The Soil and Water Assessment Tool (SWAT) is a well established distributed eco-hydrologic model. However, using the example of a mesoscale catchment in Germany it is shown that the version SWAT99.2 is not able to correctly reproduce the runoff generation in a low mountain region. The calculated contribution of the baseflow to the streamflow is far too high whereas the interflow is strongly underestimated. Alternatively, the modified version SWAT-G can be used which, as is demonstrated in this paper, yields far better results for catchments with predominantly steep slopes and shallow soils over hard rock aquifers. In the example, calibrating the model over three hydrologic years of daily streamflow, the model efficiency increases from −0.17 to +0.76. The modifications in SWAT-G allow hydrological processes to be modelled in low mountain ranges while not restraining the applicability of the model to catchments with other characteristics.