根系密度对黄土塬沟头溯源侵蚀产沙和形态演化过程的影响

Effects of root density on gully headcut erosion and morphological evolution process in gully regions of Loess Plateau

为明确根系密度对黄土塬沟壑区沟头溯源侵蚀产沙和形态演化过程的影响,采用野外"人工模拟降雨+放水冲刷"试验方法,以裸地试验小区(CK)为对照,研究冰草根系密度试验小区(株行距:20 cm×20 cm,C1;15 cm×15 cm,C2;10 cm×10 cm,C3)的沟头溯源侵蚀产沙过程、沟头溯源距离、沟道下切深度及发育面积等特征。结果表明:1)与对照小区相比,草被小区(C1~C3)产沙量分别降低64.32%、70.31%、69.92%;冰草株行距为15 cm×15 cm时,减沙效益最大。2)对照小区沟头溯源侵蚀过程主要包括沟口形成、贴壁流侵蚀、跌水侵蚀和沟岸崩塌等;而草被小区沟头溯源侵蚀则由贴壁流侵蚀、跌水侵蚀和根土复合体崩塌导致,崩塌是草地沟头溯源的主要原因;各根系密度下沟头溯源距离与时间均呈极显著幂函数关系;与对照相比,草被小区沟头溯源距离缩短75.61%~78.87%。3)对照小区侵蚀沟纵断面呈阶梯形,存在缓冲平台,沟头近似矩形;草被小区则呈梯形和圆弧状。与对照相比,草被小区沟道平均下切深度加深1.64~1.92倍;沟道面积随根系密度增加而缩小,草被小区沟道面积较裸地缩小68.0%~74.0%。结果可为该区"固沟保塬"工作的实施提供科学参考。

Gully headcut erosion become the main cause for the longitudinal and horizontal surface, surface fragmentation and area shrinkage in the gully region of the Loess Plateau in China,threatening the development of local agriculture, economy and ecological environment. At present, the vegetation hasplayed an important role in controlling the gully headcut erosion in this region. Most of the previous studies mainly focused on the influence of the above-ground parts of the vegetation on the basin and even the regional scale.However, little is known about the effects of the root density of vegetation on the gully headcut erosion. The effects of sediment production and its morphological evolution are also lacking, particularly on the loess.Therefore, a combined experiment of simulated rainfall and runoff scouring was conducted to explore the effects of the grass(Agropyron cristatum) root densities on the erosion process and morphological evolution during the gully headcut erosion.The experimental plot consist of the upstream area, gully head and gully bed. The length and width of the upstream area were8.0 m and 1.5 m. The gully headwall height was 1.2 m. The length and width of the gully bed were 1.0 m and 1.5 m. The slope of the upstream area was consistent with that of the gully bed, being set as 3°.Compared to the bare land experiment area, this paper presents the process of soil loss, gully head retreat, gully cutting and development, in the different root density around the experiment area of Agropyron cristatum(planting space: 20 cm×20 cm, C1;15 cm×15 cm, C2;10 cm×10 cm, C3). The results showed that: 1) Compared with the control plot, the sediment yield of grass-cover plot(C1-C3) decreased by 64.32%,70.31%and 69.92%, respectively. When the plant spacing of the Agropyron cristatum plants was 15 cm × 15 cm, the sand reduction benefit was the largest. 2) In the control plot, the gully headcut erosion mainly includedthe following steps: the upstream flow incision, headwall erosion by the wall flow, plunge pool erosion by the jet flow, and finally the gully bank collapse,whereas,in the grass-covered plot,the gully headcut erosion was determined by the wall flow, plunge pool erosion by the jet flow, and the collapse of the hanging soil-root matrix at the gully head. The collapse was also the main reason for the gully headcut erosion of grassland. Alternatively, the accumulation distance of gully head in the grass-covered plot was shortened by 75.61% and 78.87%, and the accumulation distance of gully head inboth plots showed a significant power function with time. 3) The longitudinal section of erosion gully in the control plot was in ladder-shaped, and the gully head was rectangular;while in the grass-covered plot was trapezoid and arc-shaped. The average incision depth of the gully channel in the grass-covered was 1.64-1.92 times as deep as that in the control plot. However, the developed area of the gully channel decreased with the increase of root density, and the area of the gully in the grass-covered plot decreased by 68.0%-74.0%compared with that in the control plot. The findings can provide a promisingapproach for the implementation of the gully stabilization and loess-tableland protection in the gully region of Loess Plateau in China.