Changes in diffuse phosphorus and nitrogen inputs into surface waters in the Rhine watershed in Switzerland

Using the regional scale, nutrient-flow simulation model MODIFFUS, an estimate of phosphorus (P) and nitrogen (N) inputs from diffuse sources into surface waters over the Swiss part of the Rhine watershed downstream of large lakes was performed for the years 1985, 1996 and 2001. The data are required for purposes of national monitoring and for reporting to international panels such as the International Commission for the Protection of the Rhine (ICPR) and the Convention for the Protection of the Marine Environment of the North-East Atlantic (OSPAR). Total diffuse P and N losses, including natural background losses, decreased from 547 tons (t) P and 19 219 t N in 1985, to 423 t P and 17 902 t N in 1996, and 393 t P and 16 610 t N in 2001. Total reductions between 1985 and 2001 of 154 t P and 2 609 t N corresponded to 28% and 14% of the total diffuse, and 38% and 18% of the anthropogenic diffuse losses, respectively. These reductions are mainly attributable due to a decrease in agricultural land and partly to measures in agricultural practice. However, discharges from point sources (wastewater treatment plants and stormwater overflows) also have fallen substantially over the period. Over the Swiss part of the Rhine watershed, the international target of 50% reduction in the total inputs into surface waters of P and N was achieved for P (reduction of 51%), but not for N (reduction of 23%). Comparison of the calculated losses of P and N from all sources with the loads measured in the Rhine near Basel shows the calculations to be realistic.