Model based impact analysis of policy options aiming at reducing diffuse pollution by agricultureda case study for the river Ems and a sub-catchment of the Rhine*

In this paper an integration of the agricultural economic model RAUMIS with the hydrological models GROWA98 andWEKUis presented. The focus lies on an area wide, regionally differentiated, consistent link-up between the indicator ‘‘nitrogen balance surplus’’ and nitrogen charges into surface waters. The model network is used to analyze the status quo situation in the year 1999 for two river catchments in Germany that feature very distinct natural and socio-economic conditions. Regarding agriculture, the study areas include regions with specializations in cash crops, in intensive livestock featuring high nitrogen surplus, and extensive livestock production on permanent grassland. Due to regionally varying hydrological conditions quite different shares of agricultural nitrogen surpluses ranging from 25 to 92% enter surface waters. Furthermore, impacts of alternative nitrogen reduction measures namely a limitation of livestock density and a tax on mineral nitrogen are quantified. Measures of the nitrogen reduction potential and costs in terms of agricultural income forgone are taken into account in the assessment. Results regarding the effects of restricting the livestock density or tax mineral nitrogen highlight that the mitigation of diffuse water pollution problems requires regionally tailored measures.