极端降雨情形下黄土区水土保持治理的减沙效益估算

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

极端降雨情形下水土流失治理的效益评估对黄河治理具有重要意义,但由于缺乏有效的针对降雨事件尺度的定量评估方法,该问题长期存有争议。2017年陕西榆林"7·26"特大暴雨致灾严重,引发广泛关注。该研究提出了一种适用于事件尺度的减沙效益计算方法,并以岔巴沟流域为研究对象,计算流域水土保持治理在"7·26"暴雨事件中的减沙效益。研究结果表明,在流域未治理时段,高强度降雨事件具有较好的降雨径流关系,并且事件平均含沙量相当稳定,藉此可估计"7·26"洪水发生在未治理时段的产流量和平均含沙量,从而计算出相应的产沙量,估算出流域治理的总减沙效益。进一步,利用坡面措施(包括梯田和植被)和淤地坝减沙机制的不同,可分割其各自的减沙贡献。计算结果表明,"7·26"暴雨中,流域水土保持治理使得洪水平均含沙量减小83%,径流减小55.1%,流域治理的总减沙效益因此高达92.4%,与2007-2017年岔巴沟流域泥沙的减幅相当。总减沙效益中有55.1%来自坡面措施,37.3%来自淤地坝措施,2种措施均发挥重要作用。该方法不仅可以计算流域水土保持治理的总减沙效益,还能够分割不同水土保持措施的效益,可广泛应用于黄土区极端降雨情形下水土流失治理的效益评估。

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.