A considerable reduction in the nutrient and pesticide inputs into the rivers and lakes of Germany is required in order to meet the “good ecological status” as demanded by the European Water Framework Directive (WFD). Subsurface tile drainage systems are one of the main pathways for such diffuse nutrient and pesticide inputs. However, the simulation of water and matter fluxes under tile-drained land on the landscape scale is still problematic in many countries, mainly due to a lack of data about the existing drainage systems. The present study examines for the first time whether an existing method to calculate the usually unknown proportions of tile-drained areas could be transferred to a large river basin, for which minimal data about drained areas is available. The study area was the Saale river basin (24,000 km²) in central Germany, with a broad variety of soils and site characteristics. The share of tile-drained areas in the Saale river basin was calculated to be 11% of the agricultural area. Apart from that, the calculated proportion of tile-drained areas corresponded satisfactory with the statistical data of the meliorated areas of the former German Democratic Republic. The successful application of the promising method is considered as an important step towards the calculation of the proportion of tile-drained areas for the whole Germany and Europe.