Estimation of sediment reduction benefit by soil and water conservation under extreme rainfall in a loess watershed

Quantitative assessment on the effects of soil conservation measures under extreme rainstorms is crucial for the watershed management in the Middle Yellow River Basin of China. Most previous efforts have focused mainly on a long period evaluation. The developed methods were applicable to the multi-year time scale, but not to the storm-event scale as the fluvial processes at the short time scale are more complex and stochastic. As a result of the absence of an efficient method, there has long been considerable controversy surrounding the role of soil conservation measures under extreme events in the Middle Yellow River Basin. On 26 July, 2017, an extreme rainstorm occurred in Yulin City (the so-called "7·26" storm event), Shaanxi Province, China, causing serious flood hazards and attracting extensive public attention. The purpose of this study is: 1) to develop a new approach to evaluate the effect of the soil conservation measures at the storm-event time scale; 2) used the method for the "7·26" storm event in the Chabagou Watershed, a 205 km2 loess watershed on the Loess Plateau. The results showed that: 1) after removing the non-high-intensity rainfall events (with a runoff coefficient lower than 0.2), there was a close relationship between rainfall and runoff for the high-intensity rainfall events (r2=0.87) during the non-management period (1959-1969); 2) since most flows approached the flush-limited maximum sediment concentration, the mean sediment concentration of a flood event was considerably stable, leading to a proportional relationship between event runoff and sediment yields. The resultant proportionality coefficient can be used as an estimation of the mean sediment concentration of a flood event. The two results allowed us to effectively evaluate the runoff yield, the event mean sediment concentration, and then the basin sediment yield when assuming that the "7·26" event had occurred during the non-management period. In this way, we obtained the total sediment-reduction effect of the soil conservation measures. Previous studies have shown that the slope measures (including terracing, afforestation, pasture establishment) reduced basin sediment yields simply by reducing runoff, whereas the check-dam construction (channel measures) by reducing both runoff and sediment concentration in the runoff. Based on the difference, we can further partition the respective impacts of the two measures. Our calculations showed that during the "7·26" event, the mean sediment concentration reduced by 83%, the runoff by 55.1%, and thus the basin sediment yield by 92.4%, as a result of the implementation of soil conservation in the Chabagou Watershed. Among the total sediment-reduction effect, 55.1% was attributed to the slope measures and 37.3% was attributed to the check dams. The calculations suggest an enormous sediment-reduction effect of soil conservation practices even under extreme storms on the Loess Plateau, and that both the slope measures and the check dams play important roles. The proposed method cannot only figure out the total amount of the reduced sediment yield by soil conservation measures in a basin, but also can figure out that by an individual measure. The method has good applicability and data requirement, and is suitable for the evaluation of the sediment-reduction effect by soil conservation measures at the storm event scale in the middle Yellow River Basin.