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

An irrigation scheduling model (SimISP, Simulating Irrigation Scheduling for Potatoes) was developed to determine the irrigation requirements of potato crop. The model uses daily radiation, temperature, humidity, wind speed and precipitation and crop and soil parameters to estimate, evaporation (E), transpiration (Tr) and crop growth. Dry matter accumulation is based on transpiration and atmospheric vapour pressure deficit using a water-use-efficiency coefficient; and tuber yield on a proportioning of dry matter accumulation to leaves, stems and tubers throughout the season. Model output (the number of irrigations required, E, Tr and tuber dry matter yield) was tested for sensitivity to changes in four model parameters: the minimum stomatal resistance, the soil water potentials used to trigger irrigation and to limit transpiration, and the maximum allowable daily transpiration. Outputs were most sensitive to changes in the minimum stomatal resistance.