Proportions of subsurface drainages in large areas—methodological study in the Middle Mulde catchment (Germany)

High nitrogen (N) input to rivers requires measures for the reduction of diffuse N pollution. Beside the groundwater, artificial subsurface drainage systems are the main pathways of diffuse N input into rivers. Nevertheless, the N discharge via subsurface drainage systems is one of the main missing links for modeling, especially because of the lack of data bases of subsurface drainage areas. We introduce a method to calculate the normally unknown proportions of drained areas in arable lands improving the existing method by Behrendt et al. (2000). The method is applied for the catchment of Middle Mulde river (area: 2,700 km 2 ) in Saxony/Germany. The data records of the mesoscale soil mapping are allocated to the subsurface‐drainage areas digitalized in representative areas using ARC/INFO GIS. In this way, it is possible to establish a differentiated record of the proportion of subsurface‐drainage area of each regional site type. The results were extrapolated to the entire area by transferring the proportions of subsurface‐drainage areas to areas where no information on drainage areas was available. The approach is well‐suited for future model approaches on a regional scale. By creating and integrating new data sets derived from modern GIS operations, the approach reduces the uncertainty of modeling N and water fluxes.