Application of a conceptual catchment scale nitrate transport model on two rural river basins

A conceptual catchment scale model for simulating nitrate transport in rural river basins has been developed. The model consists of a hydrologi- cal part based on the TOPMODEL and a nitrate-leaching module. This module consists of two functions: the SLIM-concept controlling the release of nitrate from the soil root zone and second, a transfer function based on the 'flushing hypothesis' controlling the appearance of nitrate in the stream channel network. The SLIM-concept divides the soil into layers with mobile and immobile categories of water and dissolved solids. Movement of water and dissolved solids between the different layers occurs in response to effective rainfall. The transfer function defines a proportional relation between the decline in catchment mean groundwater table and the nitrate concentrations in the stream channel network. Comparison with field data shows reliable simulation results for two different rural catchments, although improvements can be expected when topographical data and field data on the nitrate content in the soil profile at the onset of winter drainage are taken into account.