MODELING SOIL-WATER STATUS FOR IRRIGATION SCHEDULING IN POTATOES .2. VALIDATION

Scheduling irrigation for the potato crop requires information not pro vided by tensiometers and other soil water measuring devices. The mode l SimISP described by Singh et al. (part I, this issue) allows for inp ut of such specified parameters as minimum stomatal resistance. It was applied to irrigated and rainfed conditions at a farm site. where the soil properties, weekly soil water content, rainfall and irrigation a mounts were measured. Estimates of seasonal water use was less than 4% lower than measured at irrigated site for a 60 cm soil profile when t he estimated drainage was subtracted from measured rainfall and irriga tion. Under rainfed conditions the estimates of water use were higher than measured by about 10%. Estimates for individual weeks were less p recise due to inadequate accounting of drainage after irrigation or he avy rainfalls. The available soil water in the 60 cm profile and soil water content in the individual layers were estimated within the range of measurements more often for rainfed than irrigated conditions. Yie lds were underestimated in a year when soil water was plentiful early in the growing season with a shortage of rainfall late in the season a nd were overestimated in a year when conditions were the opposite. Thi s was apparently due to inadequate accounting of dry matter partitioni ng to the leaves, stems, roots and tubers.