MODELING SOIL-WATER STATUS FOR IRRIGATION SCHEDULING IN POTATOES .1. DESCRIPTION AND SENSITIVITY ANALYSIS

An irrigation scheduling model (SimISP, Simulating Irrigation Scheduli ng for Potatoes) was developed to determine the irrigation requirement s of potato crop. The model uses daily radiation, temperature, humidit y, wind speed and precipitation and crop and soil parameters to estima te, evaporation (E), transpiration (Tr) and crop growth. Dry matter ac cumulation is based on transpiration and atmospheric vapour pressure d eficit using a water-use-efficiency coefficient; and tuber yield on a proportioning of dry matter accumulation to leaves, stems and tubers t hroughout the season. Model output (the number of irrigations required , E, Tr and tuber dry matter yield) was tested for sensitivity to chan ges in four model parameters: the minimum stomatal resistance, the soi l water potentials used to trigger irrigation and to limit transpirati on, and the maximum allowable daily transpiration. Outputs were most s ensitive to changes in the minimum stomatal resistance.