Hydrological impact analysis of land-use and climate change

Land-use and climate change are increasingly threatening water resources worldwide. To select appropriate mitigating or adaptive actions, assessing the hydrological impact of land-use & climate change on both global and local scale has become an important research topic. Until recently, hydrological impact assessments primarily focused on flood events and oversimplified the groundwater component. However, current climate change simulations predict an increase in severity of drought events in large parts of the world, including Belgium. The focus of this study is therefore on the impact landuse and climate change on the groundwater system. Furthermore, the importance of hydrological model uncertainty in climate change assessments is investigated. The Kleine Nete basin, situated in the North of Belgium is used as a study area. Land-use changes are frequently indicated to be one of the main human-induced factors influencing the groundwater system. In the first chapter of this thesis we coupled a groundwater flow model with a water balance and land-use change model to assess the impact of land-use changes on the groundwater system. Four future land-use scenarios (A1, A2, B1 and B2) based on the Special Report on Emission Scenarios (SRES) are developed using the CLUE-S model. The impact of these four land-use scenarios on the groundwater system is assessed using a steady state MODFLOW model in conjunction with the WetSpass model. Results show that the average recharge decreases with 2.9, 1.6, 1.8 and 0.8% for scenario A1, A2, B1 and B2, respectively, over the 20 covered years. The predicted reduction in recharge results in a small decrease of the average groundwater level in the basin, ranging from 2.5 cm for scenario A1 to 0.9 cm for scenario B2, and a reduction of the baseflow with maximum 2.3% and minimum 0.7% for scenario A1 and B2, respectively. Although these averages appear to indicate small changes in the groundwater system, spatial analysis shows that much larger changes are located near the major cities in the study area. Hence, spatial planning should take better account of effects of land-use change on the groundwater system and define mitigating actions for reducing the negative impacts of land-use change. Results of the first chapter indicated that, at least in the near future, urbanization is likely to be the land-use change with the largest impact on the groundwater system. To increase our understanding on how impervious surfaces influence hydrological processes Chapter 2 illustrates a remote sensing based methodology that allows integrating the heterogeneity of the impervious surface fraction of urban grid cells into a water balance model. The method combines a technique to estimate the impervious sub-pixel fraction of historical and recent medium resolution remote sensing data with an adapted water balance model that incorporates spatially distributed impervious fraction maps. The impact of impervious surface cover changes, derived from an old (1986) and more recent (2003) remote sensing image, was analyzed. The results show that impervious surface fractions can be obtained from medium resolution remote sensing data with reasonable accuracy: the mean absolute error is 17.7% for the recent image and 18.6% for the older image, the bias is close to zero. Comparing the impervious surface fractions of 1986 with 2003 indicates that most urbanization occurred by densification of existing urban areas. Integrating the impervious surface fraction into the water balance model shows a strong spatial variation of simulated runoff and groundwater recharge within the urban area. The urbanization between 1986 and 2003 increase