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Nitrogen inputs in ground and surface waters in the middle Mulde catchment
As the EU Water Framework Directive (WFD) is translated into national legislation, the nitrogen (N) concentration in many rivers will have to be significantly reduced. In order to implement suitable measures, the relevant pathways and input quantities of nitrogen discharge have to be identified. For the Middle Mulde catchment in Saxony, Germany (2,700 km2), a pathway-related model is developed to quantify point and diffuse nitrogen discharges. Calculated N-input via point sources is 2,400 Mg entering the rivers in the study area every year. The main point input representing over 50% of the total is via municipal sewage treatment plants, followed by the industrial direct dischargers and the combined sewer overflows, amounting to approx. 20% (462 Mg and 444 Mg) each. The diffuse nitrogen discharges from the soil zone of the arable land amount to a total of 8,050 Mg per year in the 1980s and 2,815 Mg in the 1990s. The inputs via the groundwater flow make up the main proportion for the 1980s with 5,303 Mg, followed by 1,480 Mg from tile drainage and 1,267 Mg from direct runoff. In the 1990s the nitrogen discharge fell sharply with the considerable fall in the surpluses in the agricultural nitrogen balance after German reunification with 1,934 Mg (groundwater flow), 454 Mg (drainage runoff) and 410 Mg (direct runoff). Sensitivity analyses show that the parameter ‘‘total atmospheric deposition’’ represents the highest uncertainties, because of its uncertain quantification and the high sensitivity of the results to this variable.
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