Water in a main controlling factor for economic development and eco-environmental protection in the inland river basin of arid northwest China. In order to reduce the environmental problems of oasis induced by water resources management and use, this paper proposes a concept of ecological water requirements (EWR). It is defined as three grades for EWR, including critical EWR, optimal EWR and saturated EWR. Critical EWR are the Mater regimes needed to maintain vegetation survival at a lowest level of risk Optimal EWR are the water regimes needed to maintain vegetation growth and ecological values of oasis ecosystem. Saturated EWR are that water is enough and is not a controlling factor for vegetation growth. Based on the definition, the methods for calculating the amount of ecological water requirements are determined, which is that the theory of water balance, meteorological datum, data of soil moisture content are employed to calculate ecological water requirements and analyze ecological water shortage. In the case study of the middle reaches of Heine River, the water requirements are divided into three parts, including arbor forest, shrub forest and grassland. The results of calculation show that the critical EWR in the region is 1.22×10{sup}8m{sup}3year{sup}(-1), optimal EWR is 2.37x10{sup}8～2.89x10{sup}8 m{sup}3year{sup}(-1) and saturated EWR is 6.53×10{sup}8～8.00×10{sup}8 m{sup}3 year{sup}(-1), availability precipitation of corresponding areas is 0.44×10{sup}8 m{sup}3 year{sup}(-1). The water requirements of arbor forest, shrub forest and grassland account for 60%, 8% and 32% of the amount of total ecological water requirements, respectively. Furthermore, in the process of water resource adjusting, it was necessary that not only consider spatial heterogeneity but also pay attention to season variability of ecological water requirement. This paper show that ecological water shortages of April, May and June are the most serious in the middle reaches of Heihe River.
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