Climate change impact on hydrological extremes along rivers in Belgium

A methodology to analyze potential climate change impacts on hydrological extremes along rivers in Flanders (Belgium) has been developed. The results show that hydrological modelling techniques driven by climate modelling techniques and climate change scenarios enable a prediction of the long-term evolution of the hydrological system of the studied area. The hydrological system behaviour of the river Dender basin in the Scheldt River Basin District is modeled for an observed historical period and for a future change from the control period (1961-1990) to the predicted period (2071-2100) under forcing of a modified (predicted) climate. The climate induced impact on hydrological extremes is assessed through the comparison of key variables of the hydrological system for the two periods (e.g., runoff peaks, low flow values, overland flow and potential evapotranspiration). The modelling procedure is completed through 24 climate model simulations highly resolute (derived from the PRUDENCE climate project), local scale lumped conceptual hydrological models (NAM of DHI), hydrodynamic models (MIKE11 of DHI) and models for topographical information (DEM: Digital Elevation Models) and risk calculation models covering the studied area. An appropriate downscaling method has been developed counting for variable statistical properties as intensity and frequency. Follo wing this method, potential climate change scenarios for Flanders have been created based on sequences of low, mean and high variation factors for rainfall and potential evapotranspiration. The modelling results show significant reduction of the low flows due to a considerable hydrological regime modification. As for hourly high flows (flood risk), the results range from increasing to decreasing depending on the climate change scenario and counting for a large uncertainty. Overland flow follows similar patterns as for the high flows while evapotranspiration shows systematic increase as a result of regional warming. A statistical method has been implemented for the quantification of the modelling uncertainties induced by the created climate change scenarios. Also a sensitivity analysis has been performed on the climate change scenarios to assess their degree of sensitivity to the process used to generate them and the degree of sensitivity of the hydrological response in turn. has potentially a major impact, especially on economy and human life. These problems are enhanced by a climate change induced modification of the frequency and intensity of heavy rainfall events as well as periods with low rainfall volumes. The Scheldt River Basin District is likely to be sensitive to potential climate change impacts where the hydrological regime is strongly influenced by water accumulation variation throughout the different sub-basins. A modification of the prevalent climate, especially the variables of rainfall and evapotranspiration, can considerably affect this regime and induce important impacts on the water management (Burlando et al., 2002; Jasper et al., 2004). This could have a significant impact on water uses highly dependent on the hydrological regime, such as navigation or irrigation, but could also increase water related risks such as floods and low flows (Willis and Bonvin, 1995; Loukas et al., 2002). The prediction of climate change impacts has consequently an evident socio-economic interest. Based on the European climate project PRUDENCE, this paper presents the overall modelling procedure leading to the assessment of climate change impact on the hydrological extremes at a sub-basin scale. The Dender basin in the Flanders area of Belgium, and part of the Scheldt River Basin District, has been chosen as case study. 2 THE CASE STUDY AREA: THE SCHELDT RIVER BASIN DISTRICT The International Scheldt River Basin District extends from northwestern France, via Belgium to the Netherlands. Its total area is about 36416 km2. It is one of the most industrialized river basin districts in Europe with the densest population (12.8 million inhabitants). The International Scheldt River Basin District morphology is hilly where rivers are mostly lowland watercourses characterized by varying slopes. The Flanders part (Belgium) of the District is mainly flat with a highest altitude of 157m. Agriculture dominates land use in this District with 61% of total area formed mainly with livestock and arable farming, but the basin is also highly urbanized (13%). Less than 10% of the area is covered with forests. Figure 1 presents the location of the Dender river basin subject of this study in the Belgian area. Figure 1. Location of the Dender river basin in Belgium. 3 THE MODELLING APPROACH Climate change impact assessment is mainly made by the modelling science through climate models. Climate models are numerous and different in concepts as in spatial and temporal resolutions and in the integrated processes. Assessing climate change impact on the hydrological field would lead to deal with two complex physically based systems: the climate system and the hydrological system. Climate models are forced by different emissions of greenhouse gases to produce different possible pictures of future climate. The outputs of the climate model are used as forcing inputs for the hydrological models. The interface link between the two systems should allow the right transfer of the climate signal to the hydrological system and is known as the “Downscaling process”. During this last and while empirically analyzing the climate outputs that are valuable for the hydrological field, some climate models results would not be consistent with real observations and, therefore, cannot be valuable for impact analysis. This procedure allows a feedback from the interface between the two systems to the climate system and helps choosing climate models most representative of the real Belgian (Flanders) situation. Figure 2 gives an overall summary on the modelling procedure in assessing climate change impact on hydrological extremes. 4 THE PRUDENCE REGIONAL CLIMATE MODELS PROCESSING The PRUDENCE project (http://prudence.dmi.dk) has been chosen as the climate data support necessary for the present and future climate investigations to cover the studied area with different spatial resolutions and different time aggregations. To do so, PRUDENCE provides a Wallonie Flanders