基于水沙关系框架的黄土区不同水保措施减沙贡献分割方法

Partition of reducing sediment for various soil and water conservation measures of Loess Plateau in China based on runoff-sediment relationship

黄土高原水土保持治理已取得巨大成效,但常规的水文法和水保法均不能可靠地进行不同水土保持治理措施的减沙贡献分割。该研究首先分析了不同治理措施的减沙机制,认为在流域尺度上:1)由于坡面水下沟后继续冲刷,植被、梯田等坡面治理措施仅通过减少地表径流来减沙;2)淤地坝既通过减少地表径流来减沙,也通过降低地表径流含沙量来减沙;3)降雨对产沙的影响机制与坡面措施类似。基于此,该研究提出了一种新的针对黄土区的流域减沙贡献分割方案,给出了降雨、坡面措施以及淤地坝的减沙贡献计算公式。验证结果表明,这3个公式计算结果与实际观测结果均非常一致,其中淤地坝公式准确计算出了大理河、清涧河、延河流域的淤地坝拦沙量(R^2>0.9)。水文法被认为是一种较可靠的降雨变化和人类活动贡献分割方法,该研究提出的方法与水文法的计算的结果几乎一致。水保法计算结果为坡面减蚀率,因而会高估减沙贡献,而该研究方法考虑了减沙效应的空间尺度变异,计算结果为流域减沙率。该研究提出的方法具有较好的理论基础,对黄河流域减沙的归因分析以及未来治黄策略的制定具有重要意义。

Watershed management for soil conservation has great beneficial effects on the Loess Plateau in China,which can be characterized by a very high specific sediment yield(SSY)of>10000 t/km^2·a.The major conservation measures include the check-dam construction(channel measures)and terracing,afforestation,pasture establishments(slope measures).In various conservation measures,it is still lacking efficient method to partition their synergistic impacts,as well to reduce SSY.Here,this paper presents a new method to monitor the relative effects of rainfalls and different conservation measures on the SSY change in the Loess Plateau.In the framework of the runoff-sediment yield relationship,the sediment-reduction mechanisms of rainfalls and various conservation measures can be expressed by:SSY=CR,where R denotes the surface runoff and C the mean sediment concentration in surface runoff.The results show at the watershed scales:1)Slope measures can only reduce SSY by decreasing R as the flows entrain some sediment from upland slopes to gullies;2)Rainfalls has the same effects as the slope measures because the rainfall cannot reduce the available sediment,where the mass movement is very active on the Loess Plateau;3)Check dams can reduce SSY by decreasing both R and C.Based on the differences in the sediment-reduction mechanism,the equations were developed to calculate the SSY-reduction effects of rainfalls(ΔSSYP),slope measures(ΔSSYSM,)and check dams(ΔSSYCD).The resultant equations are:1)ΔSSYP=C0ΔRP,where C0 denotes the C value for the reference period(P0),where soil conservation practices were the minimum,andΔRP denotes the R change due to the rainfall change;2)ΔSSYSM=C0ΔRSM,whereΔRSM denotes the R change due to the slope measures;3)ΔSSYCD=C0ΔRCD+RiΔC,whereΔRCD denotes the R change due to the check dam establishment,Ri denotes the R value for the ith evaluation period(Pi),andΔC denotes the C difference between the P0 and Pi.The term C0ΔRCD represents the reduced SSY by check dams reducing R,and the term RiΔC represents the reduced SSY by check dams reducing C.The results showed that there was a good agreement with the observed and calculated equations(R^2>0.9).The developed equations are different from those in the“hydrology”and the“soil and water conservation”method,where there is no the SSY-reduction effect at the watershed outlet,but the erosion-reduction effect occurs on slopes due to neglecting the variations of the SSY-reduction effects with spatial scales.The proposed method considers the spatial scale variation of the SSY-reduction effects,to obtain the SSY-reduction effects at the watershed outlet.This method can also provide a sound theoretical foundation to monitor the relative contributions of various conservation measures to the SSY change on the Loess Plateau.