Trends in normalized difference vegetation index time series in differently regulated cascade reservoirs in Wujiang catchment, China

Dam construction is economically and ecologically important for power generation and water capture. However, the effects of cascade dams with various regulation types (i.e., daily, weekly, seasonal, yearly) on vegetation coverage are still need to be clarified. In the current study, we investigated the effects of dam construction on vegetation coverage and predicted the dynamic trends of vegetation at the sub-catchment (i.e., the catchment removing the overlapping catchment of the upstream reservoir) and buffer scale of each reservoir. MODIS Terra normalized difference vegetation index (NDVI) composites (MOD13Q1, 2000-2019) were used to study spatial and temporal changes in riparian vegetation coverage. At the sub-catchment scale, the long-term mean NDVI increased significantly from upstream to downstream from 2000 to 2019, and the NDVI mutation point (i.e., turning point) was consistent with the construction year of the corresponding dam. At the temporal scale, the vegetation trend predictions indicated a greater degree of degradation (23 %-61 %) than in the past (2000-2019; 3 %-32 %). Our findings also showed that reservoir vegetation degradation was associated with regulation type, i.e., daily reservoirs experienced a higher degree of vegetation degradation. Each cascade reservoir exhibited higher past vegetation degradation within the 0-1 km buffer zone compared to the sub-catchment. However, future degradation was predicted to be much higher in the sub-catchment than in the 0-1 km buffer zone, indicating the potential impact of damming on vegetation from adjacent areas (i.e., 0-1 km buffer) with gradual expansion to more distant areas (i.e., sub-catchment) over time. A potential cascade effect of upstream dams on sub-catchment vegetation of downstream dams was also observed. This study provides a reference for watershed management and ecological civilization construction. Relevant laws and policies should be established in areas surrounding reservoirs to help mitigate dam-induced degradation of vegetation communities, especially in res-ervoirs with short retention times.