Accurately quantifying the evaporation loss of surface water is essential for regional water resources management, especially in arid and semi-arid areas where water resources are already scarce. The long-term monitoring of stable isotopes (δ18O and δ2H) in water can provide a sensitive indicator of water loss by evaporation. In this study, we obtained surface water samples of Shiyang River Basin from April to October between 2017 and 2019. The spatial and temporal characteristics of stable isotopes in surface water show the trend of enrichment in summer, depletion in spring, enrichment in deserts and depletion in mountains. The Surface Water Line (SWL) has been defined by the lines: δ2H=7.61δ18O+14.58 for mountainous area, δ2H=4.19δ18O-17.85 for oasis area, δ2H=4.08δ18O-18.92 for desert area. The slope of SWL shows a gradual decrease from mountain to desert, indicating that the evaporation of surface water is gradually increasing. The evaporation loss of stable isotopes in surface water is 24.82% for mountainous area, 32.19% for oasis area, and 70.98% for desert area, respectively. Temperature and air humidity are the main meteorological factors affecting the evaporation loss, and the construction of reservoirs and farmland irrigation are the main man-made factors affecting the evaporation loss.
Studies based on basins or regional scales often ignore the uniqueness of recycling moisture in mountain areas, and little effort has been made to understand the impact of the local recycled moisture on precipitation in mountain areas. We collected and analyzed a series of samples (stable isotope of precipitation, soil water, plant water, runoff, and groundwater) in the Qilian Mountains, northwest of China. Based on the isotopic mixing model, the characteristics of recycled moisture in the Qilian Mountains were assessed. The results showed that lateral advection moisture is the primary source of precipitation (83.5~98.38%). The contribution rate of recycled moisture to precipitation was higher in spring, summer, and autumn (2.05~16.5%), and lower in winter (1.62~3.32%). The contribution of recycled moisture to precipitation in the high-elevation areas (>2400m) was higher than that in the foothills area (2300~2100m). The contribution of vegetation evapotranspiration (fTr) to precipitation in the east of Qilian Mountain was higher than that of the land surface evaporation (fEv).
The dissolved organic carbon(DOC) content of rivers is the most active part of the carbon cycle migration in the basin under consideration, and it is the basis for a comprehensive understanding of the regional carbon cycle. In this study, we periodically took samples from four monitoring stations in the Xiying River Basin of the Qilian Mountains in the northern Qinghai-Tibet Plateau. We calculated the fluxes of organic carbon in the rivers within the study area and will discuss the influencing factors of Dissolved Organic Carbon concentration in these rivers in this paper. Our results showed that: (1) The DOC concentration and output flux in the inland river runoff area are basically the same as those in the Heihe River Basin, but far lower than those in the low-latitude monsoon climate zone and most of the basins in the Eurasian Arctic region. This is mainly due to the small river runoff and low DOC concentration in the area. (2) The Dissolved Organic Carbon concentration and transport flux of the rivers show significant seasonal changes, with the Dissolved Organic Carbon content in summer and autumn being higher than in winter and spring. (3) The larger runoff causes higher concentrations of dissolved organic carbon in rivers. Runoff is the primary means of carbon migration in the Inland River Basin. There are significant carbon migrations from the upstream to the middle and downstream sections of the Inland River Basin.
Due to the increase in industry, agricultural production and domestic water consumption, a common practice is to improve water use efficiency by building reservoirs. However, the construction of reservoirs has the effect of an increase in the evaporation area of the water surface. Observations show that due to the evaporation effect of the reservoir, the precipitation and precipitation distribution will change in some area. The Xiying Reservoir is a typical manmade mountain reservoir in the Qilian Mountains in the upper reaches of the Shiyang River Basin. Based on the data collected by the multi-water body stable isotope observation network in the Shiyang River Basin, the study under discussion here used the isotope mixture model to quantify the impact of reservoir evaporation in this region. It was found that the advected water vapor of the Xiying reservoir mainly comes from southeast, northeast and northwest. The d-excess value of precipitation around the Xiying reservoir is significantly higher than that of other regions in the Xiying River Basin, which is characterized by the mixing of water vapor generated by the evaporation of surface water and advected water vapor, indicating that the evaporation water vapor of the reservoir has a certain impact on local precipitation. It was calculated that 3.86-11.86% of the precipitation around the Xiying reservoir comes from the evaporation of reservoir water and that about 4.39% (8.06×106 m3) of the Xiying river water was consumed by evaporation. The background of atmospheric water vapor content is the main influencing factor responsible for reservoir evaporation. The local atmospheric movement determines the influence range of reservoir evaporation on precipitation.