Spatial distribution of rock fragments on steep hillslopes in karst region of northwest Guangxi, China

Rock fragments (> 5 mm in diameter) at the soil surface and within the topsoil have a large effect on the intensity of various hydrologic and geomorphic processes. However, little information is available on the spatial distribution of rock fragments in subtropical regions. The objective of this paper was to investigate the relationship between the spatial distribution of rock fragments and landforms on two different steep karst hillslopes in northwest Guangxi, southwest China. On the first hillslope (a disintegrated landslide failure) with the presence of several large rock outcrops (> 2 m in height), the spatial distribution of rock fragment cover had no obvious relationship with topographic position except that the mean cover percentage of small rock fragments (5–20 mm) decreased from bottom to top. On the second hillslope (an avalanche slope) without the presence of large rock outcrops, the mean total rock fragment cover (5–600 mm) increased from bottom (5%) to top (21%) with decreasing variability and rock fragments with various sizes (5–20, 20–75, and 75–250 mm) showed a similar increasing trend. The mean total rock fragment cover increased linearly with slope gradient on the second hillslope and tended to increase and then decrease with gradient but their relationship was not obvious on the first hillslope. This indicated that the spatial distribution of surface rock fragment cover had a close relationship with the presence of large rock outcrops and slope gradient. However, the median diameter (D50) of the surface rock fragments had an increase–decrease trend with slope gradient but there was no obvious relationship on both hillslopes with low overland flow. Therefore, the dominant factor for the spatial distribution of rock fragment cover and size at the soil surface might not be soil erosion by water, but slope gradient, vegetation and geomorphologic condition of the slope. The mean total volumetric rock fragment content (5–250 mm) within the topsoil (10–20 cm thick) increased linearly from bottom (16%) to top (39%) with slope gradient on the first hillslope, and had a logarithmic increase from bottom (10%) to top (27%) with gradient on the second hillslope. This suggested that rock fragment content within the topsoil was mostly controlled by slope gradient and topographic positions and had not a close relationship with the presence of large rock outcrops.

Research Highlights
► The spatial distribution of rock fragments was different on hillslopes 1 and 2.
► The dominant factor for surface rock-fragment distribution might not be water erosion.
► Rock fragment content within the topsoil was primarily controlled by slope gradient.