Abstract: After the national decision to conduct the unified water resource regulation in 2000, the ecological environment in the middle and lower reaches of the Heihe River is improved. However, an objective and impartial ecological environment evaluation method is lacked. Because of the large area and little data in the middle and lower reaches of the Heihe River, the method to calculate vegetation fractional coverage (VFC) by extracting normalized difference vegetation index (NDVI) during growing season from satellite remote sensing images is proposed, and the spatial distribution, area proportion and trend of VFC are analysed. This research finds that there is a transformation from low-cover vegetation to other types of vegetation in the middle and lower reaches of the Heihe River during 2000 and 2017. Besides that, the area of high-cover vegetation increases and the overall vegetation coverage is good. The conclusion is consistent with the ecological environment change in the middle and lower reaches of the Heihe River, which proves that the proposed method is reliable to assess the regional ecological environment.

Key words: satellite remote sensing, middle and lower reaches of the Heihe River, dimidiate pixel model, vegetation fractional coverage, normalized difference vegetation index