WATER QUALITY MODELLING OF AN AGRICULTURAL FIELD: FROM SOIL SURFACE TO SURFACE WATERS

摘要

Environmental problems related to agriculture is an important issue, and this since several decades. Leaching of nitrate from the soil to groundwater and surface waters became a major problem, especially in Belgium and The Netherlands. The fate of nitrogen in the soil is of major concern because of the potential hazard that nitrogen, applied in excess of the natural decomposing capacity of the soil, contaminates shallow and deep aquifers. For the prediction of the nitrogen behaviour in soils and subsurface drainage systems simulation models are used in combination with field measurements. In this study the transport and fate of nitrate-nitrogen within the soil profile and the leaching to the subsurface drains were analysed by comparing historic field data with the simulation results of three mathematical models, the WAVE, SWMS_2D and DRAINMOD models, with application to two case studies. In the first case study, the WAVE and DRAINMOD models were calibrated using experimental data of the Hooibeekhoeve, situated in the Kempen (Belgium), and collected in the period 1992 to 1995. In the second study, the water flow and nitrate transport to a subsurface drain, using a simplified and detailed model, the SWMS_2D and DRAINMOD models, are simulated for the specific hydro-geological conditions of the Lovinkhoeve, The Netherlands. For the calibration of both models use was made of the data of an in-situ leaching experiment, conducted in 1991-1992. Thereafter, the calibrated models were used to simulate the nitrate leaching for the period 1992-1995. Comparison between measured and simulated state variables indicated that the nitrate-nitrogen content in the soil and the nitrate-nitrogen leaching are primarily controlled by the fertiliser practice, the initial soil conditions and the rainfall depth and distribution. Furthermore, the research revealed that the models used give a fair description of the nitrogen dynamics in the profile rootzone and subsurface drainage system. From the comparative analysis, between experimental data and simulation results, it can also be concluded that the models after calibration are useful tools to optimise as a function of the combination 'climate-crop-soil-bottom boundary condition' the nitrogen application strategy, i.e. the derivation of that package that results into a maximisation of crop yield and minimisation of nitrate leaching.