5.1 Impact of reservoir evaporation on precipitation.
The precipitation around the Xiying Reservoir is characterized by the
mixing characteristics of the reservoir evaporation water and the
stratospheric transport water. The calculation results of the
contribution rate of the recycled water vapor at each site of the
Shiyang River show that the contribution of the ground evaporation rate
around the Xiying Reservoir is more than 23% (the ground evaporation is
more than 9%, vegetation transpiration is more than 15%) , which is
the highest in the whole Basin (Zhu et al., 2019). Many studies have
confirmed that large lake or reservoirs will increase the proportion of
water vapour recirculation under certain weather conditions, with a
higher recirculation rate of water vapour often being due to the
contribution of large open water bodies (Edwards, 2002; Bowen et al.,
2012). The Xiyingwugou site of 4.4 km from the Xiying Reservoir also
clearly displays the characteristics of the mixed water of the reservoir
and the transport water of the stratosphere. However, at the Huajia Town
sampling point 20.48 km in the south of of Xiying Reservoir, the mixed
characteristics of the reservoir evaporating water and the stratospheric
transport water are greatly weakened. Furthermore, there are no mixed
characteristics of reservoir evaporating water and stratospheric
transport water in the Xiying Town sampling point 18.46 km to the north
and at the Huling sampling point 38.22 km in the south of Xiying
Reservoir. Local airflow is mainly from a low altitude to high altitude
in the Xiying River Basin (Pan, 2019) and the influence range of
reservoir evaporation is strongly influenced by the local water vapor
movement path. Therefore, the evaporation at the Xiying Reservoir has a
greater impact on high-altitude precipitation.
Ressarch in different regions of the world demonstrates that lake
evaporation can increase the contribution rate of local precipitation.
The contribution of lake evaporation to atmospheric precipitation is
4.6-15.7% in the Great Lakes of North America (Gat et al., 2013)
(Fig.4). Lake evaporation losses are generally greater than the 25%
experienced in the Yinchuan Plain of China (Qian et al., 2013) (Fig.4).
The annual contribution rate of evaporation to precipitation is 23.42%
at the Qinghai Lake of China (Cui and Li, 2015) (Fig.4). Although the
mountainous reservoirs generally have a small surface area, the
influence range and contribution of water vapor evaporation remains
limited. However, almost every mountain runoff has a built reservoir,
and its comprehensive impact cannot be ignored in the arid regions. This
effect may greatly increase the amount of precipitation in the Basin,
thus increasing the accumulation of water resources in the high-altitude
areas of the Basin.