Knowledge journal / Edition 2 / 2019


Water Matters: research with a view to a practical application

In front of you is the eighth edition of Water Matters, the knowledge magazine of the journal H2O. You will find ten articles about a variety of subjects, written by Dutch water professionals on the basis of thorough research.

Repeated appeals and a personal approach have resulted in sufficient proposals for articles in order to enable the editorial board to make this selection. The board mainly looked at a clear relation with the daily practice in the water sector. Research, results and findings must be new and produce articles which present new knowledge, insights and techniques with a view to a practical application.

This edition also covers a wide range of topics: from a drought risk analysis, the damage caused by extreme rain showers, to the water storage capacity of compacted soils. Furthermore, underground fertigation for efficient use of water in agriculture, CO2 capture by submerged aquatic plants, forelands such as dyke protection and the ecological effects of effluent discharges from WWTPs.

Like the journal H2O, Water Matters is an initiative of the Royal Dutch Water Network [Nederlands Waternetwerk (KNW)], the independent knowledge network for and by Dutch water professionals. Members of KNW receive Water Matters twice a year as a supplement with their H2O journal.

The publication of Water Matters is made possible by prominent players in the Dutch water sector. These Founding Partners zijn ARCADIS, Deltares, KWR Watercycle Research Institute, Royal HaskoningDHV and Stichting Toegepast Onderzoek Waterbeheer (STOWA). With the publication of Water Matters, the participating organizations wish to make new, applicable water knowledge accessible.

You can also read Water Matters in digital form at H2O -online ( This publication is also available as a digital magazine in the English language via the same website or via The English publication is partly made possible by the Netherlands Water Partnership (NWP), the network of approximately 200 collaborating public and private organizations in the area of water.

The articles in English can also be shared from the digital magazine at H2O -online. Furthermore, articles from previous editions can be found on the site.

We hope you enjoy reading this edition. If you have any comments, please let us know via (

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


Presenting: Water Matters!

Knowledge journal / Edition 2 / 2019

Can water bodies dominated by submerged aquatic plants capture CO2 from the atmosphere?

Aquatic ecosystems that contain many submerged water plants are potential hotspots for capturing organic material. Aquatic plants store carbon and nutrients in their biomass, and dead plants and other detritus form an organic layer at the bottom of the water body. Could such systems lower the amount of CO2 in the atmosphere? How would the carbon capture be impacted if the water warms up due to climate change?

Mesocosm: a large water tank containing Eurasian watermilfoil; with an inflorescence of this plant.

Not only do aquatic plants provide nutrition and habitat for other (aquatic) organisms, they also store carbon and nutrients in their biomass. In addition, patches of aquatic plants can reduce water flow, which aids dead organic material to settle and prevent resuspension. Aquatic plants also provide structure and surface area for the development of, for example, biofilms that in turn can die and sink to the sediment.
The organic layer in the sediment of the water forms a substantial carbon and nutrient storage and is particularly interesting for both the objectives of climate agreements and for preventing surface-water eutrophication. However, there is still insufficient understanding of how much carbon and nutrients these kinds of systems are able to capture.
A large portion of the plant material that sinks to the lake bottom is broken down by microbes before being recirculated. This mineralisation process is dependent, among other factors, on the temperature of the water. Climate change is expected to lead to a temperature rise in the Netherlands of around 3.5° C by 2100. With higher temperatures, aquatic plants grow faster and can form more biomass. Given that this biomass consists for a large part of carbon (C), nitrogen (N) and phosphorus (P), more of these elements may be incorporated at a higher temperature. However: sedimentation and the decomposition of organic material can also increase. How the balance of build-up and breakdown at higher temperatures will look like is still unknown.
In a mesocosm system — controlled experiments in water tanks under semi-field conditions — we followed the effect of a 4° C rise in temperature on the plant biomass formation of Eurasian watermilfoil (Myriophyllum spicatum) and the sedimentation and decomposition of organic material.

Study design

Eight 1,000-litre mesocosm tanks were filled with sediment and Eurasian watermilfoil from a mesotrophic, shallow pond in Wageningen as well as tap water. The one-year experiment included two temperature treatments: 1) a natural Dutch seasonal water temperature cycle (control) and 2) the same seasonal cycle, only four degrees warmer (heated). The sedimentation of organic material (dead pieces of Eurasian watermilfoil and other detritus) with sedimentation traps was determined on a monthly basis. The microbial degradation of Eurasian watermilfoil was determined by incubating litter bags filled with dried leaf material above the sediment and harvesting them after the ½, 1, 2, 4, 6 and the 8-month mark. At the end of the experiment, the biomass of Eurasian watermilfoil was harvested. Plant, sedimentation and decomposition material was dried at 60° C before being analysed for dry weight and the C, N and P contents. The degradation rate k and the remaining fraction s (after mineralisation) of these elements were approximated using a two-phase decomposition model.
The storage of C, N and P was calculated with a budgeting model (Velthuis et al., 2018). This model approximates the dynamics of the degradable and recalcitrant material in the sediment during the experiment, assuming that the calculated recalcitrant fraction s and degradation rate k are representative of the mineralisation and storage of the respective element within the sediment. The storage of C, N and P was calculated as the sum of the degradable and recalcitrant material at the end of the experiment.

Increased growth and greater decomposition

The temperature increase led to 80% more biomass of Eurasian watermilfoil in the mesocosms, and the amounts of incorporated carbon and nitrogen were also significantly higher (+83 and 52%); this did not apply to phosphorus (Illustration 1). The sedimentation of organic C and N to the sediment was more than 1.5 times greater. The mineralisation of C and N during decomposition more than doubled; here too, P was the exception.
As the positive temperature effects on the sedimentation and decomposition cancelled each other out, the temperature increase had no effect on the storage of carbon and nutrients in the sediment of the water body (determined with the measured sedimentation and the modelled decomposition).

Figure 1: Pools and fluxes of C, N and P under the current situation (control) and with a higher temperature (heated). Incorporation in plant biomass g/m2; sedimentation, mineralisation and burial in g/m2/y; significant differences bolded (P<0.05, t-tests).

The results also suggest that larger plant biomass at a higher temperature can lead to increased fluxes of N and P to the sediment. This follows from the positive correlation between plant biomass and the amount of N and P in this biomass, as well as the amount of sedimented N and P (Illustration 2). The trend between plant biomass and sedimented C was also positive, but not significant (P=0.07).

Figure 2: Linear correlation between the biomass of Eurasian watermilfoil at the end of the experiment and the incorporated (green) and sedimented (red) organic C (a), N (b) and P (c) across temperature treatments.


In our study, carbon and nutrient capture in systems dominated by aquatic plants was independent of temperature treatments. This can be explained by the increased biomass and the sedimentation in warmer water which can be compensated by increased mineralisation.
In both temperature treatments, carbon and nutrients were captured in larger quantities in the sedimentation than in the plant biomass. In warmer water, microbes can rapidly decompose 50 - 75% of these sedimented carbon and nutrients. Nevertheless, a considerable portion remains in the sediment, apparently independent of the temperature. This illustrates the importance of having systems dominated by aquatic plants for the storage of organic C, N and P.
The quality of the sedimented material also plays a role. Surprisingly, there was no observable effect of the elevated temperature on the incorporation of P into plant biomass, while the total biomass of Eurasian watermilfoil was increased. Aquatic plants are known to be flexible in their nutrient-use efficiency and their P content, therefore, varies depending on the environmental factors. Warmer water can enable plants to deal more efficiently with P; as a result, they may require less of it to achieve the same growth. This can lead to a reduced incorporation of P in relation to C.

Further research and applications

Our one-year experiment did not include the long-term effects of the temperature increase. For example, how these results can be linked to hydrosere processes and also methane emissions (which is a stronger greenhouse gas than CO2) is a necessary question for further research.
If we assume that these results can be translated into a field situation, a large biomass of submerged aquatic plants combined with a low degradation would be desirable for the optimal uptake and burial of carbon and nutrients. These plants would then accumulate at the bottom of the water body, which would eventually lead to hydrosere succession. As part of a potential application, research
• can be conducted on whether it is possible to design a system with plant species that has a naturally low degradation rate;
• how dredging and mowing of the aquatic plants impact these processes. These measures are likely to have a negative effect on carbon capture and storage;
• whether it is possible to achieve increased storage in deeper ponds in which the temperature and the oxygen content of the sediment are low throughout the year or in shallow ponds with lots of emergent vegetation.
Bearing these critical notes in mind, our research results argue for the conservation and, where necessary, restoration of submerged aquatic plants for the storage of carbon and nutrients.

Mandy Velthuis
(Wageningen University & Research: WUR, Netherlands Institute of Ecology)
Sarian Kosten
(Radboud University Nijmegen)
Sabine Hilt
(Leibniz-Institute of Freshwater Ecology and Inland Fisheries, IGB)
Piet Verdonschot
(Wageningen University & Research)
Liesbeth Bakker
(Netherlands Institute of Ecology)

This article stems from the project ‘Stimuleren van CO2-opname door algen en waterplanten in zoetwatermeren’ (Stimulating CO2 uptake through algae and aquatic plants in freshwater lakes), led by Ellen van Donk and financed by the Gieskes-Strijbis Fund and the NWO Veni grant 86312012 of Sarian Kosten. The authors would like to thank the Limnotron Consortium (Ralf Aben, Garabet Kazanjian, Edwin Peeters, Thijs Frenken, Nico Helmsing, Lisette de Senerpont Domis and Dedmer van de Waal) for the collective execution of the experiments.


Water bodies containing many submerged aquatic plants might be hotspots for carbon (and nutrient) storage, particularly in the sediment. Management geared towards maintaining or restoring these aquatic plants can contribute to achieving the objectives of the Paris Climate Agreement. In a mesocosm experiment with Eurasian watermilfoil, we found that a four-degree higher water temperature resulted in a larger plant biomass and increased sedimentation. The decomposition of this plant material, however, was enhanced as well. The net result was that the storage of carbon, nitrogen and phosphorous in the sediment of the water body remained similar. These results suggest that water bodies with thriving submerged aquatic plants can capture a portion of the anthropogenic emissions regardless of the water temperature.


Benfield (2006), Decomposition of leaf material, in Hauer & Lamberti, 2006, Methods in stream ecology, San Diego, Verenigde Staten. p. 711-720.

van Dam (2009), Evaluatie basismeetnet waterkwaliteit Hollands Noorderkwartier: trendanalyse hydrobiologie, temperatuur en waterchemie 1982-2007. Hollands Noorderkwartier Water Board, report 708.

Harmon et al. (2009), Long-term patterns of mass loss during the decomposition of leaf and fine root litter: an intersite comparison. Global Change Biology, 15(5): p. 1320-1338

van den Hurk et al. (2014), Climate Change Scenarios for the 21st Century - a Netherlands Perspective. KNMI scientific report WR 2014-01.

Velthuis et al. (2018), Warming enhances sedimentation and decomposition of organic carbon in shallow macrophyte-dominated systems with zero net effect on carbon burial. Global Change Biology, 24(11): p. 5231-5242.

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Contribution to climate agreement

Knowledge journal / Edition 2 / 2019

Foreshores for flood risk management

The Netherlands is home to hundreds of kilometres of flood defences that require reinforcement. Constructing vegetated foreshores, such as salt marshes, represents one option for reducing hydraulic loads on dikes. This results in lower flood risks and higher ecological value.

Flood defences that utilise natural dynamics require a different approach to design, testing and management than traditional water defences. This article describes this innovation from the perspective of hydraulic engineering, morphology, ecology and governance (Illustration 1).

Figure 1. Vegetated foreshores: processes and functions (Source: Jeroen Helmer/ARK Nature)

Integrated assessment

In principle, a dike with foreshore does not have to be as high and strong as one without. Multidisciplinary research, however, is required to quantify this. This took shape within the research project BE SAFE (Bio Engineering for Safety).

The Safety sub-study entailed calculating wave reduction during extreme storms. Calculation models and field measurements of salt marshes in the Western Scheldt and Wadden Sea demonstrate that foreshores can significantly reduce the wave impact on a dike, depending on the water depth, the foreshore width and the vegetation.
The next questions are whether wave damping also occurs during very extreme storms, and whether the vegetation remains intact. Calculations have shown that during storms which the dikes should be able to withstand, vegetation in large bodies of water like the Western Scheldt and Wadden Sea will largely degrade. While this does not completely cancel out the wave damping, it does limit it to the effects of the foreshore geometry and its rough, stubbly surface with remnants of vegetation.
Finally, methods have been developed to estimate how much a foreshore can help in reducing the risk of a dike breach due to wave impacts and wave overtopping, both within the short and the long term. Foreshore variants appear to lower this failure probability and, under certain conditions, are more cost-effective than traditional dike heightening or the construction of a hard cover. Foreshore also reduces the risk of the dike collapsing due to piping or shearing of the outer slope.

The Biogeomorphology sub-study researched variations in foreshore characteristics, such as salt marsh width, bed level and vegetation, and the consequences thereof for wave damping. The long-term variations were researched by analysing historical data over a period of 60 to 70 years, which covers the typical lifespan of dike reinforcement projects.
In addition, field measurements were used to determine how foreshores will retain their shape over a one-year period.
Many foreshores consist of a high-lying salt marsh abutting a dike with lower-lying bare mudflats farther offshore. The salt marsh contributes the most to wave damping towards the dike. Wave damping variations in space and time are primarily the result of lateral displacements of the salt marsh edge. These displacements depend on the stress caused by the inundation time, as well as erosion and sedimentation. If the erosion and sedimentation are limited, young plants can tolerate a longer duration of inundation. If the soil dynamics are very high, the plants are not as resistant to inundation. On a seasonal timescale, soil dynamics are a greater determinant for the survival of young tidal marsh plants than the direct influence of waves.

Three questions were researched for the Ecology sub-study: i) the reliability of foreshore solutions for coastal protection, ii) the natural dynamics of foreshores and iii) the compatibility of goals for nature with those for safety.
An analysis of historical storms (1717, 1953) shows that foreshore reduce flood risk by reducing the occurrence of breach formation and confining the dimensions of the breach.
Also the strength of different plant species was measured in order to predict their wave damping effect and the moment of plant degradation. For this, measurements were taken along a succession gradient (from low-lying to higher salt marshes) and along a salinity gradient (from saline to freshwater foreshores). The differences between the plant species turned out to be many times greater than the differences between the seasons.
To understand the natural dynamics of foreshores and how to successfully construct them, the critical establishment conditions for different pioneer species were researched and the decline of foreshores was examined. Species variation appeared to cause significant differences in foreshore dynamics. Some plant species ensure stable foreshore width over the long term while for other species, the foreshore width expands and shrinks cyclically on a timescale of roughly sixty years.

The Governance sub-study looked at the decision-makers’ considerations when implementing natural solutions for flood risk reduction. This approach combines several functions, which results in multiple parties participating in the decision-making. Although foreshores do promote safety, and dike managers are obligated to include foreshores in their activities and to forge agreements with the users of adjacent areas, dike-foreshore coalitions do not happen spontaneously. Differing interests and responsibilities in tandem with historical factors complicate the efforts at collaboration.
What was the reasoning in deciding whether or not to go with an integrated natural solution for the Sand Engine (‘Zandmotor’) or the Markermeer and Afsluitdijk dike reinforcement projects? Game-theory models of such projects show that the institutional context is largely responsible for the final design. A multifunctional pilot provides more opportunities for natural solutions than a typical dike reinforcement project. Beyond this, it appears that the more the natural alternative is able to meet the safety objectives, the greater the chance for its implementation.
To support decision-makers, this game-theory approach has been applied using the ‘Added value of Collaboration’ tool. The underlying argument is that players will only work together if doing so benefits them more than their current strategy of non-collaboration. The parties learn what they are able to achieve alone and what they can achieve through collaboration. This provides insight into everyone's interests and contributes to a joint future perspective and to the actual implementation.

Foreshore solutions for Koehool-Lauwersmeer

For the Koehool-Lauwersmeer programme in Friesland, research was conducted to determine whether a foreshore solution was a good alternative to dike reinforcement. The cover and stability of this dike section is no longer sufficient and the dike has been rejected. The water defence should be strengthened for a total of 47 kilometres, and the costs are estimated to exceed €300 million. Salt marshes are already present at several points along the dike, or development of foreshores is possible using a natural method.
The BE SAFE team explored how a foreshore solution contributes to safety. Along with the parties involved (dike and nature managers, farmers and governments), consideration was also given to the realisation and management of foreshore solutions.
It appears to be technically feasible to construct a salt marsh in front of the dike. One unanswered question, however, is how to deal with changes to the natural environment. What do you gain or lose by turning a mud flat into a marsh? From an ecological point of view, developing a marsh on the inside of the dike would be interesting – by constructing a tidal inlet and a second dike – since no mudflats will be lost in that case. From a cost perspective, however, this is a less attractive option than transforming a mud flat into a marsh on the outside of the dike.
BE SAFE’s field measurements demonstrated that, during storms, the waves near the dike were much lower with tidal marshes than with bare mud flats. Probabilistic assessments show that constructing vegetated foreshores allows for a roughly 0.5 metres lower Wadden Sea dike. In particular the wave loads on dike covers of grass, asphalt and stone paving fell sharply (up to 50%); this is often the most expensive part of the dike.
BE SAFE and the project ‘POV Wadden Sea dikes’ have worked on the collaboration between the parties in and around the ‘Noard-Fryslân Bûtendyks’ foreshore (4,200 ha) to reinforce the dike. Five parties (Wetterskip Fryslân, It Fryske Gea, Nordeast Fryslân Municipality, tenants and the Directorate-General for Public Works and Water Management) have developed solutions that have the support of all parties. One of the researched themes was rewetting summer polders on the salt marshes, which affects safety, livestock farming and nature, and where joint action can lead to gains for all parties.
The effects, costs and benefits of a foreshore solution are compared with a standard dike reinforcement, which consists of raising and widening the dike and replacing its cover (see table).

A comparison of the foreshore solution with standard dike reinforcement


Foreshores can be a cost-effective solution for reducing flood risk, which also offers ecological and scenic added value. Moreover, under the right conditions, foreshores can grow with sea level rise, which makes them attractive for climate adaptation. Implementing a measure like this calls for a multi-disciplinary approach. Ecological and morphological processes interact and determine to what extent foreshores are able to reduce the wave load on the dike.
Foreshore solutions for safety offer added value for ecology, recreation and the scenery. At the same time, new forms of collaboration and financing are necessary for their implementation. In concrete terms, there are opportunities in the Netherlands to apply solutions like this along the Wadden Sea coast, in Zeeland and in and around the IJsselmeer and Markermeer. Foreshores and their vegetation can also contribute to the safety of river dikes, at locations where wave attacks occur.
Finally, the methods developed in this project can also contribute to the international development of innovative natural strategies. In many parts of the world, ecosystems such as wetlands and mangroves (Illustration 2) play or played a vital role in coastal defence.


BE SAFE is financed by the Netherlands Organisation for Scientific Research (NWO). Additional (financial) support has been provided by Deltares, Royal Boskalis Westminster N.V., Van Oord, Rijkswaterstaat, the World Wildlife Fund, HZ University of Applied Sciences, HKV Consultants, Staatsbosbeheer, Natuurmonumenten, It Fryske Gea, Ecoshape, STOWA and HWBP.

Vincent Vuik
(Delft University of Technology, HKV Consultants)
Bas Jonkman
(Delft University of Technology)
Pim W.J.M. Willemsen
(University of Twente, Royal Netherlands Institute for Sea Research or ‘NIOZ’, Deltares)
Bas W. Borsje
(University of Twente)
Stephanie K.H. Janssen
(Delft University of Technology, Deltares)
Leon M. Hermans
(Delft University of Technology)
Tjeerd J. Bouma
(Royal Netherlands Institute for Sea Research or ‘NIOZ’, Utrecht University)

Background picture:
Dike at the Western Scheldt estuary with a salt marsh.


The construction of vegetated foreshores such as salt marshes can be a cost-effective measure to increase safety and provide added value to the ecosystem and society. Foreshores with salt marshes provide adequate dike protection. New forms of collaboration and financing are necessary for their implementation. Recent research conducted for the Koehool-Lauwersmeer dike reinforcement project in Friesland offers guiding principles for other projects within the Netherlands and abroad.


Publications on safety bundled in the following dissertation:

Vuik (2019). Building Safety with Nature: Salt Marshes for Flood Risk Reduction. Delft University of Technology, March 2019.

An overview of other publications by BE SAFE is available at:

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Flood risk management

Knowledge journal / Edition 2 / 2019

Natural climate buffers combine nature development with water storage and greenhouse gas capture

Due to climate change, there is a growing need to try concepts like the natural climate buffer approach. The approach leads to multifunctional land use. The evaluation of three climate buffer projects highlights the learning points for future projects.

A recent evaluation identified success factors and learning points in the realisation of more than 70 natural climate buffers initiated by the Natural Climate Buffers Coalition (Coalitie Natuurlijke Klimaatbuffers, CNK)*.
Dutch water management has gained substantial experience with climate adaptation using natural processes, and the CNK programme ‘natural climate buffers’ is a significant example. CNK defines natural climate buffers as areas where climate policy objectives are achieved by giving space to natural processes that contribute to, among other things, water retention, the prevention of water shortages and the reduction of greenhouse gas emissions, in combination with other spatial functions. The evaluation (Veraart et al., 2019) was carried out with the aim of providing guidelines for implementation of the climate buffer approach in spatial policy.
This article is based on the evaluation of three illustrative climate buffer projects: De Onlanden, Hunze and Anserveld/Leisloot. These initiatives served as a catalyst for new project development by linking water management, nature development and the use of public space. The learning points for future projects are mapped out on the basis of contributions to water storage, greenhouse gas benefits and costs.


The water storage realised and the associated costs have been determined based on available environmental impact assessment research. The costs per hectare and per cubic metre were calculated to provide insight into the cost effectiveness.
The greenhouse gas emission reduction factors used (6 and 24 tonnes ha-1 year-1) were based on emissions measured at comparable locations in the Netherlands with and without elevated water levels. The chosen bandwidth does justice to the spatial and temporal variabilities that play a significant role. A hypothesis has been formulated about the achievable annual greenhouse gas reduction (kilotons), based on the emission factors and the surface of the climate buffer, adjusted for the presence/absence of peat (Figure 1) and the extent of drainage in the original situation (Knotters et al., 2018).
Climate buffers can only be used to generate CO2 Credits for the free market (GDNK, 2018), with the maximum carbon price in 2018 being around €5 tonne-1 CO2-eq (Hamrick & Gallant, 2018). This value was used for the monetarisation of the carbon credits.

De Onlanden

De Onlanden (2,500 ha) was a peat area with agriculture (grassland), and an unnatural water level with fluctuations between 0.25 to 0.80 –mv. Growing problems due to soil subsidence and frequent flooding that even threatened the city of Groningen formed the reason for the realisation of a water retention area combined with the development of nature and areas for recreation. After the land acquisition, quays and barriers were constructed in the area, peat pits were dug and the natural water level dynamics were restored.


This climate buffer (approx. 785 ha) concerns a brook valley and consists of three sub-projects: Bonnerklap, Torenveen and Tusschenwater, which were carried out from 2010 to 2019. Besides nature development, peak water retention and water conservation are also important goals. The three sub-projects are complementary to previously implemented nature restoration projects. The project areas mainly involve peat and are partially on marshy soil (Figure 1). Before implementation of the proposed measures, the groundwater levels varied between 0.60 m -mv (winter) and 1.20 -mv in the summer. The average water levels can increase by 0.55 m in both seasons with the taken measures (Spoolder, 2013).


Anserveld/Leisloot (approx. 150 ha) is part of the Dwingelderveld National Park (approx. 4,000 ha). Important objectives included combating the dessication of nature and reduction of flooding risks at Meppel. The hydrological restoration measures have led to rewetting of the heathland and to recovery of wet soil layers that slowly drain excess rainwater. The soil sub-layer consists of boulder clay and surface sand. Peat soil is present locally where the development of growing bog is possible, provided that the groundwater level remains within 0.05 and 0.20 –mv (Schunselaar et al., 2014). Prior to the redesign, the ground-water level was much lower.

Figure 1. Peat and bog areas in the Northern Netherlands Mineral soils (peat layer < 15 cm); Marshy soils (peat layer 15 - 40 cm); Thin peat soils (40 cm > peat layer < 120 cm), thick peat soils (frequently deeper than 120 cm –mv), marsh (no peat) and water.


Table 1 summarises the contributions of the three climate buffers to national climate policies, water management and nature development objectives.

Table 1. Summary of the contributions per climate buffer

The development of nature with peak water storage and water conservation

The discussed imate buffers are examples of successful collaboration between the nature area managers, water boards, provinces and municipalities. The realised water retention (Table 1) concerned a substantial portion (> 20%) of the total water storage needs of the water boards involved and the Dwingelderveld National Park. These projects have contributed to risk reduction for flooding in the surrounding agricultural areas and cities (Groningen and Meppel) together with the realisation of 1,550 ha for the National Nature Network and recreational facilities. Groningen Water Company has invested in water conservation and the development of nature in the Hunze brook valley. The abiotic conditions have been improved for wetland nature (De Onlanden), wet heath and bog (Anserveld) and stream valley nature (Hunze).
Although time series of groundwater levels are available , there are no studies that determine the precise effect of these climate buffer projects on water conservation. The monitoring of hydrology, nature and water quality was inadequately coordinated to quantify the impact on water conservation of climate buffers and the derived advantages and disadvantages for agriculture and nature.

The capture of greenhouse gases in climate buffers

Before constructing the climate buffers, the Hunze and De Onlanden were still partly in use for agriculture and, therefore, a net source of greenhouse gasses. The most optimal hydrological conditions for reducing greenhouse gas emissions have been achieved in De Onlanden. There is a thick layer of peat (Figure 1) in a continuous area (2500 ha) with minimal drainage. In the Hunzedal, the effect per hectare is smaller because there is less peat soil and an optimal increase of water tables is not possible everywhere due to the adjacent agriculture.
In Anserveld/Leisloot, the greenhouse gas emissions in the initial situation were already low (sandy soil). Locally, peat bog development is possible for fens (marginal, Table 1). Wet heathland captures fewer greenhouse gases per hectare compared to peat bog. In all climate buffers, emissions from methane can partly offset the capture of CO2 within the first years (Fritz et al., 2017).

Economic aspects

The cost per cubic metre of water retention is a measure of cost effectiveness, and it varies between 2 and 4 €m-3. The interviews show that the benefits of the climate buffer approach were often more decisive for those involved, or that the alternative plan was more expensive (De Onlanden).
At the current market price, the potential financial benefits of carbon capture for the landowner are between €30 - 120 ha-1 yr-1. At a price of €25 tonne-1 CO2-eq or higher, carbon capture is a realistic form of co-financing for the implementation of future climate buffers (€150 - 600 ha-1 yr-1). If the benefit for nature, the drinking water supply and recreation is also translated into financial returns, the feasibility will increase further. When determining carbon credits, financial risks can be made manageable with multi-year agreements (15-50 years).


Natural Climate buffers have been a driving force in realising the development of nature and climate adaptation in an economically efficient way with opportunities for recreation, residential areas and drinking water extraction. Due to climate change, there is a growing need to work with concepts such as the climate buffer approach. Mainstreaming the climate buffer approach in spatial policy offers an opportunity to efficiently implement the administrative agreement on Spatial Adaptation and the Climate Agreement (Ruimtelijke Adaptatie en het Klimaatakkoord).
The objectives is set within the Climate Agreement to reduce emissions up to 1.5 Mton CO2 eq by land use change on mineral soils and peat. About 20 - 30% of this can be achieved with rewetting measures (Vertegaal et al., 2019). The peat meadow area (2,400 km2 in 2004) decreases due to peat oxidation, which leads to soil subsidence. A lot of peat meadow areas can still be saved by using the climate buffer approach.
To get a grip on the financial risks of this type of spatial investment, innovation and the linking of existing monitoring networks are needed for the benefit of an integrated evaluation of climate adaptation, the greenhouse gas capture potential of ‘wet’ nature and the certification of CO2 allowances.

Jeroen Veraart
(Wageningen Environment Research)
Paul Vertegaal
(the Dutch Society for Nature Conservation)
Marjolein Sterk
(Wageningen University AEW chair)
Judith Klostermann
(Wageningen Environment Research)
Rob Janmaat
(De Lynx Communication Agency)
Boukelien Bos
Tim van Hattum
(Wageningen Environment Research)
Michael van Buuren
(Wageningen Environment Research)

* The Natural Climate Buffers Coalition consists of ARK Nature Development, LandscapesNL, Nature and Environmental Federations, the Dutch Society for Nature Conservation, Staatsbosbeheer (a Dutch forest management agency), the Netherlands Society for the Protection of Birds (Vogelbescherming Nederland), the Wadden Association and the World Wildlife Fund. The evaluation was made possible with financing from LIFE IP Delta Nature and the Ministry of Agriculture, Nature and Food Quality or ‘LNV’ (soil map).


The results from three natural climate buffers in the Northern Netherlands, namely De Onlanden, Hunze and Anserveld/Leisloot, are discussed. They have acted as catalysts for new project development in which water management, climate policy, the development of nature and the use of public space have reinforced one another. Due to climate change, there is a growing need to work with concepts such as the climate buffer approach. With rewetting measures, it is possible to achieve 20 to 30% of the challenge outline in the Climate Agreement to reduce emissions by 1.5 Mton CO2 eq by land use change on mineral soils and peat. To get a grip on the financial risks of this type of spatial investment, innovation and the linking of existing monitoring networks is necessary.


CNK, 2014. Natuurlijke Klimaatbuffers - kennis en kansen - interim report 2010-2012.

Fritz, C., Geurts, J., et al., 2017. Meten is weten bij bodemdaling-mitigatie. Bodem, number 2.

GDNK, 2018. Methode voor vaststelling van emissiereductie CO2-eq - CO2-emissiereductie via verhoging grondwaterpeil in veengebieden (‘Valuta voor Veen’), Greendeal Nationale Koolstof Markt.

Hamrick, K., Gallant, M., 2018. Voluntary Carbon Market Insights: 2018 Outlook and First-Quarter Trends. Ecosystem Marketplace, Forest Trends, Washington, USA.

Hazelhorst, H.J., 2014. Maatregelenstudie droge voeten 2050 (pp. 90), Waterschap Noorderzijlvest.

Knotters, M, et al., 2018. National, up-to-date information on groundwater levels, digitally available H20 magazine, Online (November 2018), number 11.

Schunselaar, S., et al., 2014. Anserveld - Substantiation of GGOR and WB21. Groningen, Grontmij.

Spoolder, M., 2013. Natuurontwikkeling Bonnerklap. Assen, Grontmij.

Veraart, J. A., Klostermann, J. E. M., Sterk, M., Janmaat, R., Oosterwegel, E., van Buuren, M., & van Hattum, T., 2019. Heel Nederland een natuurlijke Klimaatbuffer: evaluatie en vooruitblik. Wageningen Environmental Research.

Vertegaal, P., Borren, W., & Schouten, B.C., 2019. Natte natuur in het klimaatakkoord – win win in het kwadraat. Vakblad Natuur Bos Landschap

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Three climate buffers analyzed

Knowledge journal / Edition 2 / 2019

Dry, dryer, driest

In recent years, countries and regions around the world have been hit hard by drought and consequent water scarcity, with major consequences for agriculture, drinking water supply and energy production. The Netherlands also experienced unprecedented arid conditions in 2018, with low discharge from the Rhine coinciding with a significant precipitation shortfall. Developing effective drought policy to deal with such extremes is becoming increasingly important the world over. A first step is to prepare a thorough drought risk analysis.

A drought risk analysis is an indispensable first step for decision-making and for taking measures before, during and after drought or aridity. But what information is required to get a handle on drought, drought risks and perspectives for action? There is a substantial amount of measurement data, indicators, models, online platforms and other tools available. But are these resources adequate or is some information missing? How can full insight into a drought and its impacts be obtained? Should drought analyses be carried out in the same way everywhere?
Professionals and policymakers around the world are finding it difficult to navigate these questions. This difficulty has been recognised by the World Bank. Guidelines for drought risk analysis have been developed in collaboration with a consortium of Dutch and international experts (World Bank 2019). To better support drought risk analyses across the world, drought-oriented products and data sets have been inventoried.

Aspects of drought risks

Analysing drought risk combines information pertaining to three aspects:
• 'hazard': meteorological and hydrological data on drought within an area;
• 'exposure': information about the water demand of sectors and water users;
• 'vulnerability': information on the susceptibility of those sectors and water users to be impacted and the options they have to cope with and to recover from the effects of the drought.

Global availability of drought products

The inventory lists online and open-source products and data sets that specifically target drought (see Globally, approximately 200 products were found to be available (indices, data sets, newsletters, tools, software and online platforms) to support drought risk analyses (Deltares, 2018a). This inventory is not exhaustive. New products are constantly being developed, and there are many products that contain basic data for drought analyses, but are not specifically aimed at drought. A good example of this is the Copernicus Climate Data Store (, which is constantly evolving and is becoming an increasingly important source of climate information both at European and global scale. The compiled catalogue displays the most important features and applications for each product, along with information about availability and contact persons.
Most drought products concern meteorological, agricultural and hydrological information. In particular, there is a wide range of products available for meteorological drought. There are scarcely any products on offer about the impact of drought, and there is little information about exposure and vulnerability (Illustration 1). Therefore, to establish a thorough analysis of drought risk of an area, local information on exposure and vulnerability is always crucial.

Figure 1. Drought products available per region, distinguishing between hazard mapping, monitoring and (seasonal) prediction, impact mapping and risk mapping.

The applicability of the inventoried products has been validated with historical information on droughts (Deltares, 2018b). Illustration 2 is an example of an analysis conducted for Afghanistan. This shows that global meteorological and hydrological data sets and models (like PCR-GLOBWB and WaterGAP) form a good basis for characterising drought hazard at a national scale and for setting up seasonal drought forecasts. The few data sets and products that represent the impacts and risks of drought, however, do differ significantly from one another (examples include the Global map of drought risk (Carrão et al, 2016) and EM-DAT, The International Disaster Database ( Such products can only be used for an initial insight into the main risks of drought, and additional verification with local data or information is essential.

Figure 2. Global drought data applied for Afghanistan: the SPEI3 meteorological drought indicator based on the data sets of PCR-GLOBWB (above) and WaterGAP (below). The grey areas at the top indicate the years in which a drought was recorded in The International Disaster Database EM-DAT as well as local reports. The Y-axis indicates the percentage of land surface affected by dry conditions (SPEI <-1.0), moderate drought (SPEI <-1.5) and severe drought (SPEI <-2). Source: Deltares, 2018b.

Guidance for drought risk analyses

The World Bank's new guideline (World Bank, 2019) states that an analysis of drought risk can serve a range of purposes. The analysis can be part of a regional or national survey, but can equally focus on a very specific issue, such as securing financial support for smallholder farmers. To ensure that the analysis is a good match for the objective, the guideline describes four basic principles.

Principle 1. Prioritise the impact of the drought
It is crucial to begin with an inventory of the actual and potential effects of drought. For example, if the drinking water sector is primarily dependent on groundwater, an assessment of the drought characteristics in relation to the replenishment of groundwater and groundwater reserves is important. For smallholder farmers who use local rainwater reservoirs, precipitation and evaporation are relevant (meteorological drought), while the shipping industry or a hydroelectric station would require information about river discharges and water levels (hydrological drought).

Principle 2. Choose the perspective that suits the system
A drought risk analysis should be carried out at the right scale. This depends heavily on the actual or potential consequences of drought and the vulnerability of the exposed sectors. A risk analysis of small communities that depend on local rainwater during the growing season requires a different approach than a risk analysis of hydroelectric stations that depend on river discharge from an entire river basin for a prolonged period. To obtain a clear picture of the extent to which drought risks play a role, it is vital to involve all of the relevant local stakeholders early on in the analysis. Combining local knowledge with scientific information facilitates the establishment of a common knowledge base.

Principle 3. Consider the changes in drought risks in the future
Climate change and socio-economic developments influence future droughts and water shortages. If the purpose of the drought analysis is climate adaptation or a long-term solution strategy, it is important to include these changes. While climate change mainly influences the characteristics of the drought risk, socio-economic development influences the exposure of sectors and water users and their vulnerability.
Therefore, when choosing adaptation measures, one must consider potential negative side effects. For example, drilling wells to compensate for a temporary shortage of water can alter the hydrological system, and may make a future drought more severe.

Principle 4. Effective drought policy improves both resilience and preparedness
Beyond short-term relief actions during or after a drought, good drought policy is critical. To that end, well-organised governance, clarity about priorities in the event of drought, clear sector and area-specific management objectives and drought contingency plans are very important. Preparedness is also important: early warning and monitoring are crucial for short and long-term drought management.

Risk approach to drought in the Netherlands

Especially after the 2018 drought, there has been a greater need for a risk-based approach to drought in the Netherlands. Better availability of local and national drought products is desirable. After the 2018 drought, water managers indicated that the information provided was somewhat fragmented. As in many other parts of the world, products that map the impact and risks of drought are still very much in development in the Netherlands.
Many of the products from the inventory are hardly used, if at all, and the usability of, for example, drought indicators from other countries is not yet well understood. Research is required to clarify the use of these drought products. One conclusion (see the first principle above) is that the impact of the drought must be central and that drought indicators should be selected that indicate this impact as much as possible. These impacts may differ for various sectors and areas. For example, this could mean that for the sandy soils and higher-grounds in the Eastern and Southern Netherlands other indicators are relevant than for the Western Netherlands.
Several projects and initiatives have already been initiated that focus on developing a risk-based approach and on better availability of drought products. Examples include the Freshwater Economic Analysis (from the Delta Programme), the IMPREX project and the DigiShape Drought testing ground.

Dimmie Hendriks
Marjolein Mens
Ted Veldkamp
(VU Amsterdam)
Micha Werner
(IHE Delft Institute for Water Education)


The impacts of drought are increasing worldwide, as well as in the Netherlands. Thorough drought risk analyses are becoming increasingly important. A recent inventory in collaboration with the World Bank shows that approximately 200 open-source, online information sources, data sets and other tools are available worldwide to identify drought risks. An analysis of those products demonstrates that the meteorological aspects are often considered, but the impacts of drought are not. Information on exposure and vulnerability to drought of different groups, sectors and areas is essential to conduct proper risk analyses. As in other countries, a risk-based approach to drought is recommended for the Netherlands, and some of the global products identified in the inventory could certainly be useful when considering the Dutch situation.


Carrão, H., G. Naumann, P. Barbosa (2016) Mapping global patterns of drought risk: an empirical framework based on sub-national estimates of hazard, exposure and vulnerability. Glob. Environ. Chang., 39 (2016), pp 108-124, 10.1016/j.gloenvcha.2016.04.012.

Deltares (2018a). Report 11200758-002. Hendriks, D. M. D., P. Trambauer, M. Mens, M. Faneca Sànchez, S. Galvis Rodriguez, H. Bootsma, C. van Kempen, M. Werner, S. Maskey, M. Svoboda, T. Tadesse, and T. Veldkamp. Global Inventory of Drought Hazard and Risk Modeling Tools.

Deltares. (2018b). Report 11200758-002. Hendriks, D. M. D., P. Trambauer, M. Mens, S. Galvis Rodriguez, M. Werner, S. Maskey, M. Svoboda, T. Tadesse, T. Veldkamp, C. Funk, and S. Shukla. Comparative Assessment of Drought Hazard and Risk Modeling Tools.

Scott, Michon and Rebecca Lindsey (2017). State of the climate: Global drought.
World Bank, 2019. Assessing Drought Hazard and Risk: Principles and Implementation Guidance. Washington, DC: World Bank.

World Bank (2019). Assessing Drought Hazard and Risk: Principles and Implementation
Guidance. Washington, DC: World Bank.

Global meteorological and hydrological data sets and models
• WaterGAP:

Data sets and products that show the impact and risks of drought
• Global map of drought risk from JRC: in Carrão et al, 2016
• EM-DAT: The International Disaster Database
• FAO platform:
• Aqueduct:
• African Drought Observatory:

Dutch risk approaches to drought
• The Freshwater Economic Analysis from the Delta Programme:
• IMPREX project:
• DigiShape Drought testing ground:

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Effective drought policy necessary

Knowledge journal / Edition 2 / 2019

Using deep-rooted crops to increase water storage capacity of compacted soils

Soil compaction and poor drainage due to heavy agricultural machinery is a major issue, particularly with heavy clay soils. Farmers are struggling with inundated soils and harvest losses, while water managers are contending with erosion run-off and high peak discharges. The mechanical improvement of soil has its limitations. Can deep-rooted crops ensure greater water storage capacity?

In the Netherlands, precipitation has increased with 18% compared to 1900. As a result of climate change, extreme weather will occur more frequently, with a predicted increase in precipitation of 25-108% by 2050. Furthermore, the summers are expected to become increasingly dry (Lenderink et al., 2011). Although growing seasons will become longer, these changes in weather patterns present a huge challenge to the Dutch water system. For the heavier soils, particularly the clay polders in the lowest parts of Holland such as the Haarlemmermeer, the Wassenaarse Polder and the Gelderswoudsepolder, the challenge is even larger because the permeability of these soils is already poor by nature (Van den Akker et al., 2015). This reflects a wider problem: of the soil in the Netherlands, 45% is compacted primarily due to heavy agricultural vehicles and a decrease in soil life (Van Os, 2017).
If the soil is not sufficiently permeable, water will simply stay at the surface level of these plots. Not only the drainage, also the capillary action is compromised, preventing the groundwater from reaching the roots of the plants. This can result in harvest losses of 20-40% (CLM, 2015). Due to more extreme weather (downpours, droughts), this problem is likely to get worse.

Pore volume

For soils to become ‘climate-proof’, water storage capacity has to increase. For this to happen, the pore volume must be increased, especially in the compacted layer underneath the tillage layer. Soil compaction often starts at the bottom of the tillage layer (at 25-30 cm) and can extend to a depth of 80 centimetres (CLM, 2015). Mechanical methods to combat soil compaction are expensive and mainly target the top 25-30 centimetres. Deep ploughing is often not desirable due to the large amount of iron and sulphur in the deeper layers of soil (so-called ‘cat clay’). As such, sustainable forms of soil management are required that are not conflicting with the usual agricultural practices.
Here we studied whether deep-rooting crops can increase water storage. We used six crops that were selected based on their soil improvement abilities in other studies (Reubens et al., 2010, USDA 2019): safflower, parsnip, hops and a mixture of three green fertilisers (lupine, clover and alfalfa), totalling 4 treatments. All these crops can be used in a conventional agricultural rotation. Therefore, perennial crops such as energy grasses (switchgrass, silvergrass) or horseradish were not taken into account.

Living Lab

Thirty-six soil cores of 1 meter deep and 25 centimetres diameter were taken from a plot at the “Wassenaarse polder” (52.10’51.8”N 4.39’41.9”E) with a heavily compacted soil. The cores were transported to the Living Lab ( at Leiden university. This plot represents a ‘worst case’ soil compaction scenario as harvests had been very poor in the years prior to the experiment. The idea was that any plants able to root-penetrate these soils are likely suitable for other compacted soils as well. The four treatments with crops were compared to a one-time operation with steel pins (6 mm in diameter) that were driven 80 centimetres into the soil, and a control for which nothing was done. Due to the persistent drought in the months of May to August 2018, all of the soil cores were irrigated twice a week with ditch water (8 mm), which is comparable to the normal amount of precipitation in those months.
During the trial, the infiltration rate was determined twice a week as the time required for the irrigation water to be absorbed completely into the ground. At the end of the trial, the absorption capacity was determined as the volume of moisture absorbed after an extreme downpour of 100 millimeters (10 litres per waterbed core sample) and that does not flush out. The rooting and moisture content were then determined by taking a sample at three separate depths (15, 30 and 45 cm) and determining the moisture content after removing and weighing the roots.
Besides the experiment in the Living Lab, we performed a field test on a plot in the Wassenaarse polder with 36 sample plots measuring 2x2 metres, where exactly the same treatments were carried out. As a result of the extremely dry summer of 2018 none of the crops survived despite regular irrigation. By June safflower was the only species still present in substantial densities, but by mid-June a group of pigeons took out the remaining plants of this crop.”


Parsnips and hops exhibited scarcely any growth (plant heights after four months < 10 cm). During a seven-week period, the green fertilisers and safflower reached an average height of 35 centimetres. At the start, the infiltration rate was higher in the treatments with plants than those without (mechanical treatment and control). After six weeks, the infiltration rate was the highest for safflower, green fertilisers and hops (Illustration 1A). The control and the mechanical treatment had the worst performance.
At the termination of the experiment, the moisture content of the soil for safflower (17%) was the lowest of all treatments, which were all around 25% soil moisture. Safflower, therefore, flourished better in dry soils (this was also apparent from the field trial).
The soil’s absorption capacity at the end of the trial appeared to be highest for safflower: 92% of the water was stored (Illustration 1B), which is unsurprising given the lower moisture levels in the safflower soils. This amount was considerably lower for the other crops (80-85%), while the unvegetated control, on average, only absorbed 77% of the water.
At the end of the summer, root penetration of safflower was deepest (up to > 45 cm deep). Roots of green fertilizers grew to a depth of 25 centimetres (Illustration 2) while roots were practically absent below 10 centimetres in the other crops.


Of the crops researched, safflower clearly stood out: the root penetration was far and away the deepest (up to 45 cm) and the moisture content of the soils was the lowest. Safflower, therefore, drew the most moisture from the soil. The absorption capacity and the infiltration rate were also the highest.
Safflower is a plant found in arid regions that was grown in the Spanish colonies in South America as early as the 17th century (Oliver et al., 2005). The soil conditions in such dry climates might be similar to those of the compacted clay soil of the Wassenaarse polder, given that the moisture content measured for safflower was close to the permanent wilting point for this soil type (15-20%). Based on our research, however, it is not clear whether these characteristics could ensure better drainage and increased water storage capacity out in the field. This will require trials on a larger scale (which will be carried out in 2019). In this way, we hope to find out more about the period in which these measures have an effect and what the impact is on the nutrient levels and the soil life.

Benefits for farmers

A larger pore volume of the soil has major benefits for farmers: a reduction in the loss of fertile soil and better drainage. It could lead to a higher organic matter content and healthier soil life. Of course, this only applies if the additional processing does not cancel out the positive consequences.
However, a crop like safflower must match the business operations, including all the processes. That is why it is so important to test the crop treatments at the end of the season. Combinations of crops can also be researched, such as safflower with green fertilisers or other deep-rooted crops such as fibre hemp.
Of course, the question remains as to whether safflower can be profitable. It is grown in Kazakhstan, Mexico and New Zealand for oil-bearing seeds, for the extraction of colourants (yellow and red) and for sale as a saffron substitute (FAOSTAT 2017). To our knowledge, this crop has no application or market yet in the Netherlands.

Added value for water resource management

If this agricultural approach also yields better water storage capacity in the field, this will present major advantages for water resource managers:
• a higher ground water level will require less irrigation. Furthermore, this will reduce the amount of brackish seepage water and reduce soil subsidence;
• in summer, when the strongest increase in extreme rainfall is expected, the soil’s water storage capacity is most needed;
• as more rainwater infiltrates in the soil, water will take longer to reach the ditch, which implies that the discharge peaks to the storage basin will be decreased.
• the decreased ground-level drainage reduces the run-off of nutrients and plant protection products;
• deep rooting ensures a larger clay-humus complex at depth. Because such complexes bind cations and anions this will likely reduce the loss of nutrients and minerals.

Figure 1. Infiltration Rate (A) and Absorption Capacity (B) of the various treatments. Safflower, the best performing crop, is circled in red (left Y-axis, time in seconds).

Figure 2. Safflower had far deeper root penetration than the other crops (percentage of the samples with roots, at different depths).

Maarten Schrama
(University of Leiden)
Mees de Smet
(University of Leiden)
Peter Bij de Vaate
(Rijnland Water board)
Bart Schaub
(Rijnland Water board)


Soil compaction and a reduction in drainage due to heavy agricultural machinery is a problem for both farmers (reduced harvest, flooding) and water managers (drainage, high peak discharges). Initial research has shown that safflower, as a deep-rooted crop, can increase the water storage capacity. A detailed, sustainable application will require further research to find smart combinations of crops in cultivation systems.


Akker J.J.H. van den, Hendriks R.F.A. (2015). Hoe erg is ondergrondverdichting in de landbouw? Een samenvatting en conclusies uit onderzoek naar ondergrondverdichting. Bodem 3-2015, p. 42-44.

CLM, 2015. Brochure Ondergrondverdichting. CLM, Wageningen UR, Association of Provinces of the Netherlands, Ministry of Infrastructure and the Environment. 4 pp.

FOASTAT, 2017. World production of safflower seeds in 2016; Browse World Regions/Crops/Production from pick lists. Consulted on 04 December 2018.

Lenderink G., G.J. van Oldenborgh, E. van Meijgaard and J. Attema, 2011. Intensiteit van extreme neerslag in een veranderend klimaat. Meteorologica 2: 17-20.

Oliver, S.A., 2005. Food in Colonial and Federal America. Greenwood Publishing Group. Santa Barbara, CA, USA. 248 p.

Os, G. van, 2017. Met je kop in het zand - Investeren in kapitaal onder het maaiveld. Aeres University of Applied Sciences, publication 16-003 PP.

Reubens, B., K. D’Haene, T. D’Hose and G. Ruysschaert, 2010. Bodemkwaliteit en landbouw: een literatuurstudie. Activity 1 of the Interregproject BodemBreed. Instituut voor Landbouw-en Visserijonderzoek (ILVO), Merelbeke-Lemberge, Belgium. 203 p.

USDA Natural Resources Conservation Guide. Accessed 25-02-2019.

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Water storage capacity

Knowledge journal / Edition 2 / 2019

Toward sustainable potato cultivation with drought

Precision farming can considerably increase the efficiency of crop water use. One suitable technique is underground fertigation, in which water and nutrients are dosed according to the measured crop needs.

The long-term precipitation shortfall due to drought in recent years has become an increasing problem in the Netherlands that places ever higher demands on Dutch water management. Every year, measures are taken to prevent the depletion of water reserves during the summer, but the steadily falling groundwater level is a growing bottleneck. The agricultural sector is experiencing a heightened risk of loss of earnings.
The government has made efforts to expand water storage. In search of speedy solutions, the minister made €7 million available to improve the storage capacity of sandy soils – a measure to increase the water supply. But can we do something to reduce the demand. for water? It is possible. Precision farming can significantly improve the efficiency of crop water use.
Given the agricultural sector’s substantial water needs, precision farming may even become crucial for the Netherlands to become water-robust in the future and to protect the economy against the negative consequences of climate change. Water-saving technologies have been deployed for years in arid and semi-arid areas. With regard to the Netherlands, can we learn from these technologies and use this knowledge to tackle the increasing aridity?

Increasing drought calls for a structural and sustainable approach

Open-ground crops are regularly watered with sprinklers; however, provinces sometimes temporarily forbid using sprinklers in the event of prolonged aridity. For several crops, the sprinkler system can be replaced with sensor-driven drip irrigation, preferably subsurface drip irrigation. When nutrients are dosed as part of the irrigation, it becomes an underground fertigation system. This is a sustainable technology that offers substantial advantages in comparison to a sprinkler system.
The more efficient use of water and nutrients, tailored to the needs of the crop, results in increased yields and reduces the use of crop protection agents. Shortening the irrigation time and increasing the efficiency of water use also reduces the electricity consumption and the degree of salinisation in coastal areas where the water has a relatively high EC (electrical conductivity).
Furthermore, subsurface drip irrigation prevents leaf damage caused by salt-bearing drops drying out after being sprayed via a sprinkler system. And finally, the leaching of fertilisers into the surface water is noticeably lower because less water is used and the fertilisers are dosed.
Despite the advantages it offers, the introduction of this technology for open-ground cultivation in the Netherlands has hardly gotten off the ground. Underground fertigation is commonly applied for multi-year cultivation, such as fruit farming and arboriculture, albeit rarely utilised for annual crops. There are suitable operational systems available and more and more moisture sensors are coming onto the market; however, several important knowledge gaps are impeding the large-scale introduction of the technology, such as:
• What is the optimal soil moisture regime for various crops?
• How does fertigation affect soil moisture, nutrient and salt profile at various stages of the growth in different soil types?
• Is it technically feasible to combine a system using drip hoses with crops that must be harvested?
• Is it economically viable?
• What are the trade-offs when upscaling?
Several of these questions are addressed in this article.

Knowledge found on the bookshelf

Within the Netherlands, a few knowledge development programmes (climate pilots) have been launched as part of the Delta Programme, but they are still in the exploration phase. Fertigation systems have been installed in arid and semi-arid areas for years, and people have accrued considerable knowledge and experience with regards to hardware and software, how to solve bottlenecks, and the drafting of criteria for successful implementation of the technology for farmers.
A project is currently underway in Algeria in which Wageningen University & Research (WUR) is collaborating with the business community to optimise water management in potato cultivation. Despite the aridity, a large agricultural area has emerged in the Sahara around El Oued, an ancient city home to approximately 140,000 residents, where an underground water reservoir (the Albian aquifer) is located close to the surface. The introduction of pivot irrigation around the turn of the century saw an enormous increase in potato cultivation. The current acreage comprises approximately 30,000 hectares and represents 30% of the total potato production of the country.
The level of water and nutrient use for this agricultural practice, however, is not sustainable. Water is free in Algeria, and farmers can irrigate crops without restriction. As no measurements are taken of the soil’s moisture content or nutrient reserves, farmers irrigate too much water and fertilise at will. The farmers use chicken manure while the supply lasts. The nozzles of the pivots are directed upwards to prevent them from clogging up with sand originating from the wells. This irrigation method places a great deal of pressure on the aquifer, while it remains unknown how much water is available and whether the extracted water will be replenished. No one knows how future-proof this set-up is.
Pivot cultivation stands to improve considerably with the introduction of underground fertigation. Experience with fertigation systems in Saudi Arabia has shown that this system can save up to 70% water and 25% energy, while yields can double.
In 2018, WUR introduced a computer-controlled fertigation system at a demo company in El Oued. Staff and students from the University of El Oued made observations, and workshops and field visits were organised for farmers. With soil sensors, online measurements are performed at five different depths for soil moisture content, electrical conductivity and soil temperature. A weather station measures the air temperature and radiation. And a rain and flow meter measures the precipitation and the amount of irrigation water.

Figure 1: Online (Feb - June, 2018) measurement data from the fertigation software package, as used in El Oued, with respectively (from top to bottom): millimetres of rainfall or irrigation; air temperature; EC of soil moisture at a depth of five metres; volumetric moisture content at five different depths; the total moisture content in the root zone (blue: too wet, green: field capacity, pink: too dry). © Aquagri.

The evapotranspiration of the crop was calculated using a crop growth model, and an estimate was made of the water required for the following day. The graphical representation of the total moisture content (Figure 1, bottom panel) depicts the situation in the root zone: Blue is too wet; green represents field capacity and red is too dry. This graphic displays on the farmer’s smartphone and indicates the timing of the irrigation. As soon as the curve from the green zone enters the red part, it must be irrigated. In the autumn of 2018, underground fertigation resulted in a reduction in water use of approximately 60% compared to pivot irrigation, while the resultant potato harvest was roughly 40% higher.

What can the Netherlands learn from this?

Thus far, the project has provided data on the distribution of water and nutrients in the soil, water uptake characteristics in relation to crop growth and root depth, the course of electrical conductivity (EC) over time and the relationship between the soil and air temperature (Figure 2). This allowed researchers to calibrate the software for potato cultivation in relation to environmental factors (air and soil temperature, irradiation and soil characteristics).

Figure 2: Online (01 June – 07 June 2018) measurement data of air (above) and soil temperatures at five different depths (below). © Aquagri.

The online sensor technology is also relevant for the Netherlands, which can be applied with us on a one-to-one with basis. This is because crop growth, the water and nutrient uptake by crops and the build-up of salt and water profiles are all generic processes.
The project has yielded several intriguing insights that are also relevant to the Netherlands, such as:
1. Mechanical planting of potatoes at a depth of around ten centimetres easily combines with the unwinding and installation of underground hoses. During the harvest, a single machine can roll up the hoses and the dig the potatoes. Therefore, both the installation and removal of drip hoses is not an issue with annual crops.
2. With well-drained soils, multiple and short irrigation doses are more effective and yield greater water savings than prolonged irrigation at greater intervals. In addition, short and frequent irrigation intervals lead to less salinisation within the root zone than longer and less frequent irrigation.
3. The technology offers possibilities to guide the irrigation based on tuber setting and tuber size, thereby offering added value for seed potato companies.
4. Insight into crop propagation and the water, nutrient, pesticides and energy savings make it possible to calculate the economic feasibility of the technology for each company.
5. People in the Netherlands often begin irrigating crops too late. Computer-controlled fertigation offers possibilities to guide the water and nutrient dosing on the basis of the plants themselves.
6. However, a number of preconditions must be met to successfully introduce the new technology:
• The greater the acreage, the more that demands are made on the system regarding the water level and the continuity of the water pressure.
• The system is sensitive to blockage due to the build-up of calcium or sand, so it requires a good filtration system that must be properly maintained.
• The system sets requirements for the minimum size and investment capital of the company.

As such, not only do international projects provide region-specific knowledge, but they can also offer plenty of generic applicability. For instance, the knowledge developed for dry, hot climate zones can also contribute to the development of climate-smart agriculture in one’s own country.

Greet Blom-Zandstra
(Wageningen Plant Research)


Prolonged precipitation shortfall has become increasingly common in recent years. Although measures are taken each year to limit the depletion of groundwater as much as possible, substantial savings can also be achieved on the demand side, particularly with the annual open-ground cultivation. One suitable technique is underground fertigation, in which water and nutrients are dosed according to the measured crop needs. Arid and semi-arid areas have accrued substantial experience with this technology. The article describes the lessons the Netherlands can learn from this technology.


Badr, M.A., Abou Hussein, S.D., El-Tohamy, W.A. & Gruda, N., 2010. Efficiency of Subsurface Drip Irrigation for Potato Production Under Different Dry Stress Conditions. Gesunde Pflanzen 62: 63–70

Ebrahimian, H., Keshavarz, M.R. & Playan, E., 2014. Surface fertigation: a review, gaps and needs. Spanish Journal of Agricultural Research 12 (3): 820-837

Jeuken, A., Tolk, L., Stuyt, L.C.P.M., Delsman, J.R., De Louw, P.G.B., Van Baaren, E.S. & Paalman, M., 2015. Kleinschalige oplossingen voor een robuustere regionale zoetwatervoorziening. Zelfvoorzienend in zoetwater: zoek de mogelijkheden. Stowa report 2015-30, pp 62.

Tolk, L. & Veldstra, J. 2016. Spaarwater, pilots rendabel en duurzaam agrarisch watergebruik in een verziltende omgeving van de waddenregio. Main report, the Acacia Institute, pp 52.

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Sustainable potato growing

Knowledge journal / Edition 2 / 2019

Determining damage in built-up areas due to extreme showers

Extreme showers can cause significant damage. On 2 July 2011, no less than 150 millimetres of rain fell in Copenhagen within the space of three hours. Overall costs estimated by insurers: €800 million. In recent years, extreme showers have also caused major damage within the Netherlands. Arcadis has developed a new damages model that makes it possible to estimate water damage in advance. This model can promote water awareness, aid in effective prevention and play a role in risk dialogues to develop local measures for climate adaptation.

Calculating water damage requires information about, for example, water depth, the time period of flooding, the scope of the flooded area and the water quality. In the Netherlands, over the past few years the WaterSchadeSchatter had been used. This is a tool which water managers can use to gain an impression of the costs and benefits of flood-prevention measures. Research has shown that assumptions and uncertainties about this damages model have a significant effect on the calculated water tasking and the associated damage calculated (STOWA, 2019). It is especially difficult to estimate water damage in built-up areas, which is still in its infancy.
Arcadis developed a new, more refined damages model based on the scientific research of Penning-Rowsell et al. (2013). For that study floods in the Netherlands, Germany and the United Kingdom were analysed. To quantify the various types of damage, existing damage methods were applied and new ones were developed where necessary. Collectively, these damage assessment methods form the basis for the new damages model.

Direct and indirect damage

Direct damage relates to buildings and the road network. Indirect damage is the result of traffic delays, the loss of vital infrastructure and health effects. Specific information is required to perform these quantifications (Table).
The damages model calculates the expected damage per rain shower and per year. The annual damage depends on the frequency of the extreme showers, described here as the repeat time-lap, such as a year. For comparison purposes: an extreme rain shower that occurs once every 100 years (T=100) will cause significant damage. The chance of a shower like this is minor. A shower that occurs once a year (T=1) will cause less damage, but it occurs so frequently that the expected damage over 100 years can be considerable.

Table 1. Parameters for determining direct and indirect damage

Applying the damages model

The new damages model was applied to a residential area in the Netherlands that is vulnerable to flooding. Developed during the 60s and 70s, this neighbourhood also contains a few companies and transformer vaults. It is situated in a depressed area with a lot of paving and little greenery; the shops and homes have low doorsteps.
Using an integrated 1D/2D hydraulic model, various flooding characteristics were calculated for this area. The short-duration precipitation statistics from STOWA have used precipitation events with repeat intervals of 0.5 to 1,000 years. Illustration 1 shows the results of the model for each precipitation event, distinguishing between the types of damage.
The results show that no damage is calculated for showers with a repeat interval of up to 2 years (T=2). The capacity of the below-ground and above-ground discharge is sufficient to process these showers. For heavier showers, (T=5 and higher), however, damage is calculated. For showers like this the capacity of the discharge is no longer sufficient, water runs on the streets and also flows into a few buildings.
As the precipitation events become more extreme, the damage calculated also increases. The ratio between the types of damage, however, does change. The relative share of damage to buildings decreases while the share of traffic delays increases. With higher water levels (approx. 30 centimetres) and longer periods with water on the street (hours rather than minutes), roads become impassable for a long time. Beginning from T=50, roads are almost entirely submerged and the damage to the road network doesn’t increase much anymore.
Health damages are not calculated by the new model. The number of water-borne parasites that a person could accidentally ingest was too small in this area for someone to contract an infection (Ten Veldhuis et al., 2010). Although the survey area concerns water draining from a mixed sewer, the proportion of wastewater was small compared to the volume of rainwater run-off. Depending on the type of sewer system, the number of wastewater connections, the potential transit of wastewater from other areas and the amount of rainwater, the risk of infection can be considerably higher.

Figure 1. Damage amounts for the area being studied, per precipitation event.

Validation of the new damages model

To estimate the accuracy, and thereby the value, of the damages model, a validation was performed. Validating damage caused by extreme precipitation was complex due to a lack of suitable data or missing observations. This study applies two validation methods. The first (common) method was to compare calculated damages with damage claims submitted to insurers. This approach is used to validate the damage calculated for residences. When no damage claims are known, the second method is used: a comparison with other model results. This method has been applied for damage to the other buildings and the road network. For indirect damages, there are no damage claims nor model outcomes from other damages models available.
For the first validation method, for the area under study information from insurers was used concerning the flooding with a cloudburst on 28 July 2014. During this cloudburst, for a short period of time the water level was relatively high and caused damage to the exterior and interior of buildings, primarily residences. This cloudburst was simulated with the aforementioned hydraulic model, with the results subsequently being entered into the damages model.
The model estimated the damage in the survey area to be nearly €2 million. The highest amounts involve damage to buildings and the road network – each a form of direct damage. This includes flooded basements, water against the skirting boards and the costs for clearing the roads.
The calculated indirect damage due to, for example, health effects or the limited use of infrastructure was relatively low. This was the result of using the duration of on-street water as a parameter when determining indirect damage — and water was only on the street for a short time. A calculation made with the traffic model, for example, shows that although the traffic was somewhat impeded, it was still possible to keep going (at a lower speed). After the (simulated) cloudburst the vital infrastructure was quickly accessible again, and few people were inconvenienced because of power failure or the disruption of the drinking water supply.
The first conclusion of the validation was that damage calculated for residences was comparable to the damage claims submitted to the insurers. In addition, the comparison of models showed that the damage calculated for other buildings and damage to the road network were also correct.


The results concern a specific residential area. The occurrence and the extent of damage will turn out differently for another area. For example, the traffic volume in an area with a motorway may be higher and, by extension, also the damage caused by a traffic delay. In an industrial area with a different type of buildings the damage will also be different. A sensitivity analysis led to additional insight into the occurrence and extent of the damage: damage to buildings and damage due to traffic delays can vary by a factor of approximately 2.5; damage to the road network and vital infrastructure scarcely vary.

Improved risk dialogue

Being able to apply a refined damages model is very valuable. Municipalities and water boards can use this model for tips on how to prioritise the protective measures for vulnerable locations. At present, the risk dialogue about the approach to vulnerable locations is still complex and often arbitrary in nature. Providing insight into the highest damage claims can serve as a building block for prioritising and weighing improvement measures, which can lead to increased cost effectiveness. The use of a refined damages model can also benefit insurers. They can estimate the expected damage per area and then use this 'inside information’ to reserve more targeted capital or to differentiate premiums. Residents may even be more inclined to take preventive measures.

Erwin Slingerland
Michel Moens


Due to climate change, extreme precipitation is becoming more common and the risk of flooding in built-up areas is increasing. To mitigate this risk, there is a growing need to correctly weigh investments in measures to combat flooding against accepting it. The estimation of water damage in built-up areas is complex and still in its infancy. To get a grip on this complexity, a refined damages model based on scientific research has been developed, particularly for urban environments. Quantifying water damage promotes water awareness and proactive action and it supports weighing the acceptance of flooding against improving flood management.


Penning-Rowsell, E., Priest, S. Parker, D., Morris, J., Tunstall, S., Viavattene, C., Chatterton, J., Owen, D. (2013). Flood and Coastal Erosion Risk Management – a Manual for Economic Appraisal. Routledge

Ten Veldhuis, J. A. E., Clemens, F. H. L. R., Sterk, G., & Berends, B. R. (2010). Microbial risks associated with exposure to pathogens in contaminated urban flood water. Water research, 44(9), 2910-2918.

HKV (2019). Onzekerheden bij wateroverlast - impact op berekende schades en investeringen. STOWA.

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Support with climate adaptation

Knowledge journal / Edition 2 / 2019

OWASIS: up-to-date information on soil moisture assists operational water managers

OWASIS aims to provide operational water managers with insight into the available soil moisture, as well as the storage capacity and the shallow (phreatic) groundwater level. The test users’ assessment was that the concept was useful in practice, but that seeking out improvements would be worthwhile.

Water level management revolves around controlling surface and groundwater levels and controlling the water balance. Too much or too little water translates into nuisance or damage for users. Anticipation is crucial to being able to cope with extremes. To that end, water managers monitor surface and groundwater levels and, of course, keep a close eye on the weather forecast.
An important, but unknown, parameter is the moisture content of the soil. How much moisture is available for plants? And, alternatively, how much water can be stored here temporarily in case of hard showers? OWASIS aims to provide the operational water manager with insight into this as yet missing information.
In the event of flooding and water shortages, it is important for water managers to know both the current and expected saturation rate of the soil in their entire management area. In this way, the soil reservoir can be better used to either temporarily store water or to retain the available water in a timely manner. In this article, we explain the concept of OWASIS and demonstrate the initial insights gained from applying OWASIS to the practices of water management.

Concept and elaboration

The soil moisture content, the amount of water in the uppermost 1 - 1.5 metres of the soil, determine the growing conditions for plants and crops. In-situ monitoring of soil moisture is expensive and difficult to implement due to the heterogeneity of the soil, meaning that measurements only contain information from the environment directly around the sensor. For this reason, the conditions for nature and agriculture have traditionally been indirectly evaluated on the basis of groundwater level measurements and statistics derived from them, such as the GHG and GLG (the average highest and lowest groundwater levels; respectively. For more information, see the following website:
With the arrival of advanced models, radar hydrology and satellite observations, the time is ripe to base more and more water management decisions directly on soil moisture information. The OWASIS system was developed according to this idea. The reason for this was a heavy precipitation event in October 2013 in the management area of De Stichtse Rijnlanden Water Board. At the end of the summer, the groundwater levels were low and the soil appeared to offer ample space to store the precipitation. Pre-milling or other precautions to prevent flooding proved unnecessary.
At the time, the technology to monitor soil moisture using satellites was not sufficiently developed for operational use. However, technology for deriving the actual evaporation was sufficiently far along. Since 2013, the evolution from evaporation to soil moisture has evolved into the current OWASIS through several studies. The figure below illustrates the essential components of OWASIS.

OWASIS combines various existing data sources into several new information flows. Central to this is the hydrological model, LHM (see for more information about the LHM), which combines meteorological input (based on radar, weather forecasts and satellite-based actual evaporation) with calculation rules on the flow of water in the subsurface and the unsaturated zone and with system knowledge (soil, ground level, location of watercourses, etc.) to estimate the soil moisture conditions, storage capacity and phreatic groundwater levels. This model calculates the requested output variables in time intervals of one day per grid cell measuring 250x250 metres.
Each night, a new and updated condition and forecast is calculated for the coming days based on observations made up to midnight. This allows water managers to have a 'fresh' impression at their disposal every morning on which to base decisions for that day.

Relevance for the practice

As part of the development, the calculated soil moisture and groundwater series have been validated using soil moisture measurements from the catchment area of the river Raam (in North Brabant) and groundwater level measurements within the management area of De Stichtse Rijnlanden Water Board. The standard LHM uses interpolated precipitation and evaporation data from the KNMI ground stations. For the first improvement, the precipitation data was replaced by calibrated precipitation radar images. This means that far more variation is included in the precipitation. In the second improvement, the evaporation data was ‘replaced’ (using a simple form of data assimilation) with the actual evaporation based on satellite images.
A mixed picture emerges from these comparisons. The groundwater levels were reasonably described by the original model, with r2 values of 0.5 - 0.9 (an average of 0.75). (The coefficient of determination r2 is a statistical measure that provides information about the extent to which a model approaches reality. The value can vary between 0 (no agreement) and 1 (complete agreement). Adjusting the input resulted in limited improvement in the performance of the model. A much larger improvement was observed for the soil moisture: the average r2 value for the sixteen locations increased from 0.6 to 0.8 as a result of the adjusted input. This gave the test users confidence in this phase of the project that the relatively coarse gridded LHM also provides a good basis for this new information flow, provided that this model is fed with detailed precipitation and evaporation data.

The dry summer of 2018 appeared to be a good test case for OWASIS. After the very dry and warm months of July and August, substantial precipitation fell in the western half of the Netherlands on 05 September within a short amount of time. Around 150 mm of precipitation fell in and around Bodegraven in eight hours, with peaks above 170 mm. The precipitation event was severe but localised: in Moordrecht, about 12 km to the south, only about 25 mm fell on that day. The effect on groundwater levels and the available soil retention was immediately visible: the calculated available soil retention in Bodegraven fell from 134 mm to 93 mm in one day, with a ‘normal’ September average of 30 to 40 mm. Note that the model, therefore, also takes into account that portion of the precipitation during such an intense shower that does not infiltrate into the soil, but flows superficially into surface water or the sewage.
De Stichtse Rijnlanden Water Board used OWASIS in the autumn of 2018 to determine the transition from the summer to the winter level. This eventually took place just prior to Christmas. In the winter of 2019, De Stichtse Rijnlanden Water Board implemented the OWASIS information flows in the VIDENTE decision support system. Within VIDENTE, OWASIS is used in order to follow whether we are heading back to a dry situation like last year, or whether it might be necessary to anticipate flooding.


Satellites are capable of observing the atmosphere and the earth's surface with an ever-increasing level of detail, including the uppermost centimetres of the soil. OWASIS combines evaporation data obtained in this way with a hydrological model for soil moisture data for the entire soil column. It is also possible to derive estimates of the soil moisture from satellite observations. Because these direct observations only pertain to the uppermost centimetres of the soil, combining them with a hydrological model is a necessity to obtain estimates for the entire soil column.
We expect that this combination can be further improved to enhance the accuracy and resolution of the estimates for the soil moisture and groundwater. Besides the aforementioned satellite-based soil moisture measurements, online measurements of the phreatic groundwater level are also a good source for additional substantive improvements to the OWASIS information product. By making use of the correct assimilation techniques (e.g. an ensemble Kalman filter), the accuracy can also be quantified.

At present, estimates of the current and expected moisture condition are generated on a daily basis. This temporal resolution can be increased by generating expectations several times daily. In particular, this has added value when the weather models are able to accurately predict heavy summer showers. The KNMI is constantly researching this in the context of improving the HARMONIE (HIRLAM ALADIN Research on Mesoscale Operational NWP in Euromed) model.
The combination of a model with satellite data — including weather forecasts — is very powerful because it provides us not only with soil moisture information for NOW, but also about the hydrological conditions of tomorrow, the day after tomorrow, etcetera. Given that OWASIS is a country-wide system, we use this information to create perspectives for action for all water managers in the Netherlands, allowing water boards to better anticipate wet and/or dry conditions within their management area as well as in a more supra-regional context, such as Smart Water Management; in consultation with the surrounding water boards and the Directorate-General for Public Works and Water Management.

The development of OWASIS from a practical idea into a national information product is an example of an innovation envisaged by STOWA with the SAT-WATER programme: the application of information based on satellite data combined with existing knowledge and tools to advance existing water management a step further. A strong, practice-oriented development path was followed in a number of steps and with the financial support of the European Space Agency (ESA).
Nevertheless, the question remains whether an innovation like this will find its way into operational water management. This ‘valorisation’ of OWASIS depends on the added value within the working process of the hydrologist and water level manager when making decisions in the event of water shortages and imminent flooding. The experiences at De Stichtse Rijnlanden Water Board can certainly be called good. Not only was OWASIS used last year to (partially) determine the moment of transition from the summer to the winter level, but OWASIS was also embedded in VIDENTE 0.1, an HDSR BOS (decision-making support) system.

The availability of the input data required for OWASIS is guaranteed via the Water Board Headquarters (Waterschapshuis). The WIWB ( project and SATDATA 3.0 project regulate the availability of basic, precipitation and evaporation data and weather forecasts, covering the entire country and cross-border, as open data. The Smart Water Management Programme of Directorate-General for Public Works and Water Management and the water boards will provide OWASIS with data for all Dutch water managers as from this summer.

Matthijs van den Brink
Joost Heijkers
(De Stichtse Rijnlanden Water Board)
Hans van Leeuwen
(STOWA, Foundation for Applied Water Management Research)


Knowledge of the soil moisture content is of key importance to water managers in order to anticipate extremely wet and dry conditions. OWASIS aims to provide this knowledge by supplying historical and forecasted time series of soil moisture content, storage capacity and shallow (phreatic) groundwater levels. OWASIS combines satellite and radar based observations with water system data in a hydrological model, yielding operational data streams on a national scale. Test users state the product is a valuable addition to their daily operations, and recommend further improvements for the data and their applications.

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Usable but could be better

Knowledge journal / Edition 2 / 2019

Ranking of wastewater treatment plants based on measurements of pharmaceuticals

A measurement campaign, designed to determine the impact of pharmaceuticals discharged from seven wastewater treatment plants, provides the opportunity to ascertain whether the correct wastewater treatment plants have been designated as hot spots. In general, the measured concentrations appear to be slightly lower than previously calculated in the Hot Spot Analysis (HSA).

Many surface waters in the Netherlands contain traces of micro-pollutants: pharmaceutical residues, plant protection agents, household chemicals and industrial contaminants (Moermond et al, 2016). These substances are in low concentrations, varying from a few nano- to micrograms per litre. Although the concentrations are low, there is growing evidence that these substances negatively impact the aquatic environment (STOWA 2014-44).

A large portion of the micro-pollutants in surface water is directly related to effluent discharges from wastewater treatment plants (Moermond et al, 2016). For a better idea of this relationship, the HotspotAnalyse Geneesmiddelen RWZI’s or ‘Hot Spot Analysis of Pharmaceuticals from Wastewater Treatment Plants’ (STOWA 2017-42), hereinafter referred to as the ‘HSA’, was conducted in the Netherlands.

This analysis was based on the average removal efficiency - identically estimated for all of the wastewater treatment plants with the same method - for the pharmaceutical residues. The input for the calculations was the total load of a specific group of pharmaceutical residues derived from the measurements taken for previous studies. The pharmaceutical load of the influent was calculated on the basis of the estimated excretion per connected population unit. The concentrations in the effluent and surface water were subsequently calculated on the basis of a mass balance. The study resulted in a national ranking of wastewater treatment plants according to several indicators.

Several studies show that the removal efficiencies for pharmaceuticals vary from 0 to 99%. The efficiency here depends in part on specific substance properties. In addition, it is now known that the removal efficiency of wastewater treatment plants varies greatly (such as Watson database, Maas et al., 2017, Wubbels et al., 2018, in comparison with STOWA 2018-02, STOWA 2018-46).

The HSA ranking does not take into account the ecological impact of the discharged pharmaceuticals, but rather only the quantity: the greater the discharged load compared to the diluting capacity of the receiving water, the higher the final concentrations (and, therefore, the higher the ranking position). This article describes the results of follow-up research, whereby the current removal efficiencies and surface water concentrations, as well as the ecotoxicological relevance have been included in the analysis. This information allows us to determine whether the correct wastewater treatment plants were designated as hot spots in order for them to be prioritised for purification efficiency improvements.

This article is the first of two that deal with micro-pollutant measurements in influent, effluent and surface water, and how the results can be used to further develop the HSA Geneesmiddelen (HSA of pharmaceuticals). This article describes a measurement campaign that was designed to determine the impact of pharmaceuticals discharged from seven wastewater treatment plants.

Sample collection

We carried out the measurement campaign in the management area of the Aa and Maas Water Authority. The focus of the campaign was the relationship between the effluent discharge and the receiving surface water. For a total of twelve months (2017/2018), a monthly sample of influent and effluent was collected from each of the seven wastewater treatment plants, as well as upstream and downstream of the effluent discharge point in the surface water. The samples were then analysed for twenty-seven different pharmaceutical residues (see Annex Table 1). The analyses were performed by the Aquon Laboratory (Schoffelen et al, 2018).

Table 1. Pharmaceuticals analysed

Comparison with the HSA

Based on the results, the sum was calculated for each sample of the nineteen pharmaceuticals used in the HSA. The measured concentrations in the receiving water bodies were compared with the values calculated for the HSA (see Table 2). The individual pharmaceutical concentrations were totalled for each month and averaged for the entire year.

Table 2. Comparison of model calculations from the HSA and concentrations measured downstream of wastewater treatment plants
*Between parentheses, the numbers of substances whose PNEC (predicted no-effect concentration) was exceeded

With the exception of the Aarle-Rixtel location, these measured concentrations were lower than the calculated values from the HSA. This may be due to the fact that the HSA uses a median flow rate for the summer period. Particularly in the winter, greater dilution occurs, thereby reducing the average concentration.
The ranking that emerged from the data measured differs considerably from that of the HSA. The WWTP occupying the number one position in the HSA (Land van Cuijk Wastewater Treatment Plant) appears to score better based on the measurements; coming in at the fifth position in the new ranking. This shift is likely caused by the fact that the HSA does not include the effect of a downstream treatment phase.

According to the values measured, the Aarle-Rixtel Wastewater Treatment Plant, occupying the fourth position in the HSA, has the highest concentrations downstream. The distribution in the measured concentrations also appears to be lower than in the calculated concentrations: the highest measured value (8.5 µg/L) is 35% lower than the highest calculated HSA value (13.1 µg/L).

A closer look at the data shows that there are shifts in the relative contribution of individual substances. According to the HSA calculations, the highest pharmaceutical concentration in the surface water belongs to metformin. According to the measurements, the highest pharmaceutical concentration belongs to valsartan, while metformin occupies the fourth position (see Figure 1).

Figure 1. Pharmaceutical concentrations in surface water, downstream of wastewater treatment plants near the rivers Aa and Meuse (Maas in Dutch). The substances in yellow exceed the PNEC. The substances in blue exceed the signal value for intake at the Keizersveer production site.

Relationship with water quality of the surface water: PNECs

The total pharmaceutical load is not necessarily correlated with the toxicological impact of the discharge. Large differences per substance in the concentration-effect relationship can be expected; this is expressed, for example, with the PNEC.

Figure 1 presents the total pharmaceutical load in surface water downstream from the Aa and Maas Wastewater Treatment Plant. More than 50% of this total load consists of four substances with a relatively high concentration but with a low ecological impact, whereby the PNEC is not exceeded. Diclofenac (a painkiller), present in a much lower concentration, only occupies the tenth position in terms of load. However, the diclofenac concentration exceeds the PNEC, meaning that this substance has a toxicologically relevant impact.

The other substance that exceeds the PNEC, clarithromycin (an antibiotic), occupies the thirteenth position in terms of concentration. One or both PNECs were exceeded at four locations. It is striking that the ranking based on measured concentrations corresponds to the ranking involving the PNECs.

Relationship with drinking water production

Eventually, the effluents from all of the wastewater treatment plants included in the measurement campaign enter the river Meuse directly. Further downstream, this water is used for drinking water production. Drinking water producers do observe a signal value for micro-pollutants, such as pharmaceutical residues. Once this is exceeded, vigilance is required.

Depending on the nature of the substance, it may be necessary to stop the water intake. At all of the locations, the concentrations for five substances measured downstream of the wastewater treatment plant were higher than this signal value. This involved the following substances: valsartan, gabapentin, irbesartan, metformin and hydrochlorothiazide (see Figure 1).

Conclusions and looking ahead

1. The measurements of pharmaceuticals in surface water provide valuable information that can be used to further elaborate the HSA. The measurement campaign we carried out on seven wastewater treatment plants demonstrates that the downstream concentrations have been generally overestimated in the HSA.
2. The values measured result in a different ranking of wastewater treatment plants than the ranking from the values calculated in the HSA. The conclusion from this is that prioritisation of the locations where additional pharmaceuticals should be removed must also be based on measured values and not solely on calculated values.
3. The HSA ranking also changes when the ecological impact is taken into account using the PNECs. This ranking corresponds to the ranking based on the loads measured.
4. Taking into account the signal values for drinking water production also results in a change in the ranking. The signal values for five substances were exceeded for all of the locations covered by the measurement campaign.

The next article utilises the data from the measurement campaign to determine the removal efficiencies for each wastewater treatment plant. This data, therefore, has been supplemented by a measurement campaign carried out on eighteen wastewater treatment plants in the east of the Netherlands, in the Rhine-East area. This measurement campaign focused on explaining the differences in the removal efficiency for each wastewater treatment plant.

Herman Evenblij
(Royal HaskoningDHV)
Niels Schoffelen
(Royal HaskoningDHV)
Roel Knoben
(Royal HaskoningDHV)
Wim van der Hulst
(Aa and Maas Water Authority)


The influents and effluents from seven wastewater treatment plants were monitored for a year, as well as the quality of the surface water in the receiving water bodies. The collected data were compared with the model calculations of the HSA. The data were also used to estimate the ecological impact of effluent discharges by comparing them with the available predicted no-effect concentration (PNEC) for several pharmaceuticals. The impact on the downstream intake for drinking water production has been estimated by comparing the data with the signal value for drinking water production.

In general, the measured concentrations appear to be slightly lower than those previously calculated in the HSA. This overestimation by the HSA can be explained by the basic assumptions used. The measured concentrations also indicate several major changes in the HSA ranking, with several wastewater treatment plants performing better than expected. When the number of PNEC exceedances is used to compile a ranking, this ranking appears to correspond to the new ranking based on the measured loads. If the impact on drinking water production is estimated on the basis of the signal values, the wastewater treatment plant effluents for the same five substances appear to exceed this signal value.


Maas, P. van der; B. Bult; H. de Vries; O. Kluiving; 2017; Verwijdering van acesulfaam in rioolwaterzuiveringsinstallaties: wat bepaalt het verschil?, H2O, 17 July 2017

Moermond, C. et al, Geneesmiddelen en waterkwaliteit, RIVM, 2016-0111

Wubbels et al. Biologische fingerprinting biedt inzicht in verwijdering van medicijnen en zoetstoffen in rwzi’s, see here.

STOWA 2017-42 Landelijke Hotspotanalyse geneesmiddelen RWZI’s

STOWA 2018-46 Zoetewaterfabriek awzi de Groot Lucht: pilotonderzoek ozonisatie en zandfiltratie

STOWA 2018-02 PACAS – Poederkooldosering in actiefslib voor verwijdering van microverontreinigingen

Watson database in de emissieregistratie;

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Ranking of wastewater treatment plants


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Knowledge journal / Edition 2 / 2019


The knowledge section Water Matters of H2O is an initiative of

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