Evolution of rock cover, surface roughness, and flow velocity on stony soil under simulated rainfall

Abstract

Erosion pavements occur commonly in many semiarid watersheds due to selective erosion. However, quantitative information regarding the dynamic feedback between soil erosion, surface morphology, and flow hydraulics as erosion pavement develops is limited. In order to quantify the spatiotemporal evolution of rock cover and surface roughness, and measure their effects on flow velocities as erosion pavement develops, a series of rainfall simulations were conducted on a 2 by 6.1 m soil plot under three slope treatments (5%, 12%, and 20%) with surface elevation and rock cover measurements. The total applied rainfall volume for each experimental replication ranging from 1,400 to 2,240 mm caused the development of erosion pavement. The results showed (1) rock cover increased from 15% to 90% as rainfall progressed, and the terminal rock cover was not slope gradient dependent; (2) random roughness was positively correlated with rock cover in the upper and middle plot sections, and increasing surface roughness and rock cover reduced the flow velocity following power functions; (3) surface roughness in the lower sections did not uniformly increase with increasing rock cover due to the formation of rills; (4) the terminal surface roughness values of full plots were 5.3 and 5.1 mm, 4.2 and 4.5 mm, and 2.9 and 3.2 mm for replications of 20%, 12%, and 5% slopes, respectively, indicating that steeper slopes produced greater surface roughness; (5) flow velocities measured at the end of experiments reached a relative constant value that was a function of unit flow rates alone for a given section; (6) hydraulic resistances were correlated with flow discharge, slope gradient, and rock cover, exhibiting no unique hydraulic coefficient for a given surface condition. These results improve our understanding of the evolution of semiarid hillslopes.