Vegetation plays an important role in gully headcut erosion in China's Loess Plateau, but little is known about the effects of root density on gully headcut erosion.This study attempted to investigate the effects of grass root density on gully headcut erosion and morphological development of the gully head using plots with different grass root densities (with bare land as the control). The experiment was performed on plots with different grass root densities, and the rainfall intensity and inflow discharge increased throughout the experiment. The results showed that headcut migration in bare land was mainly activated by upstream flow incision, headwall erosion, and plunge pool erosion, whereas headcut migration in grassland was driven by soil-root matrix collapse at the headcut, headwall erosion, and plunge pool erosion. The soil loss rate (SLR), gully headcut retreat distance (GRD), gully area (GA), and upstream soil loss volume (SLV U ) in grassland plots with different root densities were 45.64-68. 45%, 66.97-85.38%, 69.26-78.18%, and 67.89-87.02% less than those in the bare land plots, respectively. However, as root density increased, the gully depth in the grassland plots increased from 19% to 59%, and the proportion of gully head soil loss volume (SLV GH ) to the total soil loss volume increased from 63.37% to 88.37%. The SLR, GRD, GA, and SLV GH showed decreasing trends as the root density increased, as described by a Hill curve. Moreover, roots with diameters of 0-0.5 mm had greater effects on soil loss and morphological evolution than roots with larger diameters. Our results provide a reference for the reasonable selection of grass root density for controlling gully headcut erosion in China's Loess Plateau.

In Lhasa River Basin (LRB), land suitable for settlement or living is experiencing a shortage of resources. Alluvial fans have the potential to alleviate this problem. However, basic information, such as the distribution and land use types of alluvial fans, is rarely studied. In this study, Google Earth, ArcGIS and visual interpretation were used to obtain the outlines, areas, quantities and distribution of alluvial fans. Meanwhile, to show the utilisation potential of alluvial fans, we analysed the land use, their distance from the roads, places (town and village) and rivers. The results showed 826 alluvial fans exist in LRB, with a total area of 1166.03 km². The number of alluvial fans with areas between 0.1 and 1 km 2 is 517, accounting for 62.59% of the total number of alluvial fans. Grassland is the dominant land use type, accounting for 68.70% of the total area of alluvial fans. The cropland area accounted for 2.16% of alluvial fans and accounted for 18.98% of the total cropland area in LRB. Exactly 93.70%, 53.63% and 61.86% of the total number of alluvial fans were located within 5 km from the tertiary road, village, and river, respectively. To sum up, our survey results showed that alluvial fans are important land resources in LRB and may have huge utilisation potential.

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Alluvial fans are an important land resource with agricultural potential in Qinghai–Tibet Plateau. The spatiotemporal variation in land use/cover is an important indicator to understand the value of alluvial fans and protect and make scientific use of such fans. In this study, the spatiotemporal characteristics of land use/cover are determined by analysing the land use/cover changes of alluvial fans in the Lhasa River Basin (LRB) at different times, counties/districts, altitudes, and gradients. Results show that the area of cultivated land and the artificial land provided by alluvial fans for LRB has continuously increased. In 2000, 2010, and 2020, 17.72%, 21.84%, and 24.17% of cultivated land and 7.89%, 7.51%, and 25.24% of artificial land in LRB were provided by alluvial fans, respectively. At all altitudes and slopes, cultivated land and artificial land are increasing but the increasing part is basically due to the massive loss of grassland. The spatiotemporal changes in all land use/cover types of alluvial fans were dominated by human activities, although they were also influenced by natural factors to some degree.

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