Toward a comprehensive assessment of the combined impacts of climate change and groundwater pumping on catchment dynamics

Under increasing anthropogenic pressure, it is becoming increasingly important to evaluate the combined hydrologic effects of climate change and groundwater overexploitation. Climate change impact studies traditionally rely on scenario projections, provided by General Circulation Models (GCMs) that are transformed via downscaling and bias correction and used to drive hydrological models. The potential impacts of climate change are then assessed by comparing the historical and projected hydrological responses. This approach assumes that downscaled GCM simulations can function as surrogates for the corresponding actual values (represented by observations or reanalysis fields), which implies a kind of stochastic equivalence. In this work we apply the concept of Stochastic Equivalence to evaluate the validity of the response of a hydrological model driven by GCM simulations that are downscaled to generate high-resolution spatially distributed rainfall fields. We then use this approach to assess the combined effects of projected climate change and groundwater pumping in the Upper Guadiana basin in Spain. Our results suggest that very significant decreases in availability of water for the future can be expected throughout the year, but most notably during a 3-month longer, extended hotter and drier summer season; overall, the soil moisture and actual evapotranspiration are reduced by 20%, recharge is reduced by 50%, and aquifer related responses (runoff generation, groundwater-surface water exchange, wetlands and streamflow) are reduced by 60%.