MODELING WATER-BALANCE AND GRASS PRODUCTION

The previously developed soil water-crop production model SWA-CROP was adapted and tested for grassland production under the weather conditions and management practices encountered on sandy soils in the Netherlands. The model, which required soil physical and crop properties as input, was driven by meteorological data. The main output of the soil water balance module (SWATRE) included actual transpiration and the distribution of soil water contents and pressure heads over depth and time. The crop production module (CROPR) calculated dry-matter yields of roots, stubble, and harvestable sward. The CROPR module was also used to calculate leaf-area index and soil cover, which formed feedback loops with the soil water balance. After initial calibration, 85% of the predicted soil water contents in the root zone fell within 10% of the measured data. Seasonal accumulated grass yield, predicted by the model under nonstressed conditions, was within 10% of the measured yield, but yields during dry periods were overestimated. Discrepancies between measured and predicted yields under irrigated and nonirrigated conditions were attributed to the model's inability to simulate growth and death rates of individual plant organs.