Abstract:

Based on hydrological data, water conservancy and conservation statistical data, annual average temperatures, and Landsat surface reflectance data from 1956 to 2016 in the Wei (upper Xianyang Hydrological Station on the main stream of the Wei River) and Jing (upper Zhangjiashan Hydrological Station) River Basins, northwest China, historical changes in the hydrological elements, temperature, and vegetation in the basins were analyzed. Mathematical statistical methods, such as double cumulative values, correlation curve method, cumulative anomaly method, ordered clustering method, Lee-Heghinan method, and rank sum test method, were used to determine the abrupt year of annual runoff and annual sediment load and the effect of precipitation and human activities on runoff and sediment reduction. The results show that (1) precipitation, runoff, sediment load, and sediment concentration in the Wei and Jing River Basins decreased significantly from 1956 to 2016. Specifically, annual precipitation in the Wei and Jing River Basins decreased by 1.41 mm·a^-1and 0.85 mm·a^-1, annual runoff decreased by 0.711×10⁸ m³·a^-1 and 0.217×10⁸ m³·a^-1, annual sediment load decreased by 0.037 t·a^-1 and 0.042×10⁸ t·a^-1, and annual average sediment concentration decreased by 0. 586 kg·m^-3·a^-1 and 0. 905 kg·m^-3·a^-1, respectively precipitation and runoff in the Wei River Basin was reduced about 2 to 3 times more than the Jing River Basin, and sediment load was reduced in the Jing River Basin was reduced by 1.1 to 1.5 times more than the Wei River Basin. (2) The measured annual runoff at Xianyang Station on the Wei River decreased from 60.16×10⁸ m³ in the baseline period (1956—1970) to 38.67×10⁸ m³ (1971—1994) and 22.38×10⁸ m³ (1995—2016). Additionally, during the two latter time periods, the runoff reduction effects due to human activity was 74.7% and 67.1%, respectively, and the runoff reduction effects of precipitation were 25.3% and 32.9%, respectively. The annual sediment load at Xianyang Station decreased from 1.937×10⁸ t in the baseline period (1956—1973) to 0.865×10⁸ t (1974—1994) and 0.233×10⁸ t (1995—2016). The sediment reduction effects due to human activity in the two study periods were 71.7% and 64.4%, respectively, and the sediment reduction effects due to precipitation were 28.3% and 35.6%, respectively. The measured annual runoff and the annual sediment load of the Zhangjiashan Station on the Jing River decreased from 17.43×10⁸ m³ and 2.556×10⁸ t in the baseline period (1956—1997) to 10.36×10⁸ m³ and 0.975×10⁸ t in the treatment period (1998—2016). The impact of human activity on runoff and sediment reduction was 94.2% and 72.6%, respectively, and the impact of precipitation on runoff and sediment reduction was 5.8% and 27.4%, respectively. (3) The runoff and sediment characteristics of the two river basins were quite different. The annual runoff, annual runoff depth, and runoff coefficient of the Wei River Basin were 2.0 to 2.4 times higher than those of the Jing River Basin. The annual sediment load, annual sediment transport modulus, and annual average sediment concentration in the Wei River Basin was only 1/2-1/5 of that in the Jing River Basin. The main reasons for these large differences in runoff and sediment characteristics between these two adjacent river basins are attributed to differences in climatic conditions (temperature and precipitation) between the river basins, and differences in vegetation coverage, and human activity such as soil and water conservation measures undertaken. The analysis of the differences in runoff and sediment between Wei and Jing River Basins clarifies the explanation people mistakenly believe that the phenomenon of a “distinct Jing and Wei” is because “Wei water is more turbid than Jing water”.

Key words: runoff, sediment, the reduction effect of runoff and sediment, Wei River Basin, Jing River Basin