Impact of climate change on spatial and temporal patterns of groundwater head and flux.

Climate changes are likely to have an impact on the spatial and temporal hydrological patterns within a watershed. This study presents a methodology to assess the impact of climate change scenarios on the spatial and temporal patterns of groundwater recharge, head and flux. The developed methodology includes coupling a distributed hydrological model with a transient groundwater flow model and is tested for the Kleine Nete basin, Belgium. The groundwater recharge and river heads are estimated with the WetSpa model using a daily timestep to incorporate the impact of changes in rainfall intensity. The groundwater flow is modeled with a MODFLOW model with a temporal resolution of two weeks. For both the WetSpa and MODFLOW models the spatial resolution is chosen 30 by 30 meters. Potential future hydrological changes are calculated by comparing the hydrological state corresponding to 1960-1991 with future scenarios developed for 2070-2101. Since the uncertainty in the prediction of the future climate components such as potential evapotranspiration (PET) and precipitation is still high an ensemble of 28 climate scenarios were chosen from the PRUDENCE database. For each of these scenarios the recharge, river stage, groundwater head and groundwater flow are estimated for 32 years with half monthly timesteps. Evapotranspiration and precipitation are the two major climate components influencing the groundwater system. Comparison of the original measured PET with future PET shows that the PET during summer rises in all future scenarios with about 1 mm day-1. For winter conditions the scenarios predict little change in PET. Future precipitation shows an increase in precipitation during winter and a decrease during summer. Future groundwater recharge decreases on average with 20 mm year-1, the highest decreases are simulated from July until September. Average groundwater heads indicate an average decrease of 7 cm. Groundwater levels on the interfluves generally show a higher decrease up to 30 cm. The mean lowest groundwater level decreases on average with 6 cm, while the mean highest groundwater level decreases about 3 cm. On average the groundwater discharge reduces with 4%, from 5 to 4.8 m³ s-1.