Scenario analysis of nutrient management at the river basin scale

A new river basin model (TRANS) for studying the transport, removal and accumulation of nutrients in rivers, lakes and riparian areas has been developed and tested on data from a 115 km(2) river basin in Denmark (river Gjern). The model combines catchment information on soil type and land use with a physical hydrodynamic modelling system and several semi-dynamic empirical models on diffuse nutrient loading and nutrient retention in rivers, lakes and riparian areas. Three main river basin management scenarios were defined for this basin and the environmental effects analysed using the model: 1. Improved sewage treatment at sewage works; 2. 20% set-aside of arable land; 3. river system rehabilitation. Combining the three scenarios in an overall river basin management strategy reduced annual total nitrogen export from the river basin by 93 tonnes, corresponding to a 53% reduction in nitrogen export during the hydrological year 1994-95. Similarly, annual total phosphorus export was reduced by 2.6 tonnes corresponding to a 46% reduction in phosphorus export.