EVALUATION OF THE RZWQM FOR THE SIMULATION OF WATER AND NITRATE MOVEMENT IN LEVEL-BASIN, FERTIGATED MAIZE

This study concerns the evaluation of the root zone water quality mode l (RZWQM) to simulate the seasonal water and nitrate movement in a lev el basin irrigated corn field under three different nitrogen (N) ferti lizer treatments. The three N treatments, superimposed over a split ba sal dose applied before and at planting, were: a single broadcast appl ication of 150 kg N/ha as urea (100% amidic form), a single fertigatio n application of the same N as UAN (50% amidic, 25% ammonium and 25% n itrate) with the first irrigation, and multiple UAN fertigations with three irrigations. Certain variety-specific maize crop parameters in t he model were obtained by fitting these parameters to field data from the single fertigation treatment. The model was then evaluated on wate r and N results for the treatments. The model adequately simulated the water and nitrate transport for the season, with the seasonal average s of measured and predicted values differing by less than 5%. The most significant differences between measured and simulated water and nitr ate occurred near the soil surface (15 cm depth), mostly during the da ys when the soil was extremely wet following irrigations. With the soi l hydraulic properties estimated by simple means, the model tends to o verestimate downward water fluxes and related nitrate transport throug h a compacted layer; however, it is found to be a useful tool to study the relative impacts of alternate nitrogen fertilizer and irrigation practices on root zone water quality. (C) 1998 Silsoe Research Institu te.