A complex approach has been developed for estimating mesoscalic nitrogen discharges via drainage systems using spatial information about land use, drainage areas, nitrogen balances and soil and site characteristics. Determining the total drainage area involves certain difficulties for larger areas, as on the one hand, the available databases are incomplete, and on the other hand the localisation and digitalisation of large subsurface drainage areas is a very timeconsuming process. Knowledge of the history and causes of drainage systems in landscapes is required. To solve this problem a method has been developed to calculate the drainage areas for large catchments. In order to obtain a complete data set of subsurface drainage areas, representative areas were selected to enable the proportion of subsurface drainage area to be determined for various soil and site characteristics. These proportions were extrapolated to the entire area and the approach tested in the Mulde River Catchment Area in Germany.The rate of drained arable land is about 25.2% of the total area, which can be broken down into grassland (19.0%) and arable land (27.4%). The results differ for sandy soils with up to 8% drained areas and 57.8% for stagnant soils. This shows that the proportion of drained land is highly dependent on the nature of the soil in the catchment area, which has profound implications for approaches to nitrogen modelling.Average nitrogen discharge for the whole catchment area via drainage water was 33 kg ha À1 yr À1 in the 1980s and 10 kg ha À1 yr À1 in the 1990s. The nitrogen discharge varies from one soil type to another: in regions with sandy substrate (11,900 ha) discharge was 34 kg ha À1 yr À1 in the 1980s (14 kg ha À1 yr À1 in the 1990s), while in areas with loess lessive´soils (89,200 ha) it was about 26 kg ha À1 yr À1 in the 1980s (9 kg ha À1 yr À1 in the 1990s). The reduction can be explained by the complete change in farming strategy since the demise of the former German Democratic Republic (GDR).The approach shown is well suited to 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 water and nitrogen modelling. This gives us a better understanding of nitrogen discharges into surface and groundwater and temporal discharge dynamics. The discharge data are highly valuable to predict environmental protection measurements for streams, lakes, coastal waters and groundwaters.

PNSS P162/1B SummaryÐZusammenfassungHigh 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. Hohe Stickstoff (N)-Einträge in Gewässer erfordern Maûnahmen zur Reduktion von Einträgen aus diffusen Quellen. Künstliche Drainagen bilden neben dem Grundwasserpfad die Hauptquelle des diffusen N-Eintrags in Gewässer. Wegen fehlender Datengrundlage bleibt der N-Austrag über Drainagen jedoch bei der Modellierung unberücksichtigt. Der Beitrag erläutert eine Weiterentwicklung der von Behrendt et al. (2000) vorgeschlagenen Methode zur Abschätzung der normalerweise unbekannten Drainageflächenanteile für landwirtschaftliche Nutzflächen. Die Methode wurde für das 2.700 km 2 groûe Einzugsgebiet der Mittleren Mulde (Sachsen) angewandt. Dazu wurden die Drainageflächen in repräsentativen Gebieten ermittelt, digitalisiert und mittels des GIS ARC/INFO mit Daten der mittelmaûstäbigen Bodenkarte verknüpft. Damit konnte ein Datensatz mit Informationen zum Flächenanteil der Drainageflächen für alle vorkommenden Standortregionaltypen erstellt werden. Diese Ergebnisse werden auf Flächen extrapoliert, für die keine Daten zur Verfügung stehen. Als Ergebnis ergibt sich eine differenzierte Karte der Anteile drainierter landwirtschaftlicher Nutzflächen im Untersuchungsraum. Die Methode ist gut geeignet für künftige Modellansätze auf der regionalen Maûstabsebene. Mithilfe der Erstellung und Integration neuer Datenebenen durch moderne GIS-Operationen wird die Unsicherheit bei der Modellierung von N-und Wasserflüssen verringert.