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

Authors: LI, LI - University Of Arizona; Nearing, Mark; Polyakov, Viktor; Nichols, Mary; Cavanaugh, Michelle

Submitted to: Journal of Soil and Water Conservation Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/26/2020
Publication Date: 9/25/2020
Citation: Li, L., Nearing, M.A., Polyakov, V.O., Nichols, M.H., Cavanaugh, M.L. 2020. Evolution of rock cover, surface roughness, and flow velocity on stony soil under simulated rainfall. Journal of Soil and Water Conservation Society. 65(5):651-668. https://doi.org/10.2489/jswc.2020.00086.
DOI: https://doi.org/10.2489/jswc.2020.00086

Interpretive Summary: Soil erosion on a stony, rangeland soil was studied using a rainfall simulator. As erosion by rainfall occurs, fine soil material from the soil is washed away, leaving a relatively dense rock covering that both protects he soil from further high rates of erosion, but also slows down the overland flow velocities when it rains. The velocities slow down because the surface gets rougher as the stone cover forms. In this experiment we looked at how this process happens, and how the rock cover forms, by raining on a large, 20-foot-long soil box filled with a stony rangeland soil from Arizona. We found that in this case the rock cover came to a final percentage of cover that was independent of the slope of the box, but interestingly, the roughness of the steeper slopes was greater than for the shallower slopes. The upshot was that the flow velocities were essentially the same on both the steepness gradients of plots used. These results improve our understanding of the evolution and erosion of semi-arid hillslopes.

Technical Abstract: Erosion pavements occur commonly in many semiarid watersheds due to selective erosion. Recent research has shown that a hillslope can evolve in such a way that steeper slopes have both greater physical and hydraulic roughness, resulting in independence of flow velocity relative to variation of slope gradient between hillslopes. The objective of this study was 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 m by 6.1 m soil plot under three slope treatments (5%, 12% and 20%) with surface elevation and rock cover measurements. The results showed: 1) rock cover increased 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) steeper slopes had 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 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.