A numerical hydrological model has been developed for a 450 km2 Danish catchment using comprehensive field data. The model integrates a simple evapotranspiration model, a lumped flow model for a phreatic aquifer found in till, and a traditional two-dimensional groundwater model for a confined fluvio-glacial aquifer. A minimal but adequate number of model parameters were calibrated by trial and error to make the model fit 29-year time series of hydraulic head and stream runoff data.

By simulating a “semi-natural” hydrological situation unaffected by withdrawals it is demonstrated that groundwater development can change the water balance considerably. In the actual case withdrawals induce a 25% increase in leakage from the phreatic to the confined aquifer, and reduce stream base flow by up to 30% in normal years, and up to 35% in dry years. On the other hand the reduction in base flow is considerably smaller for the upper stream catchments.