A methodological framework based on the concept of ANSWERS (Areal Non point Source Watershed Environmental Response Simulation), a continuous, distributed parameters surface model for simulating infiltration, runoff and evapotranspiration, has been coupled with a GIS (GRASS) in order to evaluate the expected effects of climatic changes and land use management on the water balance, particularly drainage below the crop root zone. We show that a doubling of CO2 in the atmosphere will probably have a negative impact upon groundwater resources by decreasing the recharge and by increasing the irrigation requirements.