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.