Evolution of rock cover, surface roughness, and its effect on soil erosion under simulated rainfall

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

Submitted to: Geoderma
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/24/2020
Publication Date: 12/1/2020
Citation: Li, L., Nearing, M.A., Polyakov, V.O., Nichols, M.H., Pierson Jr, F.B., Cavanaugh, M.L. 2020. Evolution of rock cover, surface roughness, and its effect on soil erosion under simulated rainfall. Geoderma. 379. Article 114622. https://doi.org/10.1016/j.geoderma.2020.114622.
DOI: https://doi.org/10.1016/j.geoderma.2020.114622

Interpretive Summary: Scientists often measure the roughness of soil surfaces for a variety of reasons, including when attempting to understand how erosion occurs on a soil surface and how much soil is eroded during a rainstorm. On rocky soils, such as we find in the southwestern part of the United States, when soils erode the fine soil particles tend to be eroded first, often leaving a lot of rocks on the soil surface. These rocks are a big part of what constitutes the roughness of those surfaces. In this experiment, we applied artificial rainfall to a 6 by 20-foot soil plot and measured how the surface roughness and erosion changed over time. We found that steeper slopes evolved to a greater roughness than did the less steep slopes, but erosion evolved to be approximately the same on the steep and shallow slopes. We also found that erosion rates were less with greater rock cover and roughness. These results help to better understand the nature or erosion in semi-arid, rocky landscapes, and in particular when soils are most prone to erosion, such as after fires. It is when soils are out of equilibrium that they erode, becoming less erodible as the hillslopes return to an equilibrium state.

Technical Abstract: The dynamic interaction between erosion, surface morphology and flow hydraulics, causes steeper slopes to develop greater physical and hydraulic roughness, such that the slope can evolve toward to a state of equilibrium wherein runoff velocity is independent of slope gradient. This study tests, under controlled condition, the hypothesis that erosion rate may also evolve toward a state wherein the erosion rate is uniform across slope gradients after slope-velocity-equilibrium is established. A series of rainfall simulations (intensities of 59 and 179 mm hr-1) were made on a2 m by 6.1 m stony soil plot under three slope treatments (5%, 12% and 20%, replicated) with surface elevation, rock cover, flow velocity and sediment measurements. The results showed: 1) rock cover, and both surface physical and hydraulic roughness increased as rainfall progressed, leading to reductions in flow velocities and soil loss rates; 2) steeper slopes developed greater surface physical and hydraulic roughness; 3) The final soil loss rates ranged from 0.87 to 1.28 g min-1 m-2, and from 5.36 to 16.01 g min-1 m-2, which were approximately 6% to 15% of the initial maximum values, under low and high rainfall intensity, respectively; 4) soil loss rate was inversely correlated with rock cover while exhibiting no correlation with the random roughness index; 5) the linear coefficient of slope gradient relative to erosion rate measured on the most evolved surface were only 6.5% and 7.3% of those on initial surfaces under low and high rainfall intensity, respective, implying that erosion rate evolved toward being less sensitive to slope gradient; 6) flow velocity and effective shear stress were found to be appropriate predictors for soil loss rate.