4.2.3 Actual Evaporation changes in the sub–basins
During the flood season, the detention of runoff due to the constructed reservoirs resulted in an increase in evaporation in the basin, particularly during change period I. This increase in evaporation was attributed to the larger surface area covered by water in the basin, the abundance of water sources, and the growth of vegetation resulting from improved water and soil conservation measures. Furthermore, the rates of evaporation in the middle and lower reservoirs were higher than those in the upper reservoirs, which is consistent with the temperature changes in the basin. The temperature increased faster in the lower reaches of the basin than in the upper reaches. During change period II, although the overall evaporation in the basin decreased compared to the baseline period, the conversion ratio of precipitation to evaporation increased from 0.37 to 0.44. Upon comparison, the evaporation rates in the entire basin and the sub-basin where the reservoir was located were found to be similar. However, the average E/P (E/P=Evaporation/Precipitation) of the sub-basin where the reservoir was located was higher than that of the entire basin. The evaporation associated with the No. 5 Reservoir was lower than the average value for the basin due to human activities, such as the conversion of cultivated land and grassland to construction land in the lower reaches of the basin. The limited forest land cover in the area further explains the lower evaporation rate.
A comparison of the five sub-basins containing reservoirs revealed that the upstream area had higher evaporation rates than the downstream area, despite the opposite trend in temperature and precipitation. This finding suggests that the water and soil conservation measures were more effective in the upstream area, and the transpiration capacity of vegetation played a significant role in enhancing the changes in evaporation. Moreover, the evaporation in the sub-basins where the reservoirs were located was better than that of the entire basin. Figure 7 depicts the simulation results of evaporation for a reservoir-controlled sub-basin.The detention of runoff due to the constructed reservoirs resulted in an increase in evaporation rates in the basin, particularly during change period I. Although the overall evaporation decreased during change period II, the conversion ratio of precipitation to evaporation increased. The sub-basin where the reservoir was located had higher average E/P than the entire basin, and the evaporation rates were higher in the upstream area than in the downstream area. The water and soil conservation measures played a crucial role in enhancing the changes in evaporation.