Combining GIS and a Quasi-Two Dimensional Mechanistic Model to Assess the Nitrate Balance at Regional Scale

Fertilised cropland is a potential non-point source of nitrogen contributing to the nutrient enrichment of surface water ecosystems. Nitrogen is present in drainage outflow primarily because of the addition of fertilisers. The purpose of this study is to model the nitrate leaching to surface waters using a quasi twodimensional mechanistic flow model (DRAINMOD) in combination with a GIS. The GIS is used to describe for the catchment the spatial distribution of soil type and land use, and to present the simulated nitrate leaching over the catchment. This paper summarises the method used to model the nitrate leaching at field scale and how the GIS was applied to present the spatial distribution of nitrogen loss by natural and artificial drainage. The method was applied to the Witte Nete catchment in Belgium having a total catchment area of 40.7 km 2 . The study shows great advantages of the association between a GIS and a nitrate simulating model where spatial data management, taken care of by the GIS, is combined with temporal variable simulation, taken care of by the mechanistic model. Furthermore, the study illustrates that a GIS in combination with a mechanistic field scale model is a powerful and suitable tool for modelling and analysing nitrate leaching at regional scale. The resulting vulnerability map depicts the nitrogen leaching as a function of soil type and land use and gives the decision maker the opportunity to diversify the nitrogen leaching standards.