Abstract
Risk assessment of future land use changes with respect to their ecological impact is still an unsolved problem. The implementation of sustainable land use concepts for mesoscale catchments requires a methodology that is able to quantify the effects on the water balance of this ecosystem. Paired catchment experiments on this scale are either not feasible or too time‐consuming and expensive, and the results are usually not transferable to other regions. Thus the application of hydrological models can clarify the processes involved and support well‐balanced management decisions.
The GIS‐linked software package IOSWAT consisting of the pre‐processing tools TOPAZ, SUSAT and SWATGRASS, the ecohydrologic model SWAT‐G and the post‐processing program OUTGRASS were tested in the German Aar watershed (60 km^2^), which is located in a low mountain range area in the federal state of Hessen. The aim of this modelling exercise is the development of a method for land use change analysis in order to derive sustainable land use options. The impact of different average field sizes on the spatial distribution of agricultural production systems was analysed by the economic model ProLand. Spatially distributed land use scenarios for an average field size of 0·5, 0·75, 1, 2, 5, 10 and 20 ha have been provided in the form of raster maps with a resolution of 25 m × 25 m and were analysed with the IOSWAT modelling system with regard to their hydrological effect. In general, the model predicts reasonable annual trends for the water balance components of the scenarios under investigation. However, the absolute changes due to deforestation, augmentation of grassland and field crops remain much smaller than expected. The peak flow rate is especially affected by the changes in land use. The strongest impact can be observed in the spatial pattern of water balance components within the catchment. Copyright © 2005 John Wiley & Sons, Ltd.