4 Results
4.1 Characteristics of stable isotope composition of water bodies in the Xiying Reservoir during summer
The deuterium excess value of precipitation in the Xiying Reservoir is much higher than in the Wushao Mountains, Yinchuan and Zhangye (Fig. 2). The deuterium excess value of the surface water of theXiying Reservoir and the runoff of the Xiying River is also higher (Fig. 2). The deuterium excess value of the precipitation in the upper wind direction stations (Wushao Mountains, Zhangye, Yinchuan) reflects the characteristics of more moisture and less evaporation. The deuterium excess value of the precipitation in the Xiying Reservoir shows that the low moisture humidity, fast evaporation and water vapor experience a strong imbalance in evaporation. Therefore, the surface water with a high deuterium excess in the Xiying Reservoir has entered the precipitating water vapor. The phenomenon of d -excess in summer precipitation around the Xiying Reservoir is the result of the mixture of reservoir evaporation and advected water vapor.
Using the measured data of δ D and theδ 18O, the Surface Water Line equation of the Xiying Reservoir was obtained (Fig.3),δ D=7.14×δ 18O+9.15, R2=0.95. Furthermore, applying the Local Meteoric Water Line equation asδ D=7.54×δ 18O+11.74, R2=0.96 reveals that the surface water points all fall near the LMWL (Fig.3), indicating that the surface water is recharged by atmospheric precipitation. However, due to the open surface of the reservoir and the slow velocity of the reservoir water, the surface water is affected ya certain of non-equilibrium evaporation. Therefore, the slope and intercept of the SWL are smaller than that of the LMWL.
In the Xiying Reservoir, δ 18Opv and δ Dpv in summer are -15.24 ‰, -106.20 ‰ and 15.75 ‰, respectively (Table 2). It can be seen therefore that the value is obviously higher than that of other sampling sites and lies between deuterium excess of local precipitation and surface water, it also reflects the mixing of lake evaporation and advection water vapor over the Xiying Reservoir in summer.
Table 2. Estimation of isotopic values of precipitation
Fig. 2The track of air mass in the Xiying Reservoir in summer
Fig. 3 Relationship between δD and δ180 in precipitation and surface water of the Xiying Reservoir
4. 2 The path of air mass affecting the Xiying Reservoir in summer
The air masses affecting the Xiying Reservoir mainly come from the northwest, southeast and northeast (Fig.2). In June the air masses from the northwest mainly come from the southern foot of the Kunlun Mountains, the eastern section of the Tianshan Mountains and the arid area of Central Asia. The northeast air masses mainly come from the western Mongolia Plateau. The southeastern air masses come from the Sichuan Basin, the Guanzhong Plain, the Qinling Mountains and other places. In July, the air masses northwest mainly come from the eastern part of the Tianshan Mountains. The northeast air masses come primarily from the Mongolian Plateau and the Loess Plateaus. The southeast air masses come mainly from the Sichuan Basin, the Guanzhong Plain and the Qinling Mountains, but at least one of these masses can extend to the northern margin of the Yunnan-Guizhou Plateau. In August, the air masses in the northwest come from the eastern part of the Tianshan Mountains and the central Mongolian Plateau while the northeast air masses mainly come from the Loess Plateau and the eastern part of Mongolia Plateau. The air masses in the southeast mainly come from the Sichuan Basin, the Guanzhong Plain and the Qinling Mountains. According to result of the cluster analysis, the tracks of the two air masses in the southeast direction account for 51.1%, the air masses in the northwest area account for 23.9%, and the air masses in the northeast direction account for about 25% of the total number. The movement distance of westward air masses is longer, but the continental character is more obvious, and the nature of the air mass is dry. The movement distance of the air masses in the southeast direction is wetter.
In this research, the precipitation stable isotope data at the sampling site on the path of air mass movement was used to deduce the isotope value of the advected water vapor. Thereafter the contribution rate of reservoir evaporation in the research area was estimated (Table 1).
4.3 Contribution rate of the Xiyingreservoir evaporation toreservoir and reservoir around precipitation
The influence of three air mass directions of external water vapor transport on the Xiying Reservoir is now considered here respectively. The precipitation water vapor isotope data is estimated based on the precipitation isotope data of three sampling sites (equation 4), and the isotope values of advected water vapor are estimated (equation 7). The isotope value of the evaporation water vapor is estimated based the isotope value of the advected water vapor and surface water (equation 6). It was estimated that the average contribution rate of the Xiying reservoir evaporation is arouond 3.86%~11.86% in summer (Fig.4, Table 3). The contribution rate of reservoir evaporation to precipitation is relatively low when the air mass comes from the eastern direction, which is 3.86% and 6.76%, respectively. This is mainly due to the relatively abundant water vapor content in the eastern air mass, resulting in continuous precipitation and insufficient mixing with locally evaporated water vapor. When affected by an air mass from the west, the contribution rate of the recycled water vapor is 11.86%. This is mainly due to the relatively low water vapor content, which leads to the existence of adequate mixing with the local evaporation water vapor.
Table 3. The isotope value of advection water vapor, the evaporating water vapor, and the contribution rate of reservoir evaporation
Fig. 4 Contribution rate of surface water evaporation in each research area
4.4Xiying Reservoir evaporation to inflow ratios in summer
According to the stable isotopic model of lake water balance, the ratio of runoff evaporation in the Xiying Reservoir is estimated to be 4.39%, while the remaining 95.61% of the water is stored in the Xiying Reservoir or continues to be injected into the middle and lower reaches of the Shiyang River.
The daily runoff at the entrance of the reservoir in summer is within 1×106m3~2×106m3, the inflow runoff is closely related to the precipitation events in the upper reaches of the mountainous area; there are several peaks in summer (Fig. 5). The total runoff in summer is 1.84×108m3. about 8.06×106 m3 of the inflow runoff is consumed by evaporation.
Figure 5. The daily and monthly potential evaporation of the Xiying Reservoir area, in summer