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