THE EFFECT OF SPATIAL VARIABILITY IN OVERLAND-FLOW ON THE DOWNSLOPE PATTERN OF SOIL LOSS ON A SEMIARID HILLSLOPE, SOUTHERN ARIZONA

In this study we investigate the relation between soil loss and slope length in an interrill portion of a gentle, plan-planar, semiarid hillslope in southern Arizona. Simulated rainfall experiments were conducted on an 18-m-wide by 35-m-long runoff plot, the upper end of which coincided with the hillslope divide. Sediment loads were measured at two cross sections, S1 and S2, located 12.5 and 21 m, respectively, downslope from the divide. From these data, section soil losses under equilibrium runoff conditions were computed. Average section soil loss was found to increase from the divide to S1 and to decrease from S1 to S2. To explain this downslope pattern of soil loss, a simulation model was developed for soil detachment by raindrop impact and sediment removal by overland flow at a cross section. Input data for the model consist of overland flow depths and velocities measured at closely spaced points along a cross section. The model was applied to three cross sections located at 3.3, 12.5, and 21 m downslope from the divide and predicted a downslope pattern of soil loss similar to that observed. The success of the model suggests that in the interrill portions of a wide range of gentle arid and semiarid hillslopes, downslope patterns of soil loss are controlled in large part by downslope changes in across-slope distributions of overland flow depth and velocity.