1 Introduction
Due to the equilibrium and kinetic fractionation of stable hydrogen and oxygen isotopes in water from different sources during the hydrological cycle process, stable hydrogen and oxygen isotopes will exist in different proportions in the natural water body, different isotopic composition characteristics will be embodied (IAEA, 1983; Mook, 2000; Gat, 1996). In recent years, the stable isotope technique has been widely used in tracer basin hydrological process and lake water volume change and balance process; these tasks are difficult to achieve through traditional hydrological methods (Liu et al., 2009; Cui et al., 2016) . Evapotranspiration, an important part in the hydrological cycle process, is the aspect most directly affected by climate change and is the main water expenditure item. Terrestrial moisture produced by evapotranspiration is called recycled moisture. Some researchers have divided local water vapor sources into advection water vapor, surface water evaporation and vegetation transpiration, and have used the three-end-member linear mixing model to estimate the contribution rate of each of these sources (Peng et al., 2011; Wang et al., 2016; Li et al., 2016). But in arid areas with sparse vegetation or lakes with a large water surface area, the effect of vegetation transpiration is mostly excluded since it is considered that the precipitating water vapor above the lake is mainly composed of the advection water vapor and lake evaporation, and the contribution rate of each source is estimated by using the linear mixing model with deuterium excess parameter(Gat and Matsui, 1991; Gat et al, 1994; Xu et al., 2011; Kong et al., 2013; Aemisegger et al., 2014). For a lake in a relatively stable state, the ratio of evaporation to inflow can be determined by using the method of stable isotopic mass equilibrium, and the evaporation situation of external water inflow can also be determined (Cui et al., 2016; Dinçer, 1968; Zuber, 1983; Gonfiantini, 1986; Gibson and Edwards, 2002; Gibson et al., 2002; Liu et al., 2009).
The construction of reservoirs in the Basin under discussion here will increase the total water area, which will lead to a change in the water and heat conditions between the land and the water, and the transfer and exchange of water vapor and heat between the earth and atmosphere. And local aerodynamic characteristics will also be changed. All these changes will have a certain impact on the local climate of the reservoir and its surrounding area (Straškraba et al., 1993). The reservoir contributes water vapor to the atmosphere by evaporation, and this process has an important impact on the regional climate. The Xiying Reservoir is a typical reservoir in the Eastern Qilian Mountains situated on the northeast edge of the Qinghai-Tibet Plateau and serves as the main water supply for production and livelihood for the Wuwei and the Minqing Oasis. The local atmospheric circulation system is diverse, and the local hydrological cycle process is relatively unique. The isotope technique has been widely used in Basin hydrology in recent years; the influence of strong evaporation background on water bodies such as lakes, reservoirs and wetlands in arid areas has often been neglected in the relevant research undertakings.
The Xiying Reservoir was selected as the case to carry out an analytical research based on the stable isotope technique to determine the effect of evaporation on the reservoir water and the precipitation of the around reservoir. The research results can provide reference and can be a source of accumulated raw data for the lake water balance and hydrological cycle in this area. It also has the potential to improve the level of local water resources management and utilization.