4.2.1 Runoff depth changes in the sub–basins
During the flood season in the Qinhe River Basin, runoff decreased continuously in all three periods, with a remarkable decline from 8.51 to 5.92 mm, which represents a decrease of more than 30%. This reduction occurred rapidly and can be attributed to various factors. Analysis of the sub-basins hosting the five studied reservoirs during the baseline period indicated that the runoff and R/P (Runoff/Precipitation) values were higher than the corresponding average values for the basin. In change period I, the average R/P value for the sub-basins that host the five reservoirs was 0.58, with the relatively low value observed in the sub-basin containing the No. 4 Reservoir attributed to low precipitation. In change period II, the average runoff and R/P for the five sub-basins hosting the studied reservoirs were 7.14 mm and 0.59, respectively. However, the total basin recorded 5.92 mm and 0.47 mm for runoff and R/P, respectively.
These findings indicate that, under the background of reduced precipitation, the basin’s runoff has decreased to varying degrees. Nevertheless, the sub-basin where the reservoir is located has experienced a significantly lower decrease in runoff than the basin’s average. Figure 5 illustrates the simulation of runoff for a typical sub-basin hosting a reservoir. In summary, the study reveals a continuous decline in runoff during the flood season in the Qinhe River Basin, with various factors contributing to the reduction. Furthermore, the sub-basins hosting the five studied reservoirs have experienced relatively lower decreases in runoff than the basin’s average, suggesting that the construction of reservoirs has contributed to retaining water in these sub-basins during periods of reduced precipitation.