中文摘要：

坡面流是坡面土壤分离与泥沙输移的驱动力与载体,退耕还林还草工程实施后,可能导致近地表层特征发生改变,集中体现为植被覆盖率提高、植被茎秆及枯枝落叶层出现、生物结皮发育、根系系统的形成,进而引起坡面流水动力学特性的变化。然而,目前对退耕引起土壤近地表层特性变化对坡面流水动力学特性的影响尚不清楚。因此,通过野外放水冲刷实验（坡度12°-14°;单宽流量0.002-0.006 m^2/s）,系统研究了退耕草地近地表层特征（茎杆-枯枝落叶层、生物结皮、植被根系）对坡面流流速的影响。结果表明：坡耕地撂荒7 a后,土壤近地表层特征对坡面流流速影响显著,随着土壤近地表层特性各作用（如根系、生物结皮、植物茎秆及其枯落物）的依次叠加,水流流速依次递减,分别较黄土母质（T4）减少了12.76%、41.53%和66.78%,且平均流速随枯落物干重、根系重量密度的增大呈幂函数形式下降。研究成果对于理解退耕草地坡面侵蚀动力的变化及其机制,具有重要的意义。

英文摘要：

The implement of the ＂grain for green project＂may cause great changes in near soil surface characteristics,which are represented by increasing in vegetation stem and litter cover,developing in biological soil crusts,and forming in plant root system. Overland flow is the driving force for soil detachment and sediment transport and its velocity is probably affected by the changes in near soil surface characteristics greatly. However,the quantitative impacts of the near soil surface characteristics of natural restorated grassland on velocity of overland flow are still unclear. This study was performed to investigate the effects of the near soil surface characteristics（ stem-litter,biological soil crust,plant roots） on overland flow velocity in a natural succession grassland under different slope gradients（ 12° - 14°） and flow discharges（ 0. 002 - 0. 006 m^2/ s）. The results indicated that the near soil surface characteristics influenced overland flow velocity significantly. As the near soil surface factors（ stem-litter,biological soil crust,plant roots） overlapped successively,the flow velocity decreased correspondingly. Compared to velocity of Loess soil,the mean flow velocity of stem-litter,biological soil curst,and plant roots reduced by 68. 78%,41. 53%,12. 76%,respectively. The velocity of overland flow decreased as a power function with litter dry weight and root mass density. The results of this study have vital significance to understand the variation and mechanism of erosion dynamics in the natural succession grassland.