Vegetation response to 30 years hydropower cascade exploitation in upper stream of Yellow River

The accumulated response of vegetation successive dam constructions and operations is an important concern, but the systematic assessment of impacts induced by cascade hydropower exploitation over long periods are seriously lacking. Using remote sensing data, the variations in grassland, the principal land cover in the upper catchment of the Yellow River, were investigated for eight dams constructed during the period 1977-2006. Two different scales-watershed scale and on-site area-were used to compare the changes in grassland and water area. Correlation coefficients from regression analyses showed that grassland area had more significant interactions with hydropower exploitation indicators in on-site scale than in watershed scale. The hydropower exploitation indicators had a more complex correlation with water area in watershed scale than in on-site scale. Consequently, observations of grassland area responses to successive hydropower exploitations were focused on the on-site region. The Normalized Difference Vegetation Index (NDVI) and the standardized NDVI, which can be used to analyze inter-annual climatic differences, were applied to identify the most heavily influenced vegetation zones. For different hydrological and micro-climatic conditions, the vegetation zones around reservoirs and along the main stream of Yellow River were analyzed, respectively. Two NDVI spatial principles at varied distances from the water demonstrated that the vegetation NDVI was recovering from 1994 to 2006. For distance of less than 10 km from water, the vegetation around reservoirs was better as the higher NDVI in 2006 than in 1977. The inter-annual NDVI comparison demonstrated that the critically affected vegetation zone was concentrated at distances of 0.1-0.4 and 1-6 km from the water. In on-site region, the grassland was further analyzed with elevation and aspect information, which indicated that grassland in sunny aspects was much disturbed. Detailed information about grassland response with water distance and the degradation characteristics provide the comprehensive assessment by cascade hydropower exploitatio