黄土丘陵沟壑区植被对不同空间尺度水沙关系的影响

根据晋西离石试验站的观测结果和相关文献的数据,研究了黄土丘陵沟壑区植被对坡面小区、全坡面、小流域及中大流域4个空间尺度水沙关系的影响。对梁峁坡坡面小区言,由于植被提高了土壤抗蚀性,因此植被不仅通过减水来减沙,也通过改变水沙关系来减沙。但在全坡面尺度,土壤侵蚀以沟蚀为主,由于植被措施难以改变沟道的各种水利参数,也难以有效控制切沟沟壁的重力侵蚀,导致水流进入沟道后仍然可以获取充足的泥沙,因此认为植被不会改变全坡面尺度的水沙关系。同样对于各级流域,由于植被措施难以改变沟道的输沙能力和黄土丘陵沟壑区流域泥沙来源充沛的特点,因此植被措施也不会改变其水沙关系,植被的减沙效应仅通过减水来实现。对离石试验站的一对水土保持对比沟的研究表明,即使在沟道已有茂密植被生长的情况下,高含沙水流的输沙能力也没有改变,这使得两者的水沙关系统计上可以认为完全一致。由于大流域水沙关系主要取决于沟道或河道的特性,而植被等坡面措施很难改变沟道或河道特性,因此认为流域尺度越大,植被越难以改变其水沙关系。

Effect of vegetation on runoff-sediment relationship at different spatial scale levels in Gullied-hilly Area of the Loess Plateau,China

The loess gullied-hilly area of the Loess Plateau in North china is one of the most severely eroded regions in the world with a mean annual soil loss rate more than 10000 t/km2.Base on data from gauging station and the published data from other Chinese research,we discuss whether vegetation reduces soil loss by reducing runoff volume,or rather by changing runoff-sediment yield relationship at four scale levels:plot,entire hillslope(including both inter-gully and gully slope),subwatershed and watershed.At the plot scale level,vegetation helps reduce soil loss not only by reducing runoff volume,but also by changing the runoff-sediment yield relationship,leading to higher sediment-reduction efficiency than runoff-reduction efficiency.However,at the entire hillslope,where gully erosion and mass wasting process are dominant,vegetation measures are insufficient to control local mass movement,implying that sediment supply is abundant so that flow can achieve its sediment transport capacity even after vegetation is established.It is also hard for vegetation to change gully topography and other hydraulic parameters,by which transport capacity of water flow is largely determined,so it is inferred that vegetation cannot change runoff-sediment yield relationship at the entire hillslope scale.Similarly,at the sub-to large watershed scale levels,due to an unchanging channel transport capacity and consistently sufficient eroded material caused mainly by mass movement before and after vegetation is planted,vegetation reduces soil loss only by reducing runoff volume,leading to a sediment-reduction efficiency that is equal to runoff-reduction efficiency.The analysis of data collected during a 15-year monitoring period shows that two neighboring sub-watersheds(one of which was cultivated with no conservation practices,and the other was cultivated with hillslope and vegetation measures)have the same runoff-sediment yield relationship,meaning that given the same runoff volume,the same sediment yield will be observed at the outlets of these two watersheds.Further,the larger the watershed,the less influence the hillslope process exerts on the runoff-sediment yield relationship.Because there is little chance for vegetation to change channel features,it is concluded that the larger the watershed,the more difficult it becomes for vegetation to change the runoff-sediment yield relationship.