Although water has the central function of the bloodstream in the biosphere espe- cially in arid or semi-arid regions such as Yan’an region in northwestern China, yet the very limited attention is paid to the role of the water-related processes in ecosystem. In this re- search, based on continuous nearly 50-year data including runoff volume, sediment discharge as well as sediment accretion from hydrographic stations, and 10-year information of water quality from pollution monitoring stations, the method for measuring in-stream flow require- ment has been put forward supported by experiential models and GIS spatial analysis. Addi- tionally, the changes of in-stream flow requirement for environment and economic develop- ment have been addressed from spatial-temporal dimensions. The results show that: (1) According to the central streams in Yan’an region, mean annual in-stream flow requirement reaches 1.0619 billion m3, and the surface water for economic exploitation is 0.2445 billion m3. (2) Mean annual in-stream flow requirement for sediment transfers in flood period occupies over 80% of the integrated volume in a year. (3) From the 1950s to 1970s, in-stream flow requirement for sediment transfers is comparatively higher, while from the 1980s to 1990s, this requirement presents a decreasing tendency.
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