Experimental investigations of the evolution of step-pools in rills with heterogeneous soils in Yuanmou Dry-Hot Valley, SW China

Abstract As a common type of microtopography in rill channels, the formation and dynamics of step-pools are prerequisites for the occurrence of rills. However, the evolution of step-pools in a rill (SPRs) is still unclear. Scouring experiments in field plots were conducted to explore the dynamic evolution of SPRs and the effect of steps on pools under conditions of strong runoff and soil-like materials aggregated with gravels in Yuanmou Dry-Hot Valley (SW China). Using three-dimensional laser scanning, spatial analysis of a GIS (geographic information system), and statistical analysis, 11 parameters were used to characterize step-pool morphology. The results showed that SPRs formed after several scourings at particular locations, and some of them disappeared in the late stage. A diverse range of SPRs formed, with geometrical parameters initially increasing, then stabilizing, and finally decreasing. During scouring, pool shape transformed from a deep V-shape to a shallow V-shape and then to a U-shape. The evolution of a typical pool included initial, active, stable and fading stages. There were 59 oval-shaped pools (88.06% of the total number of pools) in plane form and 37 deep pools (55.22%) in cross-section, and significant differences in the plane form and profile morphology of a pool presented themselves during its evolution. SPRs developed more easily in dry red soil-like materials compared with vertisol-like materials. The geometrical parameters of pools correlated with step size; and pools became deeper with increasing step height, and longer with increasing step height and decreasing step slope. The spatial heterogeneity of soil-like materials, as well as hydraulic conditions, led to complex and nonlinear SPR evolution. It is helpful to further understand the mechanism of rill incision and evolution.