Did water diversion projects lead to sustainable ecological restoration in arid endorheic basins? Lessons from long-term changes of multiple ecosystem indicators in the lower Heihe River Basin.

Ecological water diversion projects (EWDPs) are crucial for dryland ecosystem restoration in arid endorheic basins. Comprehensive and in-depth evaluations of ecological effects and especially the potential risks of EWDPs on long-term ecosystem changes were limited in previous studies. In this study, spatio-temporal variations in multiple indicators (land use/cover, landscape pattern and vegetation cover) derived from Landsat images were integrated to detect ecosystem changes from 1990 to 2015 in the lower Heihe River Basin (HRB), China, where an EWDP was conducted by the Chinese government in 2000. The results showed that the EWDP prevented ecological degradation and effectively restored the arid oasis ecosystem. From 2000 to 2010, the transformation from unused land to natural vegetation and wetlands, was the major land use/cover change; the degree of landscape fragmentation decreased, and the regional normalized difference vegetation index (NDVI) increased significantly. However, the EWDP did not restore the ecosystem to the status in 1990, and some vegetation degradation occurred after 2010, with approximately 38% of the study area showing significant decrease in NDVI. Human interventions, especially overexpansion of cultivated oases, which restricted ecological water use, and construction of artificial canals, which reduced groundwater recharge, resulted in the browning trend from 2010 to 2015. Therefore, the EWDPs did not definitely lead to sustainable ecological restoration and ecosystem development. The quantification results can provide significant implications for water resources management and ecosystem planning to prevent ecological degradation in the study area. The multi-indicator method with long-term and high-resolution dataset used here can be adapted to other arid regions for comprehensively evaluating the ecological restoration ability of the EWDP.