. The following simplifications are possible:
• More straightforward processing of wave loads and water level changes. Now, very different events (storm, flood, combinations of these) result in one normative combination and therefore an unrealistic load;
• a simpler failure definition. Currently, the dike 'fails' when the erosion depth is greater than 50 centimetres after failure of the grass cover. In reality, the remaining clay or sand core of the dike will also have to erode before flooding occurs. This residual strength is not yet taken into account.
In case of an unrealistically high failure probability, the dike manager is advised to follow a probabilistic approach for water level and wave propagation, and to apply the residual strength model to include further erosion.

Case study 'Model uncertainties Hydraulic Load'

In the assessment and design of flood defences in the Netherlands, the model uncertainty is incorporated into the water levels, by considering it as an additional stochastic variable. The current water level frequency lines, including model uncertainty, do not always correspond to physical insights, which may lead to an unrealistic increase of standard water levels (up to 0.5 m). The corresponding exceeding risk of a given water level can therefore be more than a factor of 100 higher than without this model uncertainty. This probability factor subsequently affects the assessment of, for example, piping and macro-stability.
Recently, Rijkswaterstaat has looked into this matter in the research project 'Knowledge for flood defences'. This has led to a sharpened and generally lower standard deviation of the model uncertainty in the water level. This results in different water level frequency lines and thus different flood probabilities of a flood defence. Our research shows that in the current semi-probabilistic approach, the effect of these 'new' model uncertainties hardly affects the calculated failure probability. That is because of the approach itself. With a different, probabilistic approach, this effect can indeed be taken into account, resulting in a lower failure probability. We recommend that, in areas where sharpened model uncertainties are available or failure probabilities are perceived to be high, the new model uncertainties are used probabilistically.

Case study 'Length effect’

The length effect means that the failure probability of a long flood defence is always greater than that of a comparable short flood defence. There are several reasons for this. Applying WBI 2017 at the level of entire dike sections (dozens of km) often leads to (unrealistically) large flood probabilities, especially for piping and inward macro-stability. In the BIG5 study, we analysed how the length effect within dike sections and the correlations between dike sections influence the probability of flooding at dike section level. First, we combined the length effect with proven strength at a cross-sectional level. In the past, some of the Dutch flood defences were regularly subjected to heavy pressure without dike breaches. Taking into account the length effect, a form of proven strength at dike section level can thus be derived for a failure mechanism.
Secondly, it was found that while dividing larger sections into smaller ones may result in a clearer picture, it also leads to interdependence of the dike sections, which in turn overestimates the failure probability at section level. It is recommended to take into account the influence of the section size on the length effect when performing an assessment.


The significance of the BIG5 research in actual practice is threefold. Firstly, a more accurate assessment of the probability of flooding by the defence manager (in particular the water board) gives a sharper picture of water safety in the Netherlands. The cases show that flood probabilities can be reduced by a factor of up to 100.
Secondly, the safety task for a reinforcement becomes much smaller and sometimes it can even be omitted. This will result in considerable savings (billions of Euros!) in the High Water Protection Programme and less inconvenience for local residents.
And finally, the insights can be used by Rijkswaterstaat (national agency for public works and water management) to improve the assessment and design instruments in the short term.

Bas Kolen
Matthijs Kok
(TU Delft / HKV)


Since 2017, in the Netherlands the standards for primary flood defences have been formulated as flood probabilities. The transition to this approach is in full swing and in this article we show what substantial improvements can be made. These improvements were immediately tested in practice, which showed that in the cases studied reductions of a factor of 2 to even 100 in the calculated probabilities of flooding were possible.


ENW, 2020. Naar geloofwaardige overstromingskansen. Expertise Netwerk Waterveiligheid, Februari 2020.

HKV, 2021. Resultaten BIG 5 en doorkijk naar lopende beoordelingen en ontwerpen, BOI en KvK. RO0916.10, 8 Januari.

Kanning, Wim en Timo Schweckendiek, 2017. Handreiking Faalkansanalyse en Faalkans Updating Groene Versie - Macrostabiliteit Binnenwaarts. Deltares, 11200575-014
^ Back to start


Five major improvements

Knowledge journal / Edition 1 / 2021

Lost in translation - use of Swiss and German key figures for the removal of micro-pollutants from sewage

The implementation of measures for the advanced removal of organic micro-pollutants from sewage water draws on experiences in Germany and Switzerland, where they are already well advanced in this field. It regularly transpires that a key figure from abroad cannot simply be applied to the Dutch situation. Some studies compare apples and oranges. There is still much to learn.

Engineers love to know figures: what are the costs per cubic metre, what is the consumption in grams of powdered activated carbon or ozone per cubic metre, and what is the energy consumption in kWh and the CO2 emission per cubic metre or population equivalent? How do the investments relate to the operating costs? These key figures help us to project experiences gained elsewhere onto our own situation. It is a translation key and usually it works very well.
Specifically in the area of micro-pollutants, much practical information is exchanged between Switzerland, Germany and the Netherlands, although this is mainly one-way traffic towards the Netherlands. In recent years, we have seen a development in the various aspects of micro-pollutants, initially learning from the Swiss in particular and later also from the Germans. Initially, the need was mainly for technological experiences. The first thing was to determine which techniques work, and what the achievable removal efficiencies are. Then the focus shifted to engineering, with questions about the facilities to be built, such as capacities and related costs. Finally, more and more attention is being paid to the impact on the user's sustainability ambitions, mainly expressed as a carbon footprint.


This article presents the key figures from the German literature and compares them with the Dutch key figures, as published in STOWA reports (see the reference list). We follow the order as outlined in the previous section: 1. Engineering and Technology; 2. Engineering and Costs and 3. Sustainability. Each time, we analyse where the differences come from and discuss the factors that determine the usefulness of the relevant key figures. For a number of key figures, a proposal is made as to how these can best be used in the Dutch situation and what the bandwidth is for the Dutch situation.
The sources for the German-language key figures are articles from German-language professional literature, the two Kompetenzzentren knowledge centres in Baden-Württemberg and Nord-Rhein Westfalen and the website of the Verband Schweizer Abwasser- und Gewässerschutzfachleute.

Oxidation and adsorption

Currently, there are mainly two technologies that are widely used for the removal of micro-pollutants: oxidation (usually with ozone), and adsorption (usually with activated carbon in powder form (PAC) or granular form (GAC). In this article, key figures of these technologies will be discussed.
In Switzerland and some German states, a removal efficiency of 80% over the entire wastewater treatment plant is targeted for a selection of guide substances. These guide substances are partly the same as the guide substances designated in the Netherlands, but not exactly the same. In addition, based on the guidelines from the Ministry of Infrastructure and the Environment, in the Netherlands we are aiming for a 70% removal efficiency. In the Netherlands, this 70% abatement must be achieved for 7 of 11 guide substances; the choice of guide substances is free for each sample taken. In Switzerland and Germany, the freedom of choice is much more limited. In general, the Swiss requirements and also the German directives are therefore stricter than the Dutch requirements. This is already an important difference that has to be taken into account when translating Swiss and German key figures.

Engineering and Technology

Table 1a lists the relevant technical and technological key figures. These determine how much ozone or activated carbon is required for the desired removal of micro-pollutants and thus the dosing capacity to be installed in kg ozone or PAC per hour.
Both ozone and PAC react with micro-pollutants, but also with other compounds present in the water like nitrite and organic macromolecules, such as humic acids. These organic macromolecules (often presented as DOC, dissolved organic carbon) are present at a concentration 1,000 to 100,000 times higher than micro-pollutants and almost entirely determine the necessary overdosing of ozone and PAC to subsequently tackle the micro-pollutants as well. For various reasons, the DOC content in Swiss wastewater appears to be significantly lower than in the Netherlands. In addition, its bandwidth is greater within the Netherlands. By expressing the dosage as grams of ozone or activated carbon per gram of DOC, this problem is overcome.
However, also when we look at the DOC-specific dosage, we see a significant difference: in Switzerland, up to 30% less ozone is dosed per gram of DOC than is necessary in the Dutch situation, according to Dutch research. This is probably caused by a different composition of the DOC, in addition to the already mentioned removal targets. Because of this higher dosage, the energy consumption and carbon footprint per cubic metre are higher in the Netherlands. Moreover, this also directly affects the formation of bromate during ozonisation. The formation of this undesirable product increases with higher ozone doses. For both PAC and GAC, a lower DOC concentration also has a positive impact on the dose rate and the GAC retention time, but the effect seems less pronounced than for ozone.

Table 1. Comparison of key figures for technology, costs and sustainability for extensive removal of drug residues from wastewater.
1 Removal efficiency of wwtp effluent including untreated bypass, versus influent.
2 Contact time 30 minutes. 3 Average mix for the Netherlands. 4 Several renewable materials: wood, coconut shells, MDF waste (STOWA 2020-19).

Engineering and costs

As an illustration, the differences between the Dutch and German key figures for costs are presented in Table 1. This immediately raises the question: why is it cheaper in Germany? Comparing cost figures is a tough job even within the Netherlands, let alone in an international context. It can go wrong in many ways. That is why, within the Innovation Programme on Micropollutants (IPMV), strict agreements have been made on the way in which costs are compared, namely within a predetermined fictitious case for a WWTP of 100,000 p.e.
When comparing construction costs, it comes down to matters such as the reserve count used, the construction method, the type of subsoil and safety facilities, which in turn are related to legal requirements and site-specific customer wishes, etc. Another important factor is where the system boundaries lie. All these factors determine the numerator of the cost per cubic metre.
Furthermore, it is important to check what exactly is included in the denominator of the key figure - which cubic metres are involved? It makes a difference whether the wastewater treatment plant under consideration is designed for dry weather flow or a higher capacity including (part of) the peak during rainy weather flow. In the IPMV, it was agreed that the costs should only be based on the actual volume treated. In foreign presentations of key figures, however, it is often decided to allocate the costs of post-treatment to the total flow rate (the treated and untreated parts together).
Finally, it appears that the allowance of investments and operational costs is very different, whereby in particular the calculation method for investment costs (depreciation method, interest rate) and maintenance costs, are very decisive. The translation of cost figures is therefore impossible without knowledge of the system used, depreciation periods, interest rates and assumptions for maintenance costs.


The most important measure of sustainability at the moment is the equivalent CO2 emission of the activity under consideration. Table 1 shows the most important key figures. This shows that the CO2 emission from electricity in Switzerland is significantly lower than in the Netherlands. This, of course, has to do with the method of generation. In contrast to the Swiss mix, the Dutch mix contains a very low proportion of green energy. The differences in CO2 emission from electricity also affect a number of other key figures such as those for liquid oxygen production and activated carbon activation and regeneration. For GAC, the main difference is related to the electricity consumption for activation and regeneration. In other aspects, such as coal mining and transport, there are practically no differences. Since PAC cannot be regenerated, the carbon footprints differ much less, these are mainly related to a different effluent composition in terms of DOC. In addition to the values in kg CO2 per kWh or kg carbon, the technological differences also strongly influence the sustainability values in kg CO2 per cubic metre sewage water. The effect of a higher ozone dose or longer GAC life time due to, for example, a higher DOC concentration, quickly worsens the sustainability score by tens of percent.


Key figures for purification technology for the removal of micro-pollutants from sewage do not have universal validity, they need translation. Working with key figures per m3 and extrapolating from this leads to incorrect choices in the selection of technologies. The authors therefore call on everyone to find out for themselves what the best choice of technology is, based on the specific local situations at the wastewater treatment plants, and what the related costs and carbon footprint would be. Key figures from home and abroad can be used, but they must be carefully translated. In this way, variants can be properly compared and investment decisions can be better substantiated.

Mirabella Mulder
(Mirabella Mulder Waste Water Management)
Herman Evenblij
(Royal HaskoningDHV)
Arnoud de Wilt
(Royal HaskoningDHV)

Background picture:
The dosing installation for powdered activated carbon in the full scale PACAS demonstration project at the Papendrecht wastewater treatment plant.


Experiences from Germany and Switzerland are used in the implementation of measures for extensive removal of organic micro-pollutants from sewage. It regularly transpires that a key figure from abroad cannot simply be applied to the Dutch situation. This article presents the key figures on engineering and technology, costs and sustainability from the German-language literature and compares them with the Dutch key figures. This shows that these key figures have no universal validity, they require translation. The authors therefore call on everyone to find out for themselves what the best choice of technology is, based on the specific local situations at the wastewater treatment plants, and what the related costs and carbon footprint would be.


STOWA 2015-27 Removal of micro-pollutants from wastewater treatment plant effluents

STOWA 2017-36 Exploration of technological options for the removal of pharmaceuticals from wastewater

STOWA 2018- 02 PACAS - Powder carbon dosing in activated sludge for micro-pollutant removal

STOWA 2018-46 Freshwater plant WWTP de Groote Lucht: Pilot study on ozonisation and sand filtration.

STOWA 2018-67 Proof of Concept and laboratory research removal of micro-pollutants from sewage effluent with the O3-STEP filter.

The feasibility studies in the Innovation Programme Micropollutants from Water, of STOWA and Ministry of Infrastructure and the Environment, see here

STOWA 2020-41 Pilot study comparison oxidative techniques effluent Aarle-Rixtel wastewater treatment plant

URLs websites:

^ Back to start


Removal from sewage

Knowledge journal / Edition 1 / 2021

Restoring a riparian zone does not end at the waterline

After restoring banks along waterways to a more natural state, often a diverse vegetation develops. However, what kind of plants will grow there after a few years and what is their ecological value? What fish and other aquatic animals will live there? And what is the impact on water quality? Nature organizations FLORON and RAVON, together with Wageningen University and Research centre, are doing a national study into the value of restored banks.

In recent decades, hundreds of kilometers of banks have been restored to a more natural state, new restored banks are added every year. Often, the goal is to improve the water quality in order to meet the targets of the Water Framework Directive of the European Union (i.e. increase the WFD score). Since 2017, FLORON, RAVON and the WUR, funded by water boards, provinces and STOWA (Foundation for Applied Water Research), have been studying older restored banks in a standardized way. So far, 61 restored banks have been investigated; all for aquatic plants (both close to the banks and in the water), 52 for fish and 30 for macrofauna.

Restored and traditional, non-restored, banks

In this study, we compare restored banks with nearby, non-restored sites (hereafter: references). This shows that especially the riparian zone benefits from bank restoration. The bank zone or riparian zone has widened with on average more plant species than in the references (Figure 1). In the aquatic zone, on average there are no statistically significant differences in the number of plant species and aquatic plant cover. In restored banks along canals the average number of macrofauna taxa is significantly higher than in reference sites, but this is not the case in ditches and streams. On average, one extra fish species was found at sites with restored banks compared to the references. This increase (as a percentage) is comparable to the increase in plant species. A recent study in the province of Noord-Holland has shown that restored banks can accommodate more juvenile fish than banks along canals supported by sheet piling.
However, it is striking that even in restored banks the transition between the riparian and aquatic zone was often quite sharp. As a result, the spatial variation in water depth was limited, both horizontally (parallel to the watercourse) and vertically (perpendicular to the watercourse) (Figure 2). This uniformity, resulting from the often ‘technical' uniform construction (rather than focusing on more natural heterogeneity), can potentially contribute to a lower diversity of flora and fauna. Furthermore, water quality (estimated on based on vegetation and macrofauna) also seems to play a role in limiting plant diversity.

Figure 1. Average number of plant species (± SE) found in the riparian zone (left) and aquatic zone (right) of restored banks (green) compared to references (yellow). Usually, banks were visited 2 years in a row: species lists of both years were combined. (Statistics: p=0.001-0.01: **; p=0.01-0.05: *; p>0.05: ns. One-sample T-test on the difference between NVO and corresponding reference (μ=0)).

Improving bank restoration

Based on our results, we recommend focusing not only on the bank zone but also on the water zone when constructing and subsequently managing a restored bank. The water zone is very important for meeting the targets of the EU Water Framework Directive, but in general benefits too little from bank restoration. This can be seen in the scores for the WFD water quality assessment, the so-called Ecological Quality Ratio (EQR), with which the ecological quality is quantified and judged. The EQR scores of restored banks on average were not much higher than in the nearby references. (Note: the EQR is not designed for the evaluation of individual banks).
In uniformly constructed banks, there are opportunities for a higher ecological value, for example by realizing more spatial variation. With different water depths, both shallow and deeper (open) water, additional habitats can be created for animal species and for different growth forms of aquatic plants. Based on the WFD, in most water types the presence of different growth forms of aquatic plants is desirable.

Management often uniform

Enquiries to some water authorities revealed that the management of the water zone at restored banks is often done in the same way as at the nearby reference. Knowing this, it is easy to imagine that there will also be relatively few differences underwater. If the helophytes are kept as a ‘boundary’ in the management of the water body and the bank, it is conceivable that eventually a homogeneous, straight strip of helophytes will form (Figure 2, right). If the bank zone is not managed, or is managed very extensively, a sharp transition between land and water may develop consequently. The bank zone can show terrestrialization, while the intensively managed water zone remains open/deep, even if the transition from land to water was gradual and/or erratic before.

Design and management

When restoring banks, it is important to define a clear goal: is the aim to create a reed marsh for birds, an ecological connection and/or a good score for the WFD? The design must be in line with the goals and local (abiotic) conditions. After construction, it is important to find out what management is needed to maintain or develop the described or visualized target. As this is difficult to determine accurately beforehand, it is advisable to consider experiments with different management frequencies and with spatial patterns of phased management (mowing and aquatic plant removal). Take plenty of time for this, at least several years. Flexibility is needed, because the effect of management can vary from year to year, for example due to a dry or very wet summer.
The water and soil quality, and thus plant productivity, can interfere with a good ecological status of a bank (based on the WFD goals). In our study, we regularly found turbid water at restored sites, resulting in little or no submerged plants. Along a nutrient-rich ditch, high helophyte vegetation is thus also expected. In such high vegetation, we often find mainly common species that are less desirable for the WFD. In the study, we even came across restored banks with lower species richness and EQR scores than in the non-restored references (which were more open, due to frequent mowing). High productivity of tall plants in the bank vegetation can cause organic matter buildup and also shading on the watercourse itself, which can further impede aquatic plant growth.
The abiotic conditions and the local species pool (including migration possibilities) will determine the ecological potential of a restored bank. Management then influences whether this potential becomes visible by means of increased biodiversity. In order to maximize the effect of the restoration, it may be necessary to do something else first, such as terminating nutrient inflows or facilitating fish migration. In short, it is important to set measurable goals and to look from a landscape viewpoint and choose realistic site-specific goals. This can vary, for example, from a reed marsh in one (dirty) spot, to open more diverse vegetation with rarer species in another (clean) spot.

Figure 2. Examples of restored banks with a lot (left), moderate (middle) or almost no spatial variation (right). The banks on the left and in the middle had a nice variety of river bank plants, while the bank on the right did not. Often the restored banks looked like the one in the middle or right-hand photo, especially when productivity was high and frequent management of the watercourse was considered necessary for water flow.


A measure that will always produce the desired result everywhere does not exist. We have to deal with different types of water, local conditions and therefore different goals.

Ditches and canals (stagnant water)
For stagnant water bodies, the advice is to construct a wider, gradually sloped bank without hard artificial bank structures. This increases the chance of a higher species diversity in the bank zone. In doing so, allow the riparian zone to extend into the aquatic zone, to allow for a more diverse flora and fauna. Where there is a risk of bank erosion, a partially open, protecting foreshore can be constructed. Behind it, deeper water (at least 20 cm) should be present for fish, as well as shallow water. This will allow, at least locally, submerged aquatic plants to grow, serving as habitat for fish and macrofauna. The desired water depth depends, among other things, on the clarity of the water and the target species.

In the Netherlands, maintaining flow in streams and small rivers was a major problem in the dry summers of 2018 and 2019. However, even in the summer of 2017 about half of the surveyed streams and small rivers (R-type in the WFD) showed no visible water flow. Substantial water flow is essential for typical plants, fish and macrofauna to occur in streams. However, we often found many generalists and/or animal species typical for stagnant, plant dominated waters, resulting in an on average low proportion of typical, stream related species, resulting in lower EQR scores. Making streams and (small) rivers climate-resilient by preserving sufficient water flow is therefore an important task for the near future.
In several streams and small rivers, we found wide restored banks along the watercourse. This can be detrimental to the flow rate. Here, bank restoration that does not widen the watercourse is preferred to prevent flow velocities from becoming even lower. By (locally) deepening and/or narrowing, higher flow velocities can be created. Narrowing may also be possible by extending the vegetation from the bank zone into the streambed.
Restoring flow, at least locally, can contribute to the typical variation in streams and small rivers (differences in flow, substrate and vegetation). However, it is important to keep an eye on the whole system, as narrowing or deepening can reduce flow velocity upstream.

Take home message

Restored banks can be successful when there is sufficient variation in abiotic conditions (layout, spatial variation) and when water quality and quantity meet the requirements of the desired species. The aquatic zone typically requires more attention: restoring banks to a more natural state does not stop at the water line.

Michiel Verhofstad
Edwin T.H.M. Peeters
(Wageningen University and Research Centre (WUR))
Jelger Herder
Jeroen van Zuidam


Since 2017, FLORON, RAVON and WUR have been investigating the ecological value of bank restoration. Compared to non-restructured banks, restored banks are on average wider with a more species rich vegetation, but in the adjacent water zone on average less improvement is observed. Moreover, many restored banks are uniform and straight, and in streams and small rivers, they sometimes cause a reduction in flow rate, which is undesirable.

Bank restoration can be a success if there is sufficient spatial variation and if the water quality and quantity meet the requirements of the desired species. The aquatic zone deserves more attention: restoring banks along waterways to a more natural state does not stop at the water line.


Verhofstad, M., J. Herder, E. Peeters & J. van Zuidam (2021). Kunstmatig natuurlijk. Een evaluatie van de meerwaarde van natuurvriendelijke oever. Gegevens: 2017 t/m 2020. Rapportnr. FL.2017.034.e2

^ Back to start


Allowing nature in

Knowledge journal / Edition 1 / 2021

More from less, producing bioplastic from wastewater

What is the effect of the presence of nutrients on the production process of polyhydroxyalkanoates (PHA) from wastewater? This is the central research question in a study that makes an important contribution to the definition of the desired wastewatercharacteristics for the production of bioplastics.

The image of wastewater is shifting more and more towards a source of energy and raw materials. Wastewater that contain a lot of organic carbon can, by means of anaerobic treatment, be upgraded to produce methane from the volatile fatty acids (VFA) present. By making use of what nature has to offer, other, potentially more valuable products can be made instead of methane from these VFA.
One of these alternative products is the polymer group 'polyhydroxyalkanoates' (PHA), better known as a bioplastic due to its properties similar to regular plastics, with the main distinction being that it is biodegradable. Two known routes have been developed for producing bioplastics from wastewater:

1. The PHARIO process, in which the natural PHA storage capacity of activated sludge from wastewater treatment plants is used.

2. The second option, which was used in this article, has been based on the cultivation of specific bioplastic-producing microorganisms.

For the production of methane, the main trick is to create an anaerobic environment; the production of bioplastic is more complex. The production process of bioplastics according to route 2 can be divided into 2 processes. In the first process, part of a VFA-rich wastewateris used to grow bioplastic-producing microorganisms. The key words that describe the selection trick in the enrichment are 'eat fast and get fat'.
An environment where this comes into its own is the so-called 'feast-famine' regime. Substrate (VFA) is fed in batches to the culture, creating periods where there is a lot of substrate (feast) and periods where there is no substrate (famine). A bacterium that eats the food present the fastest and can make enough storage to grow in the nutrient-poor period wins the competition. Hence, 'eat fast and get fat'. A champion of 'eating fast and getting fat' is the bacterium Plasticicumulans acidivorans. This bacterial species is capable of storing 9 times its own weight internally as PHA. In other words, you have a fat belly that is 9 times your own body weight!

Accumulation process

When the culture consists of as many of these champions as possible (now without a fat belly) it is sent to the second process, accumulation. In the accumulation process, the substrate is used as much as possible to become fat and as little as possible to grow. One way to force this is to take a wastewater that is poor in an essential growth nutrient such as ammonium or phosphate. The absence of ammonium and/or phosphate in the accumulation means that no growth can take place. When the substrate is added in doses to the bacterial culture, it can only go towards storage ( bioplastic production) and the bacterium feeds on it up to the aforementioned 9 times its own body weight.
There are wastewaters that are potentially interesting for the production of bioplastics due to a high chemical oxygen demand (COD). However, the presence of growth nutrients makes it more challenging to make efficient bioplastics from them because growth cannot be excluded in the accumulation. The effect of the presence of nutrients in the accumulation process was investigated to gain more insight into this topic. For this purpose, a parallel reactor system was used in which a predetermined amount of COD with a varying amount of ammonium and/or phosphate was dosed to a culture enriched with Plasticicumulans acidivorans. This will make it possible to research, among other things, whether minimal growth is really harmful to the bioplastic production process and how long the champion can last, because with growth there are also opportunists lurking.

A little growth, a lot of bioplastic

An enriched culture with Plasticicumulans acidivorans needs 8 to 12 hours in a 'classical nutrient-limited' accumulation to reach 90% dry weight (wt%) PHA. Practical research has shown that an optimum based on yield is rather around 4 hours of accumulation. However, in practice there are no ideal conditions, so growth nutrients may be present. Since it is desirable to use the VFA as efficiently as possible, it is therefore valuable to find out what effect these nutrients have on the bioplastic process.
By using a parallel reactor system, several COD:nutrient ratios were tested with an enriched P. acidivorans culture. An advantage of this is that the starting point is the same for each experiment. Since different COD:nutrient ratios have been chosen, ranging from highly nutrient-limited to mildly nutrient-limited, frameworks have been found in which bioplastic production is possible under different conditions.
Several characteristics of the process have been better mapped out with this study. Firstly, it has become clear that P. acidivorans remains dominant for a long time under different conditions. All cultures produced and retained significant amounts of bioplastic during 12-hour accumulations, with the ratio of COD:nutrient in the substrate directly influencing the purity that could be achieved - as illustrated in Figure 1.

Figure 1. The amount of bioplastic and biomass produced after 12 hours accumulates with different ratios of COD:nutrient. Highly limited means that the COD:nutrient ratio is high.

Figure 1 shows that the amount of bioplastic produced under different conditions is largely the same. However, the purities decrease drastically from 85 to 60 wt% in the presence of sufficient nutrients.
It has been shown that bioplastics can achieve purities of 60 wt% or higher regardless of the presence of nutrients. It is important that the production process does not last longer than 6 hours. After these 6 hours, the purities decrease when sufficient nutrients are present and the process becomes less profitable. This will give leeway as there is a production time that is independent of the nutrient concentration.
This phenomenon of 6-hour constant production probably occurred because the bioplastic bacteria, as in the culture, were trained to eat very quickly. In this way, they take away most of the food from opportunistic bacteria; when the bioplastic bacteria are reasonably full, they will eat more slowly and there will be more room for opportunists. In addition, the opportunists grow exponentially and it is only a matter of time before they out-compete the bioplastic culture. An example of the presence of opportunists is shown in Figure 2.

Figure 2. The same starting culture after 12 hours of accumulations in the presence of many or few nutrients.

Figure 2 shows microscopy pictures of two cultures, after 12 hours of accumulation with different substrates. In the nutrient-poor situation, P. acidivorans still seems to dominate the culture. This is not the case if sufficient nutrients are present. There seems to be another species out-competing with the bioplastic producer.

Chicken-egg situation

It can be said with great certainty that bioplastics can be produced from many fatty-acid wastewaters. This position is confirmed by several pilot studies carried out on wastewater that are highly nutrient-limited to wastewaters that are nutrient-rich. For example, bioplastic purities of 70 wt% to 80 wt% have been achieved on wastewater from a paper-mill factory, a chocolate factory and on leachate water from vegetable, fruit and garden waste (VFG). This shows that the techniques used in lab experiments can also be successfully applied under real wastewater conditions and on a larger scale.

Final thoughts

Waste flows can be nutrient-limited, which is advantageous for the bioplastic process because growth of (other) bacteria can be excluded. However, not all waste flows that are attractive for producing bioplastics from are nutrient-limited. This study researched the effect of the presence of nutrients on the bioplastic production process. The importance of a good inoculum was demonstrated as at least 60 wt% bioplastic could be obtained for a period of 6 hours regardless of the presence of nutrients. Furthermore, the ease of producing bioplastics can be better estimated in advance, based on the characteristics of the wastewater flow.

Michel Mulders
Jelmer Tamis
(Paques biomaterials)
Gerben Roelandt Stouten
(TU Delft)
Robbert Kleerebezem
(TU Delft)


Anaerobic digestion is often used to create value from agricultural waste in the form of methane-containing biogas. New processes are being explored that produce more valuable products instead of biogas from these waste flow. A promising process is the production of biodegradable plastics. In this process, the trick is to have as many bioplastic-producing bacteria as possible, and to minimise the proliferation of other bacteria. This article presents the results of a study that examined the influence of growth macronutrients (in this case ammonium and phosphate) on the bioplastic production process.

Often, wastewaters are not strictly growth nutrient-limited, allowing bacteria to choose between growing and making bioplastic. How nutrient availability affects the distribution between growth and bioplastic production was the central research question in this study. Experiments were conducted in a multi-reactor set-up, with an enriched microbial culture dominated by the bacterium Plasticicumulans acidivorans (a bioplastic production specialist). Different amounts of growth nutrients (ammonium and/or phosphate) were added to a constant quantity of volatile fatty acids.

It has been shown that with the presence of a minimum ratio of ammonium (117 gCZV:gNH4-N) or phosphate (1055 gCZV:gPO4-P) bioplastic purities of at least 80% on a dry weight basis can be achieved in the first 12 hours of the production process. As the percentage of bioplastic per unit of dry weight is a crucial factor in the economic feasibility of the process, this research makes an important contribution to the definition of the desired waste flow characteristics for bioplastic production.


Simultaneous growth and poly(3-hydroxybutyrate) (PHB) accumulation in a Plasticicumulans acidivorans dominated enrichment culture

^ Back to start


Effect presence of nutrients

Knowledge journal / Edition 1 / 2021

Experiences with the ‘Inland buffer zone’ concept in the Koopmanspolder

Due to climate change and growing water demand, the Netherlands is increasingly facing water shortages. How can we avoid them in the future? Part of the solution is sought in the polders behind the dikes of the large water bodies: there we can reserve more space for water. This so-called inland buffer zone concept, potential water basins behind dikes, has been experimented with since 2012 in the Koopmanspolder pilot. What is the impact on the ecology and water quality in the polder if we optimize the water level regime with water from the IJsselmeer (Lake IJssel) for nature?

The IJsselmeer region (the lakes IJsselmeer, Markermeer and Randmeren) is a crucial freshwater reservoir for large parts of the Netherlands. However, the possibilities for water storage within the boundaries of this main water system are limited – during the drought of 2018, an intake stop from the IJsselmeer was very close. Together with partners, Department of Public Works (Rijkswaterstaat) is therefore exploring possible measures to help us avoid water shortages in the future. We do this by looking for innovative ways to distribute water better in space and time.

One of these innovations is the so-called 'inland buffer zone' concept. Such a buffer zone is a water storage area behind the dike where, with flexible level management, water can be stored from a nearby main water system. The design and management of such a water storage behind the dikes can simultaneously make a positive contribution to ecology, economy, safety and/or quality of life. A fine example is the Koopmanspolder pilot in Andijk, in the province of North Holland. After several years of monitoring, we have more insight into the ecological effects.

Design of the Koopmanspolder

In the Koopmanspolder (16 hectares), the opportunity arose to experiment with the storage of water from the IJsselmeer. The polder was newly designed in 2012. A connection to the IJsselmeer was constructed by an intake and a fish-friendly screw pump by FishFlow Innovations. The polder was formerly used as a soil depot. By constructing a ring-shaped structure with small dikes and water, the surplus of soil deposit has been incorporated into the western part of the polder. In the eastern part, there are three grasslands separated by ditches (see opening photo).

Experiments with water level management

After the polder was re-designed, experiments with extreme water levels were carried out in the period 2014-2016 to investigate the possibilities for water storage. An examination was carried out on the effects of the water level regimes on the surface water level, groundwater level around the polder, quality of the surface water, deposition of silt after inundation, presence and migration of fish, and presence of birds, butterflies and dragonflies, mammals, amphibians and aquatic and river bank plants. Subsequently, water level management was geared towards water storage in combination with the optimization of nature values. From March 2016, a natural water level regime with spring flooding of grasslands in the eastern part of the polder was introduced in order to facilitate fish spawning. In summer too, a high level was maintained to create a suitable habitat for birds and vegetation. The high level also helps suppress the influence of brackish seepage.


In the period 2016-2020, the Koopmanspolder had a natural water regime, no grazing by cattle and hardly any mowing management. In the western part of the polder (with the small ring-shaped dikes), goose feeding is low and reed has developed abundantly along the shores. The grasslands in the eastern part of the polder are inundated for several months every spring. The middle pasture here is the lowest and therefore has been under water the longest; in 2020, it was under water for almost 5 months. However, the vegetation is only dying off to a limited extent. Few bald patches appear in the grass and they grow back during the growing season. Goose feeding has been found to have a major restrictive effect on reed development here and the grass is also kept short by the geese. The abundance of herbs in the meadows has increased enormously (Figure 1); one of the herbs that has spread greatly is greater yellow-rattle, a species typical of dynamic systems. During the plant inventories, many insects were also found here (butterflies, grasshoppers and different kinds of flies). Water mint, hemp-agrimony, common fleabane and various thistle species are particularly popular with insects.

Figure 1. Increase in number of plant species
Greater yellow-rattle has spread considerably.

The birds, too, have greatly increased in abundance and species diversity. A systematic monthly count shows a sharp increase in the number of water and marsh birds since rewetting. There is a large proportion of piscivore (fish-eating) birds. In recent years, we have seen an increase in the number of reed birds, which is a logical consequence of the vegetation development (Figure 2).

Figure 2. Increase in piscivore birds and reed birds (source: SBB, Leon Kelder)

Besides spoonbill and little egret, purple heron and bittern were also observed. Furthermore, we see an increase in meadow birds that use the polder as a foraging area. There is also limited incidence of breeding, which is particularly successful for species such as lapwing, oystercatcher and redshank.

Students of Aeres Hogeschool sampled the polder for fish in eight places. Sampling is difficult, due to the dense growth of submerged aquatic plants. However, the results suggest an increase in fish. Visual inspections confirm this convincingly: between the water plants there are bare spots with very clear water where in summer a lot of fish can be seen, especially roach and rudd. There are many small fish swimming along the banks and in shallow pools (length approximately 2-3 cm). This image was unthinkable in the period before the reorganization (2012). In 2017, professional fish sampling was carried out by Sportvisserij Nederland. The count yielded 23 fish species and the fish biomass was estimated at 346 kg/ha. This is a high value for a clear water system.

Lessons from ten years of experimentation

After the organization in 2012, the polder was given a year to settle down and the vegetation started to develop. In 2014, the polder was strongly rewetted. Subsequently, the effect of an extremely low level was investigated in 2015, and that of an extremely high level in 2016. With the extreme water levels, we deliberately pushed the limits of the inland buffer zone concept. The quantities of water storage have been determined and we have investigated when adverse effects occur. Reports on this can be found on the website of Helpdesk Water. The findings can be summarized as follows: In case of emergency storage (extremely high level), we observed barely any negative effects on water safety. After a month of inundation, there was no significant damage to the dike revetment. At extremely low levels, we observed a temporary deterioration of the water quality. Not the effects we had expected, such as botulism, fish mortality and algal blooms, but a strong decrease in water transparency and an increase in brackish seepage.

Since March 2016, there has been a long-term natural water level with spring flooding. Thanks to a network of professionals and volunteers, there are now long-term measurements. These show that biodiversity is increasing and that the drought of recent years had no negative effects on nature in the polder. The area functions well as a spawning and juvenile area for fish, and the number of fish-eating birds has increased. Reed vegetation and reed and marsh birds are also increasing. Screw pump tests have provided more insight into when which fish will migrate from the IJsselmeer to the polder.

However, there are still many questions that we would like answers to. Fish seem to like to enter the polder, but will they also want to leave again? What can such marshy areas around the IJsselmeer signify for the ecological functioning and productivity of the lake, and what will this mean for long-term planning and management? What opportunities are conceivable in addition to ecology? How can these take shape?

Possible applications

The intensive and constructive cooperation between governments, knowledge institutes, education, NGOs, the business community and volunteers has created broad support and enthusiasm for exploring the possibilities of the inland buffer zone concept together. The ambition is to develop various types of ecological inland buffer zones in the IJsselmeer region. This ties in well with the ambitions of Agenda IJsselmeer Region 2050, the Programmatic Approach Large Waters and many regional initiatives in the IJsselmeer region. Various exploratory studies are underway, including on the Frisian coast, Markermeer dikes, nature reserves Wieringerhoek, Oostvaardersoevers, the Ketelmeer and the Randmeren. The inland buffer zone concept offers a range of opportunities for combining water management with other functions in the hinterland of dikes. Examples include living and working near and on the water, drinking water, nature development, recreation, and sustainable (wet/floating) circulary farming or fishing. In principle, this will enable the development of the hinterland into a sustainable living and working area and to create new opportunities for the regional economy. In the pilot ‘Inland buffer zones Wieringermeer’, for example, research has been done into combining water storage and food production.

In this way, hinterlands can play an important role in the way we adapt to climate change in the Netherlands. In the TKI (Top Consortia Knowledge and Innovation) Programme 'Working with Waterscapes', we will be experimenting with sustainable development concepts with a large number of parties led by Wageningen University and Research Centre (WUR) and Deltares in the Koopmanspolder field lab over the next four years. The aim is to optimize soft land-water transitions for ecology. Attention will also be paid to recreation and other functions with the potential to create revenue in the vicinity.

Roel Doef
(Department of Public Works (Rijkswaterstaat))
Remco van Ek

Background picture:
Inland buffer zone Koopmanspolder (1), the ‘Swim basin' (2), the foreshore (3) and the lake IJsselmeer (4).


Water storage in areas behind the dikes of the major water bodies may help to deal with predicted water shortages in the Netherlands. This inland buffer zone concept was experimented with for almost ten years at the Koopmanspolder pilot testing ground on the edge of the IJsselmeer region.

This article focuses on the ecological effects of natural water level management in the polder through the intake of water from the IJsselmeer water. The long-term measurements show that biodiversity is increasing and that the drought of recent years has not had a negative effect on nature in the polder. The area is functioning well as a spawning and juvenile area for fish, and fish-eating birds have increased in number and number of individuals. Reed vegetation and reed and marsh birds are also increasing. Furthermore, there is more insight into when which fish want to migrate from the IJsselmeer to the polder.

However, there are many unanswered questions. What do such areas mean for the productivity of the IJsselmeer and what does this mean for long-term planning and management? What development and co-occurrence opportunities besides ecology are conceivable? How can these take shape? Enough material for further research.


Ek, R. van, R.W. Doef, K. Bruin-Baerts & A. Nierop, van, 2017. Achteroevers, lessen uit de Koopmanspolder, Landschap 2017/1 (15-23).

Rijkswaterstaat, 2008. Achter de oever liggen de kansen. WINN-werkconferentie 27, augustus 2009 Rijkswaterstaat Lef Future Center.

^ Back to start


Experiences ‘Achteroever’ concept

Knowledge journal / Edition 1 / 2021

The role of bird manure in the nutrient balance of surface waters

Many waters in the Netherlands suffer from water quality problems due to an excess of nutrients. Efforts to reduce eutrophication sometimes have limited success and it has been suggested that this may be due to nutrient inputs from large numbers of birds. Logically, this would mainly concern birds that concentrate in colonies and roosts, but forage outside the area. Indeed, some case studies in specific areas indicate a potential increase in the external nutrient load of tens of percentage points. The question is how widespread this problem is.

This modelling study, commissioned by EmissieRegistratie (Pollutant Release & Transfer Register; PRTR) and carried out by Deltares in collaboration with Sovon Dutch Centre for Field Ornithology, has attempted to paint a national picture of this problem. The study uses three sources: 1) a dataset from EmissieRegistratie, 2) bird data from Sovon Dutch Centre for Field Ornithology and 3) the model Waterbirds 1.1 from the Netherlands Institute of Ecology (NIOO).

Model calculations

The Emissieregistratie dataset gives the load of total nitrogen (N) and total phosphate (P) for each of the 2500 drainage areas into which the Emissieregistratie divides the Netherlands. Based on counts by the many volunteers from Sovon, the average numbers of birds have been calculated for all these areas. A distinction is made between (1) common breeding birds, (2) wintering and migratory birds, and birds that (3) gather in colonies and (4) at roosts in the area, but which often forage outside the area. The Waterbirds model was then used to calculate the amount of nutrients that the birds circulate within the areas (common breeding birds and wintering and migratory birds) or potentially bring in (colony breeding and roosting birds). The model works with separate calculation rules for herbivores, carnivores and omnivores, with carnivores and fish eaters excreting on average over one and a half times more nutrients per gram of body weight than omnivores and almost seven times more than herbivores. The average body weight per species was also included in the calculation rules, as smaller birds excrete more nutrients per gram of body weight.

Figure 1. The maximum relative contribution of breeding colony birds and roosting birds to the total external P load per drainage area
Each dot represents a drainage area (reference year 2015).


Circulation of nutrients
The first two groups of birds – breeding birds that do not breed in colonies, migrants and wintering birds – are present in the area 24 hours a day during their stay, where they both feed and excrete. Regardless of their number, these birds do not contribute to the external load of the drainage area, they circulate local nutrients. Therefore, the project has continued to focus on the other two bird groups.

Supply of nutrients

Breeding colony and roosting birds forage – at least in part – in other areas from which they can obtain nutrients. This increases the P and N load in the breeding/roosting area. (The fact that nutrients are removed from the foraging areas is not included in the calculations.) This contribution is variable in size and seems to decrease with the size of the drainage area (Figure 1).
According to the Emissieregistratie dataset, the median P load in the 2500 drainage areas in the reference year 2015 was 1.5 kg/ha; the N load was more than ten times higher at 17.3 kg/ha. Figure 1 shows for phosphate what percentage we can attribute to birds (breeding colony birds and roosting birds).

Figure 2. Calculated potential contribution of birds to the external phosphate load of the 2500 drainage areas in the Netherlands

Breeding bird colonies were present in 58% of the drainage areas during the 2013-2015 monitoring period, roosting birds in 20%. Both occur mainly in larger areas and hardly at all in drainage areas smaller than 100 hectares. Thus, in a significant part of the 2,500 drainage areas, there is little or no external supply of nutrients by birds. However, even in the majority of drainage areas that do have colonies or roosts, the potential increase in load is less than 1%, and this also applies to the national total. Only in a few dozen areas, the contribution is greater than 10%. These are mainly areas with breeding colonies. Breeding colonies are often fish-eaters, which excrete the largest amount of nutrients per gram of body weight.
Roosting birds are often geese, which, as herbivores, excrete relatively few nutrients per gram of body weight. Moreover, they produce a large part of their faeces while grazing and leave only about 15-20% in the roost.

Areas of high contribution
As mentioned above, a few dozen out of a total of 2,500 drainage areas may have a substantial contribution of birds to the nutrient load (Figure 1). Whether this is actually the case depends on the local situation. On the one hand, some of the food of the breeding colony and roosting birds may come from the same drainage area, so it is partly an internal circulation. On the other hand, a relatively large proportion of the excrement may end up on land, so that the water is not directly affected. The situation in these areas should therefore be examined locally to determine the extent of the problem. An actual high local contribution may occur, for example, if colonies of fish-eating birds or large roosting birds are located in relatively small lowland moorland waters with limited pollution, such as in the Vechtplassen area. Other examples are vulnerable, relatively nutrient-poor areas such as a number of dune regions and higher elevated peat bogs such as the Bargerveen. In the latter area, there is a supply from a large roost of mainly Tundra bean geese, which may add 20% to the phosphate load of the Bargerveen-North drainage area.

Most relevant species
Breeding colony birds are usually fish-eaters, which excrete a relatively large amount of nutrients per gram of body weight. In the calculations, the cormorant in particular comes to the fore, which in addition to this diet also has a high body weight and often lives in large colonies. In addition, heron and gull colonies can also play a role. However, not all of these colonies are on or above water. Grey herons can breed in dry forests and parks, herring gulls in extensive dry dune areas.
The proportion of cormorants is also high on roosts, also because roosts are often located on or above water. Geese excrete fewer nutrients, but are also heavy and gather in numbers of tens of thousands of birds, so they still score high.


We see that bird manure contributes only minimally (<1%) to the nutrient load of Dutch surface waters as a whole. Of the 2,500 drainage areas into which the EmissieRegistratie divides the Netherlands, there are a few dozen where there may be a significant contribution (> 10%). Whether this is really the case must be determined by an on-site investigation.


Limitations of this study
In specific local situations, the picture may be different than in the drainage area as a whole. If all the birds of a large drainage area are concentrated in a small pond, problems may occur locally that are not reflected in the area assessment. On the other hand, the contribution of birds may also be overestimated if they leave a large proportion of their manure in the dry parts of a drainage area.
Therefore, this method does not reveal all the problems.

Another limitation is that the major national waters are excluded (Figure 2). This is because the contribution to the load by flow between these waters is not included in the EmissieRegistratie (i.e. the bird contribution would be greatly overestimated). This will not change the picture much, because the national waters are generally nutrient-rich. However, the concentration of phosphate in these waters, in particular, has declined significantly in recent decades, as a result of which the relative contribution of birds has automatically increased. Moreover, due to the size of these waters, problems in sub-regions may be underestimated.
The high values in the dune areas (Figure 2) are probably largely due to seabirds breeding on land; these are drainage areas with little open water.

Possible measures and management
In areas where bird manure causes local problems in the form of conflicts with other natural values, targeted measures are sometimes possible. One example is the isolation of water under colonies by compartmentalisation in combination with adapted water management (flushing).
Control of the colonies or roosts themselves is usually not possible or desirable, as the waters where they are located often also have conservation objectives under the Birds Directive that concern these species. For example, the roost of bean geese, which could prevent the attainment of the improvement targets for raised bog in the Bargerveen, is itself protected by a Natura 2000 conservation target.

Ruurd Noordhuis
Nanette van Duynhoven
Marc van Roomen
(Sovon Dutch Centre for Field Ornithology)
Erik van Winden
(Sovon Dutch Centre for Field Ornithology)

Background picture:
Roost of cormorants and great egrets in the Millingerwaard. Photo Harvey van Diek.


Efforts to reduce the eutrophication of Dutch waters are sometimes only marginally successful. This study investigates the significance of bird manure for the nutrient balance of our surface waters and presents a national picture for the first time. This shows that the contribution of birds in general is small. Only in a few dozen areas is the contribution potentially significant (>10%). Whether or not there is a problem in those areas should be determined by on-site investigation. Indeed, in specific small-scale situations, the picture may differ from the assessment with the model used, due to the application on the scale of drainage areas.


Computer model Waterbirds 1.1, download from habitats

EmissieRegistratie, https:/

Hahn S., S. Bauer & M. Klaassen 2007. Estimating the contribution of carnivorous waterbirds to nutrient loading in freshwater habitats. Freshwater Biology 52: 2421-2433.

Hahn S., S. Bauer & M. Klaassen 2008. Quantification of allochthonous input into freshwater bodies by herbivorous waterbirds. Freshwater Biology 53: 181-193.

Noordhuis R., M. van Roomen, E. van Winden & N. van Duijnhoven 2021. Bird Manure in Dutch Waters. National picture of the significance of bird manure for the nutrient balance of our surface waters. Deltares report 11205268-003, Utrecht.

SOVON 2018. Bird Atlas of the Netherlands. Kosmos Publishers, Utrecht.

^ Back to start


The role of bird manure

Knowledge journal / Edition 1 / 2021

How can continuous monitoring bring ecological goals closer?

In many water bodies, the ecological water quality is insufficient and the Water Framework Directive targets are not yet met. Besides the biology, several abiotic environmental parameters are also measured. The exact cause of the poor water quality can, however, often not be determined based on these measurements. This article shows that the diagnostic value using abiotic measurements is greatly enhanced when water bodies are seen more as a dynamic system during monitoring.

In the current Water Framework Directive monitoring programme, the assessment of the ecological water quality is based on standardised inventories of fish, macro-invertebrate, plant and algal communities. In addition, the abiotic environment is characterised by measuring physico-chemical and hydromorphological parameters at certain points in time. Interim reports have thus far shown that the water quality is still moderate to poor in most water bodies.
In order to meet the Water Framework Directive objectives in 2027, restoration of these water bodies is thus necessary. To be able take appropriate restoration measures, a diagnosis of the cause behind the insufficient biological status must be made for each water body. However, the abiotic environmental parameters, as currently measured, provide limited diagnostic insight. This study addresses the question of why current abiotic measurements provide little diagnostic insight into the cause behind the insufficient ecological water quality. We conclude with recommendations on how to better monitor abiotic environmental parameters in the future.

Research on abiotic environmental parameters

Our research focused on a physico-chemical and a hydromorphological example regarding both oxygen dynamics and discharge dynamics.

Oxygen dynamics
We investigated the oxygen dynamics in three peat ditches along a eutrophication gradient. Oxygen concentrations were measured every 10 minutes for five days in each season (details in Van der Lee et al. 20181). In all ditches, a day-night regime in the oxygen saturation of the water was observed (Figure 1). During the day, aquatic plants and algae produce oxygen. During the night, all organisms use oxygen. However, plants and algae can grow excessively in the eutrophated ditches, due to leaching of fertilisers. In these ditches, more oxygen was used during the night, which led to the depletion of oxygen at night in the summer months.
Currently, the standard is to take one dissolved oxygen measurement in the surface water during 'office hours' (between 9:00 and 17:00). This means that depending on the time of measurement, any score from 'poor' to 'very good' can be obtained in eutrophic and hypertrophic ditches (Figure 1B and 1C). However, almost all organisms need oxygen to survive. By taking one measurement during the day, the degree and duration of oxygen dips during the night are not observed, while these extreme parameter values provide important diagnostic insight into why certain oxygen-sensitive organisms may not occur.

Figure 1: Oxygen dynamics in three peat ditches along a eutrophication gradient: A) mesotrophic, B) eutrophic and C) hypertrophic. Oxygen saturation is colour-based on the quality assessment of the Water Framework Directive measure. The standard time block in which field staff take an oxygen measurement is indicated by grey bars. The measurements were taken 10 cm deep during the summer (02/09/2016 - 03/09/2016).

Discharge dynamics
In natural lowland streams, water discharge is moderately dynamic; water is retained in the stream and its valley for a long time and slowly released downstream. Because many streams have been normalised, surrounding forests have been cut down and the adjacent lands have been drained, the water flows off much more quickly. This can lead to high discharge peaks during extreme rainfall events. During such a discharge peak, the flow velocity in the stream temporarily increases.
To investigate the ecological effects of these discharge peaks, we measured the discharge dynamics and population development of the caddisfly species Agapetus fuscipes in four lowland streams for two years (details in Van der Lee et al. 20202). A. fuscipes is indicative of lowland streams with good environmental quality. The data showed that a discharge peak during the vulnerable early life stages in spring/early summer can be fatal to the caddisfly population. Presumably, the small larvae are washed away during high discharge peak and end up in places where they cannot survive.
This research indicates that especially the extremes of discharge during vulnerable life stages can be an important cause for the local disappearance of species. As a discharge peak usually comes and goes within a day, there is a high probability that these incidents are insufficiently detected by only logging the water level far downstream of the ecological monitoring site or missed when only monthly measuring the flow velocity at the ecological monitoring site.

From static to dynamic systems

In the current monitoring, it is assumed that surface waters are static systems, where it is expected that a low measurement intensity provides a complete picture of the abiotic environmental conditions. Our studies showed, however, that freshwater ecosystems can be very dynamic. Abiotic environmental parameters can vary from season to season, from day to day e.g. depending on precipitation, and even within a day due to day-night light and temperature cycles. Aquatic organisms are adapted to these natural environmental dynamics.
If these natural dynamics are disrupted by human activities, it may cause stress to aquatic organisms. The example on discharge dynamics showed that the effect of this stress on the community is determined by the 'exposure' to the stressors (unfavourable environmental conditions) times the 'sensitivity' of the organisms. Measuring environmental parameters only at certain points in time thus provides an incomplete picture of the dynamics of the environment. As a result, the extremes in the abiotic environmental parameters, the bottlenecks for organisms, may be overlooked.

Options for high-frequency measurements

To obtain an integrated picture of the freshwater environment, abiotic parameters of importance to organisms should be measured continuously. Recent technological advancements in data loggers and sensors have enabled us to take and store high-frequency measurements automatically.
For most of the physico-chemical parameters prescribed by the Dutch Water Framework Directive monitoring programme, relatively cheap (often < 1000 Euro each) and easy-to-use data loggers with integrated sensors are available. This allows measuring of, among other things, water temperature, light intensity, oxygen content, pH, salinity (conductivity) and turbidity at high frequency.
The development of technology to measure nutrients in the water at high frequency is more complicated. For nitrate and ammonium, data loggers with integrated ion-selective electrodes are available (< 1000 Euro), but these are less reliable and accurate than the more expensive optical (> 10,000 Euro; nitrate only) or wet-chemical sensors (> 10,000 Euro; nitrate, ammonium and phosphate)3.
For hydrology, water level loggers and a known cross-sectional profile can provide a cost-effective and ecologically sufficiently accurate picture of the local discharge dynamics. Morphological processes, such as sediment transport and substrate movement, can also be monitored at high frequency with, for example, automated digital cameras or sensors.

Using sensors and data loggers in practice

During our research, a few issues emerged that need to be considered when using sensors and data loggers in practice. In addition, while communicating the results, we also came across other monitoring topics that are important to emphasise.

1. What is the purpose of monitoring and what do you need to measure?
First of all, the purpose of the monitoring must be determined, i.e. why are you going to monitor? The aim may be, for instance, to diagnose why a particular water body has a poor ecological water quality using data on environmental processes. However, it is impossible to monitor all abiotic parameters, if only for financial reasons. It is therefore necessary to determine in advance which parameters at the location in question may play a role in the degradation, in other words, what are you going to measure?
An indication of the possible disturbances can be based on the environmental- and habitat preferences of the species found in the water body. Species have different adaptations that determine the abiotic conditions under which they can survive or not, i.e. they can indicate the nature of the stress. Conversely, a catchment-wide ecological system analysis (SESA) can also be carried out to identify potential stressors that can then be monitored for verification.

2. When, how often in time and where is the measurement most effective?
Based on the objective, a monitoring protocol must be drawn up to measure the relevant parameters most effectively. For this, it is important to understand the temporal scale of the relevant abiotic processes over the year. Our research showed, for example, that oxygen levels follow a day-night regime and can vary per season. For dissolved oxygen it is, therefore, recommended to take high-frequency measurements during a number of hot summer days when oxygen dips may occur. It is also essential to select the most suitable set-up of the loggers and sensors in the water body. The most effective spot to measure abiotic parameters depends on the spatial variation of the relevant environmental processes and the occurrence of the indicator species.

3. How can a measurement network be realised sustainably and cost-effectively?
Our research showed that the deployment of a mobile construction with sensors and data loggers was cost-effective. A PVC pipe or steel cage can be used to protect the equipment from vandalism. It is also advisable to place the construction out of sight of busy areas. Moreover, the construction needs to be maintained periodically, especially since algal growth can limit the accuracy of the sensors.

4. How should the data be processed and interpreted?
Finally, it must be determined how the data will be analysed in order to provide useful information. Sensors and data loggers can generate large amounts of data. For the analysis, aspects such as the intensity, frequency, predictability and duration of the disturbance are relevant, and should always be linked to the question that needs to be answered. However, there is also a knowledge gap here; how the analysis and interpretation of high-frequency abiotic data can be made more accessible to the water manager requires additional research.


High-frequency measurements of abiotic environmental parameters can provide an overall picture of surface water dynamics over time; the ecosystem functioning. This allows disturbances to be detected in the water body, which only occur at certain points in time and would often be missed by snapshots. In combination with other analyses, these insights can provide a valuable contribution to a better diagnosis of the causes behind an insufficient ecological water quality.

Gea H. van der Lee
(Wageningen University and Research)
Ralf C. M. Verdonschot
(Wageningen University and Research)
Piet F. M. Verdonschot
(Wageningen University and Research)


This article was made possible by funding from the Water Quality Knowledge Impulse and the research programme Smart Monitoring (University of Amsterdam, UvA).


Current monitoring programmes often provide little diagnostic insight into the cause behind an insufficient water quality score, as surface waters are measured as static systems. In this article, we show that by deploying data loggers and sensors specifically, an overall picture of the dynamics in the abiotic environment over time can be obtained. This allows disturbances in the water body, which only occur at certain points in time and would often be missed by snapshots, to be detected.


Van der Lee, G. H. et al. (2018). Dissolved oxygen dynamics in drainage ditches along a eutrophication gradient. limnologica, 72, 28-31.

Van der Lee, G. H. et al. (2020). Persist or perish: critical life stages determine the sensitivity of invertebrates to disturbances. Aquatic Sciences, 82, 1-11.

Pellerin, B. A. et al. (2016). Emerging tools for continuous nutrient monitoring networks: Sensors advancing science and water resources protection. JAWRA, 52(4), 993-1008.

^ Back to start


Water bodies as dynamic system

Knowledge journal / Edition 1 / 2021

Climate scan of large water bodies

What consequences does climate change have for large water bodies in the Netherlands and how can we make these water bodies more climate-resilient? This question is central to the 'climate scan' that Deltares recently published as part of the Programmatic Approach to Large Water Bodies (PAGW) of the Ministry of Infrastructure and the Environment and the Ministry of Agriculture, Nature and Food Quality.
The climate scan shows what is in store for us in terms of water quality and ecology and what we can do to achieve the PAGW ambition - the realisation of ‘future-proof large water bodies where high-quality nature goes hand in hand with a powerful economy’.

During the last few decades, the Netherlands has warmed up more than the world as a whole. Especially around 1990, the increase was intensified, both in the air and in the water (Figure 1). In addition to global warming caused by greenhouse gases, two other mechanisms are at work here. Firstly, since the end of the 1980s, the wind has come more often from the west, making winters relatively mild since then. Secondly, the decrease in air pollution in summers has played a role. Due to fewer particles in the atmosphere, the amount of fog decreased significantly. This increased the amount of sunlight and the daytime temperature. These two processes are gradually being overtaken by global warming.

Climate-related pressure factors

Increasing air temperature is only one of the climate-related factors exerting pressure on the large water bodies. The climate scan identified five more pressure factors: sea level rise, precipitation and river discharge, wind climate, fog and solar radiation, and water acidification.

The effects of the various pressure factors on flora and fauna are largely due to the increase in water temperature. In warmer water, less oxygen dissolves and aquatic animals are more likely to die. The production of algae increases and starts earlier in the season. The increase in activity and growth of plants and animals does not occur to the same extent for all species. This can cause 'mismatches': food no longer being available at the right time. Northward shifts in the area where species occur can also occur in water bodies, just as on land. The chance of successful establishment of southern exotic species is also increasing at the expense of the existing species, so that diversity is decreasing. This also has implications for achieving the goals of the Water Framework Directive and Natura 2000.

Figure 1. Increase in global average air temperature, northern hemisphere (Hadley Centre) and central part of The Netherlands (De Bilt, KNMI), and in water temperature in the South-western Delta (Department of Public Works; Grevelingen, Oosterschelde, Veerse Meer, Volkerak and Westerschelde)

Results per main water system

The Netherlands has four main water systems: the Lake IJsselmeer area (the large closed off inland bay (fresh water) in the central Netherlands), the River area, the Wadden Sea area and the Southwestern Delta. Each main water system has specific characteristics, which result in marked differences in sensitivity to the six climate-related pressure factors. The climate scan visualises this in text and infographics. Table 1 provides a summary.

Table 1. Vulnerability of the four main water systems to the six medium- and long-term pressure factors of climate change

Lake IJsselmeer area
The lakes of the IJsselmeer area are characterised by a large surface area and shallow depth. They are therefore sensitive to changes in wind climate and warm up easily. On summer days with little wind, stratification of the water occurs, which can be accompanied by additional algal blooms and oxygen deficiencies. Due to the large surface area, changes in the amount of precipitation have a relatively large effect on the water and substance balance.

River area
The rivers are sensitive to changes in the discharge regime. These are caused by changes in precipitation patterns and an increase in evaporation in summer. Models predict increase of discharge in winter and decrease in late summer. The decline in late summer is gradually becoming measurable. In addition, extremely high or low discharge values and water levels occur more frequently.
The water temperature in the rivers has increased relatively strongly. At first, this was mainly due to cooling water discharges, later combined with climate change. Since 1900, the Rhine and the Meuse have warmed up by about 3°C on average. The increase is now levelling off due to regulation of cooling water discharges. The effects of changes in river discharge also interfere with a non-climatic pressure factor, namely the deeper incision of the main channel by the regulation of the rivers, which for example at Lobith has been 2 metres for the last 150 years and also affects the inundation frequency and duration of the flood plain.

Wadden area
With its large intertidal area, the Wadden Sea area is directly sensitive to sea level rise. The growth of mudflats in the western Wadden Sea is not expected to be sufficient in the future to keep up with the rise in sea level. The long-term fluctuations in wind patterns in the winter have temporarily amplified the rise in coastal water levels during the winter months. Due to the shallow water depth and the dryness, the intertidal areas are also extra sensitive to warming and to an increase in the frequency of mortality incidents of, for example, shellfish.

Delta area
The waters of the Southwestern (SW) Delta differ greatly from one another. The differences are mainly in salinity, freshwater-salt gradients, the presence or absence of tides and the degree of flow. Therefore, there is a varied combination of the factors mentioned for the other three main water systems. The most characteristic is a greater sensitivity to effects of sea level rise and changes in river discharge on the influence of salt water in the area. In addition, the open delta waters are sensitive to loss of intertidal area. The areas closed by the Delta Works are susceptible to intensifying problems of algal bloom, stratification and oxygen deficiency.

Working on resilience

The four pillars of resilience
An important part of the climate scan is the exploration of the possibilities to make the large water bodies more resilient to climate change. Shielding from the pressure factors of climate change itself by shading, compartmentalisation or deepening is hardly an option for the large water bodies. The motto is therefore to increase the capacity of the water systems to cope with changes, i.e. to make the water systems more robust and resilient. The possibilities for this are elaborated in the seven principles of resilience according to the Stockholm Resilience Center. For application in large water bodies, the climate scan distilled the four pillars of resilience from these: diversity, dynamics, connectivity (connection) and water quality. By working on these four basic factors, the resilience of the large water bodies can be increased. Within the framework of the PAGW, this is now done on a structural basis.

The large water bodies in the Netherlands are all to a large extent 'artificial', influenced by human intervention, with the result that the four pillars of resilience are no longer in good order. The proportion in which they play a role differs per water system. Adjusting the pillars is customised and depends on the functions of the water system in question.

Problems such as algal blooms and oxygen deficiencies can often be remedied by increasing or restoring dynamics and connections (connectivity). This is done, for example, by flushing the Veluwerandmeren (Lake IJsselmeer area) or, more recently, by opening the sluice doors in the Haringvlietdam, by deploying the Katse Heule culvert for the Veerse Meer and by restoring dynamics to the Volkerak and Grevelingen (all in the the SW Delta).
Creating a greater diversity of habitats and species also contributes to resilience. If the food web is more complex, it is more resistant to external pressure. If several species perform the same function in the system ('redundancy') but respond differently to pressure, functionality is better preserved. Habitat diversity can be increased by restoring land-water transitions through the creation of under-represented habitats such as shallows and marsh areas. Examples are: nature development on Tiengemeten (SW Delta), Marker Wadden and Trintelzand in the Markermeer (Lake IJsselmeer area), more gradual freshwater-salt transitions such as with the fish migration barrier in the Afsluitdijk (between Lake IJsselmeer and the Wadden Sea) or the sluice doors in the Haringvlietdam (SW Delta), or digging side channels in the River area.
Good water quality, the fourth pillar, is a basic prerequisite. Eutrophication has been reduced by sharp declines in phosphate concentrations, but nitrogen concentrations are still well above the standards.

All in all, climate change is increasing the pressure on the larger water bodies. In combination with intensive use, the artificial layout and limited hydrological dynamics, it limits the ecological diversity and resilience of these systems. Therefore, under PAGW, more measures will be designed, often in spatial cohesion. In order to test the contribution of these designs to increasing climate resilience, the ‘Climate Compass’ was drawn up in addition to the climate scan. This allows zooming in on local effects and comparing design variants. Further knowledge development through literature study, analysis of data sets and modelling studies remains important to prepare the Netherlands for the consequences of ongoing climate change on aquatic ecology.

Ruurd Noordhuis
Sacha de Rijk

Background picture:
Photo: Moniek Löffler, Bureau Landwijzer


The climate scan explores the consequences of climate change for the four main water systems in the Netherlands.

The Lake IJsselmeer area is sensitive to changes in wind direction and wind strength, combined with warming and reduced ice cover. In the river area, the sensitivity lies mainly in changes in precipitation and evaporation, which influence the discharge. The Wadden Sea area is most directly affected by rising sea levels, which can lead to the loss of intertidal areas, especially in the west. In the South-western Delta, the effects of rising sea levels on draining possibilities, salt intrusion and freshwater-salt transitions are among the issues.

To make water systems more resilient to these changes, customisation is required, focusing on the four pillars of resilience: water quality, habitat diversity, (water level) dynamics and the connection between waters and habitats (connectivity).


Climate scan. R. Noordhuis, S. de Rijk, G. van Geest, M. Maarse, S. Vergouwen & A. Boon, Deltares, Utrecht, report 11203733-000-ZWS-0006, December 2019.

Use of Climate Compass for PAGW projects. Manual. S. de Rijk, V. Harezlak & R. Noordhuis. Deltares, Utrecht, report 11205270-003-ZWS-0001, December 2020.

Effects of temperature increase on large water bodies. A literature review with data overview. R. Noordhuis, L. van der Heijden & A. de Jong, Deltares, Utrecht, report 11205270-005-ZWS-0003, January 2021.

Climate variability effects on eutrophication of groundwater, lakes, rivers, and coastal waters in the Netherlands. J. Rozemeijer, R. Noordhuis, K. Ouwerkerk, M. Dionisio Pires, A. Blue, A. Hooijboer & G.J. van Oldenborgh. Science of the Total Environment 771 (2021) 145366.

Possible consequences of accelerated sea level rise for the Delta Programme. An exploration. Haasnoot, M., Bouwer, L., Diermanse, F., Kwadijk, J., Van der Spek, A., Oude Essink, G., Delsman, J., Weiler, O., Mens, M., Ter Maat, J, Huismans, Y., Sloff, K. & Mosselman, E. (2018). Deltares report 11202230-005-0002, Delft.

Climate adaptation in the river area. Klijn, F. A., Asselman, N.E.M., Hegnauwer, M., Mosselman, E. & Sperna Weiland, F. (2019). Landscape 2019 (2): 105-113.

^ Back to start


Large water bodies

Knowledge journal / Edition 1 / 2021

Wet agriculture in stream valleys: opportunity to link nature and agriculture

Climate proofing of stream valley landscapes creates places where agricultural plots become wetter. There is potential for cultivating wet crops on wet farmland to form a climate-resilient and ecological corridor between farmland and nature.

Historically, large parts of the Netherlands were relatively wet. Originally, a lot of water was retained, especially in stream valleys and peatlands. Due to the straightening of streams and changes in landscape design, water-regulating ecosystem services have been lost in many places and drying-out is more common. In North Brabant, this drying-out is particularly visible on the high sandy soils of stream valley landscapes. Various projects are underway to make the landscape climate-resilient, whereby water is retained in an area for as long as possible. This will result in wet areas in a climate-resilient stream valley.
This new landscape design also requires other forms of agriculture and offers, for example, opportunities for wet agriculture and improving biodiversity and water quality. Wet agriculture (paludiculture) has so far mainly been studied in the peatland area. Not much is known yet about which climate-resilient business practices a landowner can apply in a stream valley landscape and what the consequences are for biodiversity.
The research group Innovative Entrepreneurship in Rural Areas of HAS University of Applied Sciences has investigated this in collaboration with farmers, the province of North Brabant, Radboud University and KWR Water Research Institute. The research question was posed: How can the land user switch to a different crop that contributes to climate-resilient land use and improvement of biodiversity? During the study, a combination of literature research, interviews, experimental research and balance calculations was carried out.

Types of crops

Bulrush has the most potential of the wet crops. It is a suitable crop for water retention areas (Veenweide innovation and knowledge centre, 2016). Wet agriculture is good for water quality. Bulrush, like reed, purifies surface water of phosphate and nitrogen (Geurts et al., 2017). However, reed evaporates more water compared to bulrush (Mueller et al., 2005) and may therefore be a less suitable crop to place in drought-prone locations. Woody crops such as willow and black alder can also be used, especially in areas that can be periodically dry (Geurts et al., 2019). Apart from the better known wet crops, there are also alternative crops grown in the greenhouses of HAS University of Applied Sciences, including cranberry, calamus and lakeshore bulrush. These crops grew well on peaty and sandy soils at different high groundwater levels, so they are potentially a good choice for wet farming (Figure 1).

Current status of sales markets

Van Duursen and Nieuwenhuijs conducted a market survey in 2016 for several wet crops, the market is not yet fully developed. Azolla (mosquito fern), bulrush and peat moss are the most promising crops to grow in peatland areas. In addition to the growing market for insulation material and fodder, bulrush pollen can also be used as food for predatory mites that are used for biological pest control. Research into new applications and sales markets is ongoing.
For wet agriculture in stream valley landscapes, azolla and peat moss seem less suitable. Azolla is an invasive exotic species and is therefore undesirable for introduction into stream valleys as a wet crop. Growing peat moss in stream valley landscapes will be a challenge due to the low acidity and trophic level that peat moss desires (Mettrop et al., 2020). These conditions are mainly found at (former) bog sites.

New sales market as potting soil

We investigated whether wet crops have the potential to be used as a sustainable substitute for fossil peat in order to create a new sales market. To explore this, a cultivation trial was conducted at Radboud University in collaboration with HAS students. The growth of cress was compared here on different potting soil mixtures (Figure 2).

Figure 1: Biomass of cress on substrates of wet crops (greenhouse test Radboud University)

In the trial, five wet crops were briefly composted and mixed with each other or with 100% fossil potting soil. Cress was sown on this and the germination and yield of cress were measured. The germination and survival of cress was lowest on the 100% fossil potting soil. Combinations of 50% fossil peat and 50% reed canary grass, peat moss or azolla produced more biomass than a peat substrate. Even substrates that consisted entirely of composted wet crops (peat moss and bulrush) achieved almost the same biomass as cress grown on 100% fossil peat. By further optimising the pre-treatment (chopping, composting and/or fermenting) and mixing of wet crops, yields will eventually improve. This will enable the creation of a sustainable sales market for wet crops. The benefits are two-fold: less fossil peat needs to be excavated, while drained peatlands are restored.


Balance calculations have been drawn up for bulrush in which crop-specific costs and benefits have been included (Table 1). It can be concluded that, under the present circumstances, the business case is not yet balanced and it is therefore not yet profitable for farmers to switch to wet agriculture. This is partly because the market for bulrush is in its infancy; farmers can use it mainly as roughage on their own farms. The market for bulrush as insulation material is growing. There are also efficiency gains to be made in harvesting.

Table 1: Balance calculations of bulrush cultivation in the current situation; Scenario 1: payment for ecosystem services and development of chain; Scenario 2: use of land positions by authorities and development of chain.
1Fact sheet Bulrush Veenweide innovation centre, 220 ton*€120, 3based on harvested reed: Nature and Landscape Management 2019 grant 2020 from the BIJ12 website, 450% DM 10 ton* €38 CO2 , 5The transport costs are €10,- per ton dry matter, 6: For a mechanised harvest, the costs are expected to be between €800 and €1100 (Van Duursen and Nieuwenhuijs, 2016).

In Scenario 1, if the market for insulation material/fibres increases or if it can be sold as sustainable potting soil, the yield of bulrush will increase. This can balance the business case (just barely) if the crops also become more efficient. If social services, such as water storage and carbon sequestration, are also included in the calculation at a rate of about €750 per hectare per year, the business case will improve significantly and there will be prospects for farmers to make the switch.
There is also another route to a balanced business case. If authorities provide land positions that are challenging due to higher water levels, subsidence, water storage, wetlands or climate adaptation, and would make these available at a lower rent, a positive result can also be achieved (Scenario 2). In this way, this land can remain in production and authorities can achieve their goals in a more or less budget-neutral way. This would be a more favourable outcome for authorities than buying out companies. Although public money is needed for this, the alternative in such places is often that farmers are bought out or compensated for wet damage. The most cost-effective approach can be determined per area, whereby an important advantage of the transition to wet agriculture is that this land will 'simply' remain in production.

Landscaping and biodiversity

The large variety of wet crops also means that there is no single predictable effect of these crops on biodiversity. Especially in the buffer zones around nature areas, wet agriculture can enhance biodiversity. Wet crops can function as a corridor for target species, with the preference being for growing native crops (Van Duinen et al., 2018). Research in reed beds has shown that birds such as the red-backed shrike and the whinchat in particular use the reed beds for foraging or resting. Depending on the mowing regime, the biodiversity can be influenced. For example, most marsh birds benefit from a mosaic mowing regime, which means that parts of the reed are not harvested each year (Korevaar & Van der Werf, 2014). The question is to what extent biodiversity can benefit or whether wet agriculture actually encourages pests such as mosquitoes. This will probably depend on the location in the landscape, the water level and the mowing management. To answer this question, research is currently being carried out at various locations in North Brabant, investigating the impact of wet agriculture on biodiversity and how the landscape ecological fitting of wet agriculture in a stream valley can be shaped.


There is potential for cultivating wet crops on wet farmland to form a climate-resilient and ecological corridor between farmland and nature. Conditions for the earnings model of wet agriculture to be balanced are that the market develops further, ecosystem services are rewarded and/or land positions are used. We are currently investigating the opportunities that the landscaping offers to better connect nature and agriculture.

Ellen Weerman
(HAS University of Applied Sciences)
Daan Groot
(HAS University of Applied Sciences)
Jeroen Geurts
(KWR, Radboud University)
Frank van Lamoen
(Province of Noord-Brabant)

Background picture:
Insect trap for monitoring insect biodiversity in a field of wet crops by students from HAS University of Applied Sciences. Photo: Ellen Weerman


When a stream valley landscape is designed to be climate-resilient, places are created where agricultural plots become wetter. These parcels of land would then require a different exploitation. Wet farming (paludiculture) can offer a solution where both farmer and nature benefit. Our research shows that many types of wet crops are suitable. Bulrush has the most potential to become a profitable crop because of its cultivation possibilities and growing sales market. New applications are constantly appearing on the market, such as processing into sustainable, peat-free potting soil. The business case for bulrush is not yet balanced. Cultivation becomes more profitable as the market matures and ecosystem services will be rewarded. How biodiversity can be enhanced with a smart landscape integration of wet crops is currently being researched. Thus, in the future, agriculture and nature can benefit even more from climate-resilient water and land planning.


Geurts J. M., van Duinen G.J., Van Belle J., Wichmann S., Wichtmann W., Fritz C. (2019) Recognize the high potential of paludiculture on rewetted peat soils to mitigate climate change. J Sustainable Organic Agric Syst: 69(1):5–8

Geurts, J. M., Fritz, C., Lamers L., Grootjans A. P., Joosten H. (2017). Paludiculture keeps the polder clean - purifying surface water and mining phosphate-rich soils with reed and bulrush cultivation. H2O-online.

Korevaar, H., & van der Werf, A. K. (2014). Reed cultivation as a possible building block for sustainable water and soil management in wet peatlands. Plant Research International Report 544.

Van Duinen, G.J., Fritz, C., de Hullu, E. (2018). Perspectives for wet farming in the Veenland International Nature Park. Pilot project on paludiculture. Report Stichting Bargerveen

Mettrop I. (2020) Better Wetter Phase 2, wet-culture trials. A&W report 3153-2 Altenburg & Wymenga ecological research, Feanwâlden

Van Duursen J., Nieuwenhuis A. (2016) Market Exploration Paludiculture; Opportunities for Agriculture in Peatland Areas while Preserving Peat.

Wichtmann, W., Schröder, C., Joosten, H. 2016. Paludiculture - productive use of wet peatlands. Stuttgart, Germany, Schweizerbart Science Publishers.

^ Back to start


Link nature and agriculture


Edition 2/2020

Edition 1/2020

Edition 2/2019

Edition 1/2019

Edition 1/2018

Edition 2/2017

Edition 1/2017

Edition 2/2016

Edition 1/2016

Edition 2/2015

Edition 1/2015


Previous editions

Knowledge journal / Edition 1 / 2021


The knowledge section Water Matters of H2O is an initiative of

Royal Dutch Waternetwerk
Independent knowledge networking organisation for and by Dutch water professionals.

Water Matters is supported by

Alterra Wageningen University
Research institute that contributes by qualified and independent research to the realisation of a high quality and sustainable green living environment.

Global natural and built asset design and consultancy firm working to deliver sustainable outcomes through the application of design, consultancy, engineering, project and management services.

Independent institute for applied research in the field of water, subsurface and infrastructure. Throughout the world, we work on smart solutions, innovations and applications for people, environment and society.

KWR Watercycle Research Institute
Institute that assists society in optimally organising and using the water cycle by creating knowledge through research; building bridges between science, business and society; promoting societal innovation by applying knowledge.

Royal HaskoningDHV
Independent international engineering and project management consultancy that contributes to a sustainable environment in cooperation with clients and partners.

Foundation for Applied Water Research (STOWA)
Knowledge centre of the regional water managers (mostly the Water Boards) in the Netherlands. Its mission is to develop, collect, distribute and implement applied knowledge, which the water managers need in order to adequately carry out the tasks that their work supports.