Water supply scarcity is the main limiting factor for sustainable agriculture development in semiarid areas on the Western Loess Plateau. Because of imbalance for rainfall distribution, yield and water use efficiency (WUE) of potato, one of the dominant crops in middle Gansu province, are relatively low. To optimize supplemental irrigation system using limited catchment rainfall for rain-fed potato in the area, field experiments were carried out in 2008-2009 to determine evapotranspiration, WUE, yield, yield traits and quality of rain-fed potato under different supplemental irrigation amounts and timings. The results showed that WUE of potato increased significantly in the supplemental irrigation treatment of 45 mm, and decreased with increasing of irrigation amount. WUE and irrigating water use efficiency (IWUE) of potato were higher with supplemental irrigation at the seedling than the tuber expanding stage. WUE and IWUE were the highest under the 45 mm of irrigation at the seedling stage. Yield, rates of bigger tuber and middle tuber in rain-fed potato were increased, and small tuber rates were decreased by supplemental irrigation. Big tuber rates were increased by supplemental irrigation at the seedling stage, while medium tuber rates were increased by supplemental irrigation at the tuber expanding period. Potato numbers per plant were decreased, but tuber yield per plant was increased by supplemental irrigation. With increasing of irrigation amount, the yield of potato was not increased significantly, but green tuber rate and blet tuber rate were increased significantly. Starch content of potato was reduced, while protein content of tuber was increased by supplemental irrigation. Supplemental irrigation of 45 mm at seedling is optimal irrigation system for rain-fed potato in the area.
