Understanding the stable hydrogen and oxygen isotope composition of river water in the basin is of great significance for identifying the evolution mechanism of water cycle and the composition of water source. Based on the test results of δD and δ 18O of river water and precipitation samples collected from Shiyang River in August 2020, this paper analyzed the composition characteristics of δ 18O and δD of river water in Shiyang River Basin during rainy period, and further revealed the spatial evolution characteristics and influencing factors. The results show that the δ 18O and δD values of the main and tributaries of Shiyang River are gradually enriched along the river, showing an increasing trend, while the excess deuterium is decreasing along the river. The effect of δ 18O and δD in river water is significant, and the variation of δ 18O altitude gradient is smaller than that of local precipitation and global precipitation. Using the evaporation loss calculation model, it can be seen that evaporation fractionation has an obvious effect on the water isotope change of Shiyang River, and the main stream of Shiyang River has the greatest effect, followed by Jinta River and Zamu River. The evaporation of reservoir water is more intense than that of river water. Precipitation is the main source of river water, and the isotope is mainly affected by the isotopic composition of river water in tributaries, and is controlled by the elevation effect, the continuous evaporation effect along the direction of runoff, and the higher degree of evaporation effect in the rainy season.

The stability and safety of water environment are the foundation of agricultural development. The possibility of salinization and desertification in the oasis agricultural area is much higher than that in other areas, for the population density, lack of water resources and high salinity. Therefore, it is necessary to study the water environment of irrigation water in this area, so as to make a reasonable water resource utilization and protection plan. In the agricultural irrigation period (from Apr. to Sep.) and non-irrigation period (from Oct. to Mar. of the next year), there were 9 sampling points set up from the source area to the oasis of the middle and lower reaches in Shiyang River Basin. Evaluating the irrigation water quality of surface water by ion concentration, SAR and end-member mixing diagram. The results shown: (1) the dilution effect of precipitation has a decisive influence on the ion concentration of surface water in the watershed. Due to the overlapping of irrigation period and rainy season, rainfall dilution makes irrigation water quality better. (2) There are spatial differences in hydrochemical types. The upstream hydrochemical type is mainly Ca-HCO3 type. The hydrochemical type of the middle and lower reaches is Ca (Na) - HCO3 mixed type. The upstream surface water is very suitable for agricultural irrigation, and the middle and downstream oasis area is suitable. (3) Surface rock weathering and evaporation crystallization are the main factors affecting the hydrochemical characteristics of surface water. Due to changes in the underlying surface environment in a short time, it is unlikely that the water quality will deteriorate. (4) In recent years, with the increase in precipitation caused by climate change and the strict environmental protection policies, the risk of deterioration of irrigation water quality is greatly reduced, surface water may be more suitable for agricultural irrigation.

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 the Xiying River basin from September 2016 to October 2017, the water chemical composition and ion source characteristics of river was studied. The results show that the river is weakly alkaline, the average values of pH is 8.01 and the TDS is 179.29 mg·L-1. With the elevation decreasing along the river, the values of 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 value of it occurs in autumn and winter. The hydrochemical type of river is CaMg-HCO3. In the river, the order of cation mass concentration is NH4+