Data of natural vegetation coverage in Shiyang River basin in 2008 was obtained by statistical analysis on remote sensing imageries using ArcGIS 9.0 software, then the ecological water requirement (EWR) of natural vegetation in each county in the study area was evaluated by phreatic evaporation method, and the prediction of the EWR in the future was made on the basis of Key Management Program of Shiyang River basin. Results showed that the minimum EWR of natural vegetation was 195.36 million m3 presently in Shiyang River basin, and the maximum one was 634.25 million m3. EWR of natural vegetation ranged from 68.55 to 251.41 million m3 in Yongchang County, from 46.93 to 131.35 million m3 in Liangzhou District, from 41.52 to 123.91 million m3 in Gulang County, from 37.43 to 123.45 million m3 in Minqin County and from 0.93 to 3.23 million m3 in Jinchuan District. It was predicted that the lowest EWR of natural vegetation and the highest one would be 214.95 and 698.11 million m3 respectively in Shiyang River basin in 2015, and the lowest one and the highest one would be 236.44 and 767.92 million m3 respectively in 2020.

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