MODELING SOIL-WATER BALANCE OF A MAIZE-SORGHUM SEQUENCE

Modeling plant-soil-water interactions for long periods of time is imp ortant for crop management strategies. Because it is difficult to asse ss soil hydraulic properties, mechanistic models are rarely used for s imulating soil water balance. Models less complicated with respect to soil water now are more often used. Although simple water balance mode ls are easy to use, however, they do not always provide good approxima tions of plant transpiration and soil-water status. Our objectives wer e to test the ability of a model, mechanistic,vith respect to soil-wat er now and empirical for soil-plant and plant-atmosphere interactions, to predict soil-water balance components for long periods of time whe n input parameters are measured or estimated independently. A data set gathered in Nicaragua during several months was used for this purpose . Soil hydraulic properties were measured independently and parameters taken from the literature were used for plant processes modeling. The model predicted reasonably well the soil-water balance for a maize (Z ea mays L.)-sorghum [Sorghum bicolor (L.) Merr.] sequence and for a gr ass sod. Periods of crop water stress were also correctly reproduced. Adjusting some plant parameters slightly increased the goodness of the predictions. Results indicated that plant water uptake was not well p redicted when topsoil layers were dry and deep soil layers wet. In add ition, the model predicted the upward water flux into the root zone an d its important contribution to plant water uptake. During drought per iods, the upward water nux reached 2 mm d(-1) while the actual evapotr anspiration of the crop was between 2 and 4 mm d(-1).