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

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 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 erosion rate is uniform across slope gradients after slope-velocity-equilibrium is established. A series of rainfall simulations (intensities of 59 and 178 mm hr−1) were conducted on 2 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 (random roughness) and hydraulic roughness (Darcy–Weisbach friction) 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, respectively, implying that erosion rate evolved toward being less sensitive to slope gradient than it would otherwise be; 6) flow velocity and effective shear stress were found to be appropriate predictors for soil loss rate. This study supports the hypothesis of erosion equilibrium, implying that erosion rate decreases as a function of erosion pavement and that influence of slope gradient on soil erosion declines due to the dynamic interactions between soil erosion, surface morphology, and flow hydraulics.