Assessment report on NRP subtheme “Impact of climate change on regional hydrology”

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The major part of the Netherlands consists of a low-lying river delta which is very sensitive to hydrological conditions in the North-Western part of the European continent. The rivers Rhine, Meuse and Scheldt carry through this delta to the North Sea annually nearly 100 km3 of fresh water. This water originates from a drainage basin of about 185 000 km2, which is 6 times the country area. The present geography of the Netherlands has largely been shaped by this river inflow and by the sediments which are carried along. Interaction of these fluxes with North Sea hydrodynamics in a period of rising sea level has produced large lowlands, which in the past millennium have been reclaimed for agricultural, urban and industrial purposes. At present an extended hydrological infrastructure is required to contain high waters and to keep ground water and surface water tables permanently under control. Otherwise more than half of the Netherlands would be permanently or frequently flooded. In the Netherlands water management is a matter of permanent concern. The abundance of water is at the same time source of prosperity and source of vulnerability. Waterways for shipping and water supply for agriculture, industry and domestic use are essential resources for economy. The Dutch wetlands also represent a great environmental value. Changes in water supply and river discharge therefore have important impacts. More frequent occurrence of low discharge is detrimental to fluvial transport and agriculture. More frequent occurrence of high river discharge affects the safety of population against flooding and causes economical damage. Additional sedimentation raises the river beds with respect to the surrounding lowlands with possible consequences for safety and river management. The previous considerations form the basis for the sub-theme “REGIONAL HYDROLOGY” of the Dutch climate change research programme NRP. Seven research projects dealing with different aspects of the hydrological system have been selected in order to study the vulnerability of the Netherlands to climate related hydrological change. Although not all components of the hydrological system could be studied, the most important processes and relationships in the hydrological system have been addressed (see Figure 1). In the present phase of research most of the results still have an indicative character. Some projects have been completed, but others have started only recently. Nevertheless several important conclusions already emerge, which are summarized below: u Recent scenarios indicate greater regional climate changes than assumed in the past. Most remarkable are the strong expected increase in winter precipitation and the increased drought in summer. With increasing CO2 concentrations forests will become more resistant to droughts. If biomass does not increase, the evapotranspiration will decrease, causing an increase in drainage to ground water and run-off. Drought damage to crop production will increase. Agriculture will be less affected in the low lying river delta than in neighbouring regions. This may yield a comparative advantage for the Dutch economy. In coastal lowlands no substantial increase in saline seepage is expected, even if the sea level rises more than one meter. The discharge regime of the Rhine will change drastically. The annual variability of the discharge will strongly increase; winter discharges will increase and summer discharges will decrease. Periods of low river discharge will become more frequent and more prolonged. This will diminish the transport function of the Rhine, with serious economic consequences. The availability of cooling water for power plants will also be affected. A shortage of Rhine water will cause further intrusion of saline water in the lower river delta. The frequency of high discharges will increase. This causes more frequent inundation of the embanked floodplains in the Netherlands. It is not yet clear to what extent this will change safety from flooding. Sensitivity of soils to erosion is more affected by the expected changes in land use than by climate change. The production of sediment by soil erosion may be substantially increased by changes in precipitation. Sedimentation rates on the embanked floodplains in the Netherlands will increase. As a consequence, polluted sediments will become an increasing environmental concern. Pollution in the catchment basins of Rhine, Meuse and Scheldt has to be kept under control. A general conclusion from the different sub-theme studies is that the most serious impacts of climate change result from a shift in extreme conditions more than from a shift in average climate conditions. At present no reliable indications can be given regarding changes in frequency of extreme conditions. This question should be addressed with priority in the future NRP. It appears that, in particular, a more frequent occurrence of periods of drought will have significant economical consequences.