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.