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Sustainability of irrigated agriculture depends primarily on the efficient management of irrigation water. Understanding crop evapotranspiration (ETc) is essential in planning the most effective use of water resources in the arid northwest China. The objective of this study is to analyze the variation of water requirement in the middle reaches of Heihe River basin. First, reference crop evapotranspiration (ET0) was calculated using eight different models including Penman-Monteith, Priestley-Taylor, Blaney-Criddle, Hargreaves, FAO-Radiation, Irmark-Allen, FAO-Penman and Penman with daily data obtained from 14 meteorological stations in and around the study area. Secondly, the crop coefficients (K-c) of spring wheat in different growing stages were adjusted according to the investigation data. Finally, the water requirement of spring wheat was estimated in the middle reaches of Heihe River basin. From the study, the results show that ETc varies from 544 mm to 769 mm during the entire growing season, the ETc in Gaotai, Zhangye, Linze, Minle and Shandan is 631.28, 660.90, 630.02, 682.66, 722.42 and 690.88 mm, respectively, the ETc of mid-season stage is the largest percentage during the entire growing season, accounting for 60.8%, 77.5%, 62.1%, 78.3% and 61.5% of total ETc at five meteorological stations, respectively.