Estimation of the nutrient inputs into river systems - experiences from German rivers

The nutrient discharges from point and diffuse sources in more than 200 German river basins were estimated for the periods 1983-1987 and 1993-1997 employing the MONERIS model. This model distinguishes between six diffuse pathways and point source emissions from waste water treatment plants and direct industrial discharges. It was estimated that the total nitrogen input into the German river systems amounts to about 819,000 t N year(-1) in the period 1993 to 1997. These emissions have decreased since the mid-eighties by about 266,000 t N year(-1), mainly caused by the reduction of point discharges. For phosphorus the emissions have been reduced by 56,290 t P year(-1) and amount to 37,250 t P year(-1) in the period 1993-1997. Based on emission data a retention module estimates riverine nutrient loads. The comparison of the model output with the observed loads shows a deviation as low as 30% and 50% for nitrogen and phosphorus, respectively. The regional resolution of the model indicates the relative importance of different pathways for phosphorus and nitrogen input into river systems.