Extreme precipitation is more common

How often does it rain, how hard and how long? Will we be facing greater precipitation extremes in the future? The answers to these questions are very important for water management in the Netherlands. Commissioned by the Foundation for Applied Water Research (STOWA) the latest climate scenarios were made applicable for water managers.

In 2014, the Meteorological Institute KNMI presented new climate scenarios in view of climate change. The KNMI and the consulting firm HKV have made these so-called KNMI'14-scenarios more applicable for water boards and incorporated them into new rainfall statistics and time series, which are representative for the future. These statistics are of great importance for water managers, as they review their water systems and determine what measures are required to reduce flooding, partly on this basis.

The rainfall statistics provide information about the amount of precipitation at a given rainfall duration (two hours to eight days), which is exceeded with a certain frequency (twice a year to once a 1000 years).
For the benefit of accuracy and representativeness, the precipitation statistics are based on the longest possible precipitation series, on an hourly basis. For the Netherlands, it is the De Bilt (Utrecht) series, starting in 1906 and ending in 2014.

The previous precipitation statistics were established in 2004. At the time, the De Bilt precipitation series served as the basis to describe the precipitation extremes of the 'current' climate until 2003. However, the average precipitation in the De Bilt series contains a clear trend (see Figure 1). This trend is not only visible in De Bilt, but also at other rainfall stations. Not only the average precipitation, the extreme precipitation also shows an increase over the past one hundred years and more.

Figure 1. Long-term trend in the measured annual totals of precipitation in De Bilt and for the Netherlands average (102 precipitation stations) for the period 1910-2013. The smooth curves are long-term sliding averages, making the trend more visible.

In 2004, this wasn't corrected and therefore the statistics derived at the time, is basically representative of the climate around 1955 (the middle of the period of the series). The water boards wanted the new statistics to be representative for the climate around 2014, in other words, the climate we have now. That is why the researchers have corrected the precipitation series of De Bilt in determining the new rainfall statistics for this climate trend.

The rainfall associated with certain return periods (and duration of precipitation) calculated for the climate around 2014 are therefore higher than that calculated in 2004. Correcting for the trend causes a large part of the differences between the 2004 study and 2014; extending the series by eleven years until 2014, hardly has any influence.

The new statistics are not only determined for the climate around 2014, but also after climate change as that might be considered around 2030, 2050 and 2085.

Increase in precipitation

The new rainfall statistics indicate that the amount of rainfall during extreme rainfall events around 2014, on an annual basis, were 10% higher on average than in the statistics to date. This average applies to rainfall events that occur less frequently than once every two years, to very extreme precipitation with a return period of 100 years. The average increase turns out to be as much as 15 percent if only the winter period was considered.

Some of the currently available statistics are included in table 1. This shows, for example, that up to now, water managers estimated an extreme 24-hour precipitation event of 79 mm at a return period of 100 years. In the new rainfall statistics for around 2014, that has become 85 millimetres. Perhaps even more interesting is the observation that about the same amount of precipitation (77 millimetres in 24 hours) now occurs once every fifty years; so twice as often.

Table 1

Precipitation (in millimetres) exceeded once every 10, 50 and 100 years for 24 hours, four and eight days in the 'current' climate and in the climate around 2050, on the basis of annual statistics for precipitation regime G a)

a) Precipitation regime G applies to all areas in the Netherlands for which the statistics of extreme precipitation is the same as for De Bilt, both for the current climate and for the future.
b) For the 2004 statistics, 'current' implies the full historical period 1906 - 2003, (without any adjustment for the trend). With the 2015 statistic, 'current' implies the climate around the year 2014 (assuming the trend in the historical period 1906 – 2014).
c) Precipitation statistics for 2050 wasn't available yet in 2004, but in this review is based on the 2050 precipitation statistic that were included in Meteobase in 2013. The displayed range concerns the scenarios G+ (lowest value) and W (highest value).
d) The displayed range reflects all four climate scenarios (GL, GH, WL and WH) plus three sub-scenarios ('lower', 'centre' and 'upper'). In most cases, the lowest value corresponds with GH_lower and the highest value with WL_upper.

The KNMI'14 climate scenarios indicate that this quantity may remain virtually the same or may increase up to 90 mm in 2050.

In conclusion: climate change is thus already visible in the statistics. Extreme precipitation events in the climate around 2014 were about twice as common as in the past, and in future the events may possibly increase even further.

Increase in evaporation

Not only is there more precipitation, evaporation will also be greater according to the latest information. This is also corrected for a climate trend in the recently completed research. On an annual basis, for the climate around 2014, this is about 7 percent higher than in the reference used to date (1906-2010), see table 2. This increase is slightly higher in the summer half-year than in the winter half-year.

Table 2

Average evaporation per year and per winter/summer half-year for the original, but homogenised de Bilt series, for the climate range around 2014

What this means for the occurrence of drought, is analysed concisely by calculating the maximum potential precipitation deficit per year. We have added the daily rainfall per year starting from 1 April, minus evaporation, and determined the maximum precipitation deficit per year. This is done for both the original precipitation and evaporation series as well as for the 2014 precipitation and evaporation series, after adjusting for the climate trend.
This shows that the maximum precipitation deficits have little or no change once every five years. The effect of the climate correction is such that the increase in the evaporation is only visibly larger than the increase in precipitation in years that are drier. The differences in precipitation deficits in the summer run from a five-year return period to 5 percent in a return period of 50 years.
It should be noted that the maximum precipitation deficit is only a 'rough' indicator and doesn't say everything about local moisture deficits, for example, and therefore, about the impact on hydrology. In addition, the timing of the increase in precipitation and the increase in evaporation is also of interest.

We have also determined the evaporation for the year 2030 and the KNMI'14 climate scenarios 2050 and 2085. Please consult the report that is included first in the literature listing for further information.

Application in water management

With the new statistics, water boards now have the best available precipitation and evaporation data (statistics and time series) on hand to create (new) analyses of their current water systems, for the current climate (around 2014) and post climate change.

The data can be used in different ways. Precipitation statistics for instance is essential for evaluating a flooding situation, because the return period of a localised extreme rainfall can simply be determined.
The information can also be used directly for the assessment of regional water systems compared to flooding standards. Water administrators often use the so-called time-series method or the stochastic method for this purpose: the new data is available immediately for both methods.

On this basis, water boards can better assess the extent to which the systems are resistant to extreme precipitation events in the climate around 2014 and the future climate. These analyses help water boards to seek solutions to the problems surrounding flooding.

The new precipitation series and statistics can be found on www.meteobase.nl, the on-line database of STOWA with precipitation and evaporation data.

The authors are indebted to Janette Bessembinder (KNMI) as co-author of the underlying STOWA-report, and Michelle Talsma (STOWA) who made this research possible as client.

Hans Hakvoort
(HKV)
Jules Beersma
(KNMI)
Theo Brandsma
(KNMI)
Rudolf Versteeg
(HKV)
Kees Peerdeman
(Brabant District Water Board Delta/STOWA)

Summary

Recently, researchers at the KNMI Meteorological Institute and HKV consulting firm, commissioned by the Foundation for Applied Water Research (STOWA) derived new rainfall statistics.

What is new is that the measuring ranges of De Bilt for both precipitation and evaporation were corrected according to the climate trend, which is visible in the series. As a result, the new statistics give a better picture of the current climate (around 2014). The picture confirms what we have already experienced in practice: extreme rainfall events are more common place and the precipitation amounts are higher (order 10 to 15 percent higher, depending on season, duration and return period).

The new statistics are also provided for the climate around 2030, 2050 and 2085 on the basis of the so-called KNMI'14 climate scenarios.

Literature

Beersma, J., J. Bessembinder, T. Brandsma, R. Versteeg en H. Hakvoort, 2015. Actualisatie meteogegevens voor waterbeheer 2015. Deel 1: neerslag- en verdampingsreeksen. deel 2: statistiek van de extreme neerslag. STOWA report 2015-10

Smits, A., J.B. Wijngaard, R.P. Versteeg en M. kok, 2004. Statistiek van extreme neerslag in Nederland. HKV and KNMI contracted by STOWA.

Buishand, T.A., R. Jilderda & J.B. Wijngaard, 2009. Regionale verschillen in extreme neerslag. KNMI publication WR-2009-01.

Versteeg, Rudolf, Hans Hakvoort, Siebe Bosch and Maarten-Jan Kallen, 2013. METEOBASE, online archief van neerslag- en verdampingsgegevens voor het waterbeheer. STOWA report 2013-02.

Auteurs

Hans Hakvoort
(HKV)

Jules Beersma
(KNMI)

Theo Brandsma
(KNMI)

Rudolf Versteeg
(HKV)

Kees Peerdeman
(Brabant District Water Board Delta/STOWA)