Assessing the impacts of an ecological water diversion project on water consumption through high-resolution estimations of actual evapotranspiration in the downstream regions of the Heihe River Basin, China

Ecological Water Diversion Projects (EWDPs) have been implemented in several endorheic river basins in the arid region of northwest China since the beginning of the new millennium to restore the deteriorated ecosystems. However, the effects of these EWDPs are difficult to evaluate quantitatively. Here, we assessed changes in water use in Ejin Oasis in the downstream region of the Heihe River Basin, which is a typical endorheic river basin in northwest China, based on the estimated evapotranspiration (ET) during the growing season (May to October) in 2000 (before the EWDP) and 2014 (after the EWDP). The ET estimates were based on the modified surface energy balance algorithm for land (M-SEBAL) model and Landsat images. The estimated ET was validated against observations using eddy covariance towers installed on different landscapes in 2014 and was found to have a root mean square error of 1.20 mm day−1 and a coefficient of determination of 0.67 at the footprint scale on satellite overpass days. The estimated ETs in two years were also compared with another remote sensing product that showed a similar spatial pattern, with a spatial mean difference of 4 mm and 0.5 mm, respectively. The estimated ET was then used to evaluate the impact of the EWDP on water consumption. The ET over different land use and land cover types increased, with a mean increase of 52% over the 15 years of the implementation of the EWDP in Ejin Oasis. The water consumption in Ejin Oasis in 2014 was approximately twice that in 2000. Among the changes, water consumption by croplands increased significantly, with a maximum increase of 264% because of cropland expansion and increased ET. The increases in water consumption by forests and grasslands were 60% and 25%, respectively. The lake area expanded drastically (from 0 to 37.46 km2), and the corresponding water consumption caused by evaporation was zero in 2000 and approximately 3.9 × 107 m3 from May to October in 2014. This work demonstrates that estimates of the ET based on remote sensing can be used as reliable indicators for comprehensive assessments of the impacts of the EWDPs.