In the central part of the Netherlands, a 20 km2drainage basin contains a morainic ridge of Pleistocene sandy deposits and a backswamp area where the sandy subsoil is covered with river clay deposits. Agricultural and “natural” ecosystems are present in both parts. A continuous groundwater flow exists from the ridge to the clay area. A channel network drains the area. For these channels a two-dimensional model for simulating discharge and water quality has been developed. In computations, discharges and concentrations in groundwater, precipitation, and overland flow are treated separately. With a model option for calculating the effects of land use changes, water quality was calculated for base-line conditions. Mean nitrate concentrations were 1/2 to 1/7 of the actual concentrations, mainly caused by manure inputs. Storm events occasionally give very high concentrations, also under base-line conditions. Therefore base-line conditions for ions cannot be described as a single steady value.