Developing a scientific understanding of hydrochemical characteristics and their controlling factors is of great significance in protecting the quality of the water environment and maintaining ecological balance. The article presents the water chemical characteristics and their controlling factors in the Xiying River Basin (XRB) during different flood seasons from April to November, 2015. The results are as follows. The river water was found to be slightly alkaline on the whole and the dominant ions were Ca² + and HCO 3 -. The hydrochemical type was Ca² + -Mg² + -HCO 3 -. Besides that, the ion concentrations showed great differences in different periods. From the pre-flood period (PreFP) to post-flood period (PoFP), total dissolved solids (TDS), Ca 2+ , Mg 2+ , Na + and SO 4 2presented a trend of decreasing first and then increasing. TDS reached its maximum in PreFP. K + , NO 3 and HCO 3 presented a trend of increasing first and then decreasing, reaching a maximum in SuFP. Rock weathering is the major controlling mechanism of river hydrochemistry, of which carbonate weathering is the main source of ions, followed by silicate weathering. At the same time, factors such as precipitation, river supply sources, human activities and vegetation cover in different times all have significant influences on river hydrochemistry. The findings in this paper show that we need to adjust cultivated land areas and their space distribution scientifically, strengthen the construction of riverbank forest belts, and make full use of the effects of forest land and grassland. These measures could improve water quality effectively and guarantee water security in middle and lower reaches.

Adjusting the agricultural planting structure is one of the important means to realize agricultural water-saving, especially in inland river basin with droughts and water shortages. In this paper, taking Shiyang River Basin in Northwest arid inland river basin as an example, on the basis of determining the crop water production function, a multi-objective optimization model was constructed, which comprehensively considered the maximization of economic benefits, the green water utilization rate and ecological benefits. The optimized planting structure was obtained by using GAMS (General Algebraic Modeling System) model solver. The results showed that: after optimization, the sown areas of wheat, tubers, vegetables and cotton all increased, of which the sown area of tubers increased the most by 8.81×104ha. The sown areas of corn, oil-bearing crops and fruits all decreased, of which the sown area of corn decreased the most by 8.03×104ha. The optimization results of planting structure were different in different counties and districts. The average contribution rate of planting structure adjustment to total water saving was 75.24%, indicating that planting structure adjustment was the key to water-saving. The water consumption of the same crop was different in different counties. Oil-bearing crops and vegetables were more suitable for planting in the lower reaches, while corn and tubers were more suitable for planting in the middle reaches.