The new watershed model SWIM was developed in order to provide a comprehensive GIS-based tool for hydrological and water quality modelling in mesoscale watersheds (from 100 to 10000 km(2)), which can be parametrized using regionally available information. SWIM is based on two previously developed tools-SWAT and MATSALU. The model integrates hydrology, vegetation, erosion and nitrogen dynamics at the watershed scale. SWIM has three-level disaggregation scheme and is coupled to the Geographic Information System GRASS. A robust approach is suggested for the nitrogen modelling in mesoscale watersheds. Model test and validation were performed sequentially for hydrology, crop growth, nitrogen and erosion in a number of mesoscale watersheds in the German part of the Elbe drainage basin. Firstly, the hydrological module was tested in five watersheds of different size with different topography, soil and spatial resolution of input data. After that the test was performed for the crop module in the state of Brandenburg, for the nitrogen module in the rural basin of Stepenitz, and for the erosion module in the mountainous Mulde basin. A comprehensive scheme of spatial disaggregation into subbasins and hydrotopes combined with reasonable restriction on the subbasin size allows the assessment of water resources and water quality to be performed with modest data requirements at the regional scale. The direct connection to land use and climate data provides a possibility to use the model for analysis of climate change and land use change impacts on hydrology and water quality. (C) 1998 Elsevier Science B.V. All rights reserved.
