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

Abstract：Gully headcut erosion become the main cause for the longitudinal and horizontal surface, surface fragmentation andarea shrinkage in the gully region of the Loess Plateau in China,threatening the development of local agriculture, economy andecological environment. At present, the vegetation hasplayed an important role in controlling the gully headcut erosion in thisregion. Most of the previous studies mainly focused on the influence of the above-ground parts of the vegetation on the basinand even the regional scale.However, little is known about the effects of the root density of vegetation on the gully headcuterosion. 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 slopeof the upstream area was consistent with that of the gully bed, being set as 3°.Compared to the bare land experiment area, thispaper presents the process of soil loss, gully head retreat, gully cutting and development, in the different root density aroundthe experiment area of Agropyron cristatum (planting space: 20 cm×20 cm, C1; 15 cm×15 cm, C2; 10 cm×10 cm, C3). Theresults 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 sandreduction benefit was the largest. 2) In the control plot, the gully headcut erosion mainly includedthe following steps: theupstream flow incision, headwall erosion by the wall flow, plunge pool erosion by the jet flow, and finally the gully bankcollapse,whereas,in the grass-covered plot,the gully headcut erosion was determined by the wall flow, plunge pool erosion bythe jet flow, and the collapse of the hanging soil-root matrix at the gully head. The collapse was also the main reason for thegully headcut erosion of grassland. Alternatively, the accumulation distance of gully head in the grass-covered plot wasshortened by 75.61% and 78.87%, and the accumulation distance of gully head inboth plots showed a significant powerfunction with time. 3) The longitudinal section of erosion gully in the control plot was in ladder-shaped, and the gully headwas rectangular; while in the grass-covered plot was trapezoid and arc-shaped. The average incision depth of the gully channelin the grass-covered was 1.64-1.92 times as deep as that in the control plot. However, the developed area of the gully channeldecreased 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 gullystabilization and loess-tableland protection in the gully region of Loess Plateau in China