Water and nitrogen transfer study through soils of a small agricultural water catchment

Water and nitrogen movements in a small watershed (32ha) located in the Belgian Lorraine (south-eastern Belgium) were monitored intensively between 1988 and 1991 to investigate at a daily time step the possibility of predicting water discharge rates and nitrate concentrations in a spring if one knows the surface agricultural practices in the catchment area. The watershed was equipped with instruments to monitor the various meteorological, pedological, agronomic and geohydrological parameters assumed to explain most of the behaviour of water and nitrogen fluxes. They were recorded at various levels, i.e., the surface, vadose zone, saturated zone, and watershed outlet. Field observations were made to cross-check fluxes at each level. This information was then used in simulation models (SOIL and SOILN) to describe the movement of water and nitrogen solutes in the vadose zone and with original programming that describes their movements in the groundwater down to the outlet. These models present many parameters, the most sensitive of which were subjected to Monte Carlo Analysis which confirmed the robustness of the approach. The performance of the comprehensive model is quite satisfactory, with a 10% error in nitrogen fluxes over a three-year period. It shows the limits of such an approach despite the particularly intense degree of observation.