5.2 Error estimate of the calculation result
The estimation of the contribution of reservoir evaporation to precipitation strongly depends on the exactness of parameters h (Relative humidity), T (Temperature), and dpv(Deuterium excess in precipitation). The dpvvalue was estimated employing the vapor-precipitation balance, while h and T of the reservoir water surface was substituted with the relative humidity and temperature of air recorded at the Xiying Reservoir. These parameters therefore have certain inevitable errors. Relative humidity (h) is the most critical parameter in the estimation.
Since only the influence of air mass from southwest direction (upwind station is Wushaoling) was considered (Table 4), the change off ev caused by the change of each parameter has a large range. Regarding the northwest directional air mass (upwind direction station being Yinchuan), northeast direction air mass (upwind direction station is Zhangye). This is mainly because the recycled water vapor activity varies greatly when the dominant air mass from different directions. When only the influence of the air mass from the northwest direction is considered, the variation of f ev (Contribution rate of local evaporation) caused by each parameter is close to the range. At this time, the recycled water vapor activity degree is larger, and the sensitivity to the change of each parameter is low. According to the effect of air mass from different directions,dpv is the main factor causing the error, followed by air temperature and relative humidity.
Table 4 Range of change in fev from the Xiying Reservoir to precipitating water vapor for a change in input parameters by ±5%