As the largest tributary of the Shiyang River, with the average annual inflow of total runoff accounting for 23%, the Xiying River has representative of mountain runoff of inland rivers in the Northwest of China. Using samples collected in this basin from September 2016 to October 2017, the water chemical composition and ion source characteristics of river were studied. The results show that the river is weakly alkaline, the average pH is 8.0 and the TDS is 179.29 mg·L-1. With the elevation decreasing along the river, the TDS of main stream tend to increase firstly and then decrease, but those of TDS of each tributary decrease, and latter is lower than the former. Affected significantly by the flow, the lowest value of ion concentration in river occurs in summer, and the highest occurs in autumn and winter. The hydrochemical type of river is CaMg-HCO3. In the river, the order of cation mass concentration is NH4+<K+<Na+<Mg2+<Ca2+, and that of anion is F-<NO3-<Cl-<SO42-<HCO3-. The sources of ions in river are mainly from the weathering of Silicates and Carbonates. With the elevation decreasing along the river, the influence of Silicates on the inflowing tributaries is gradually strengthened.

The Shiyang River Basin is located at the edge of the monsoon wind system of South and Southeast Asia. The hydrochemical characteristics of precipitation are influenced by both monsoon and arid regions. The regression analysis method, comparative analysis, neutralization factor (NF), enrichment factor (EF) and HYSPLIT4 were used to analyze the precipitation samples collected from the upper reaches of the Shiyang River from October 2016 to October 2017. In order to study the hydrochemical characteristics and ion sources of precipitation in this basin. The results, as discussed in this paper, show that the precipitation in the upper reaches of the Shiyang River is mildly alkaline all year round while the neutralization ability of Ca2+ and NH4+ in precipitation is strong. The ion concentration was higher in the dry season than that in the wet season, but the concentration of NH4+ was higher in summer. Furthermore, as the altitude increased, the electrical conductivity (EC) of the precipitation decreased gradually. Influenced by precipitation and rainy days, the wet deposition of nitrogen (N) and sulfur (S) was higher in the wet season than that during the dry season, and the wet deposition gradually increased with the elevation. In precipitation, the earth’s crust is a major source of Ca2+, K+ and Mg2+, the ocean is a major source of Cl−, the ocean and the earth’s crust are the sources of Na+, human activities are the main sources of SO42−, NO3− and NH4+, the amount of F− is very small, its sources are natural and human activities. Water vapor in precipitation mainly comes from westerly air mass circulation and monsoon circulation while the particles come mainly from the earth’s crust.

Fractionation of stable isotopes in precipitation runs through the water cycle, and deuterium excess is a second-order parameter linking water-stable oxygen and hydrogen isotopes. It is strongly influenced by under-cloud evaporation in unsaturated air, especially in arid climates. Based on the improved Stewart model, this study used 670 precipitation stable isotope data and measured meteorological element data from 11 sampling points from January 2018 to September 2019 to verify the existence of sub-cloud secondary evaporation in the Shiyang river basin and quantitatively calculate the intensity of sub-cloud secondary evaporation and its influence on precipitation stable isotopes. The study used the vapor flux and the improved Lagrangian model to track the moisture source of precipitation and analyze the influence of the moisture source of different paths on the stable isotopes of precipitation. Therefore, this study is helpful to understand the evapotranspiration loss mechanism and recharge mechanism of moisture in the watershed. The results showed that there is sub-cloud secondary evaporation in the Shiyang River Basin, and from the seasonal scale, the sub-cloud secondary evaporation is stronger in spring and summer, but weaker in autumn and winter, which makes heavy isotopes enriched in spring and summer and depleted in autumn and winter. From the perspective of spatial distribution, the sub-cloud secondary evaporation is stronger in the midstream and downstream of the Shiyang river, resulting in more enrichment of heavy isotopes. In the vertical direction, the sub-cloud secondary evaporation at 850–700 hPa is the strongest, which enriches the heavy isotope in this layer and reduces the deuterium excess. In addition, the main moisture source of precipitation in the Shiyang River Basin is the westerly air mass, and the mid and high-latitude land sources contribute more moisture to the precipitation. However, the supply of the sea source is very limited, which makes the deuterium excess of precipitation higher and does not show regional consistency and seasonality well.