optimal irrigation scheduling based on water-yield relations in soybean cultivars

water shortage in the agricultural sector is currently a major challenge in crop production. the more efficient water usage is that which increases yield without increasing water use. thus, a determination of the water-yield relationship is essential. a field study was conducted to examine water-yield relations in three genotypes of soybeans, including optimum irrigation depth, a comparison of deficit and full irrigation, and the yield response factor (ky) at gorgan research station in the 2005 and 2006 growing seasons. four irrigation treatments (w1, w2, w3 and w4) were identified according to their decline in irrigation caused by increased distance from the line source. testing was based on a strip plot design and examined the effect of fixed irrigation rates on three cultivar treatments (sahar, g3, dpx) with four replications. each irrigation treatment of each strip was divided into three cultivar treatments along the length of the laterals. results indicated that the grain yield was affected by irrigation treatment and cultivars. the highest grain yield was obtained by treatment w1 (3306 kg/ha) with the dpx cultivar (2905 kg/ha). under deficit irrigation scheduling, when effective rainfall (p) during the growing season is negligible, the optimum values for irrigation water (i) showed little difference between cultivars and were obtained at 430 mm. under full irrigation, the sahar, g3 and dpx cultivars required 437, 467 and 523 mm of irrigation water and 550, 580 and 640 mm of applied water (i+p), respectively. ky for sahar, g3 and dpx for the total growing season were 1.1, 1.06 and 0.92, respectively. the dpx cultivar had the lowest ky (0.92) and, therefore, is a more suitable response to water deficit than were the other cultivars.