4.2.4 Changes in NDVI in sub-basins
Starting from the 1970s, the Qinhe River Basin has implemented large-scale water and soil conservation measures, with a focus on afforestation of hillsides, creating forests for water and soil conservation, and transforming residual forests. In this study, the NDVI data of the Qinhe River Basin during change periods I and II were analyzed. The average NDVI of the basin increased by 5.36%, from 0.56 to 0.59. However, the average increase rate of the sub-basin where the reservoir is located was 6.77%. The NDVI growth rate of the upstream No. 1 reservoir was lower than the average value of the basin, as it is located upstream and is more affected by natural factors. Due to the good initial vegetation coverage, the growth potential in this sub-basin is low. On the other hand, the NDVI growth rate of the downstream No. 5 Reservoir was the same as the average value of the whole basin. The NDVI growth rates of the No. 2, No. 3, and No. 4 Reservoirs were 6.78%, 10.53%, and 9.62%, respectively. The sub-basin where the downstream No. 5 Reservoir is located is severely disturbed by human activities, and the land types are frequently transformed. This explains the lack of evident NDVI growth in this area.
Field investigations have revealed that to maintain the flood detention capacity of the reservoir and protect the safety of the dam site, the local government has paid more attention to vegetation restoration and water and soil conservation measures in the upstream area of the reservoir. This has partly contributed to the higher NDVI growth of the sub-basins containing reservoirs than the average value of the basin during the same period. Figure 8 illustrates the NDVI changes in the sub-basins containing reservoirs.