The influence of slope angle on infiltration rate, surface seal characteristics, and soil loss: A laboratory experiment

Critical factors governing water infiltration into a sloping surface have been observed to vary significantly with slope angle. These include rainfall input, runoff depth, and surface sealing intensity. In past studies, the relationship of each of these to slope angle and the consequent effect on infiltration rate has been studied in isolation from other contributing factors. A more integrated approach, which includes all of the processes acting interactively, is required.; This study examines the influence of slope angle on infiltration rate by attempting to quantify the relationships between slope angle and runoff depth and pattern, surface seal characteristics, rainfall input, and soil loss; these variables are then used to develop an integrated theory describing water infiltration into a sloping surface for a soil susceptible to surface sealing.; Rainfall simulations were carried out on a sandy loam in a laboratory using a rainfall intensity of 40 mm h{dollar}sp{lcub}-1{rcub}{dollar} 100 x 40 x 10 cm soil trays for collecting discharge and sediment load, microtensiometers for pressure head measurements, and image analysis for the micromorphological analysis of soil pores.; The results show that infiltration rate decreases with increasing slope angle due to a decrease in surface storage volume and runoff depth. Although runoff depth has a negligible effect as a pressure head, it increases infiltration by increasing the spatially averaged surface seal hydraulic conductivity. The seal itself varies considerably both spatially and vertically, but there is no systematic variation in surface sealing intensity associated with slope angle. This was confirmed by both the pressure head measurements and pore micromorphology. Soil loss increased less than linearly with increasing slope angle, and best predictions were obtained using a non-linear equation relating erosion rate to discharge.; The results of the laboratory experiment are consistent with field observations in different parts of the world, and they suggest that for surface sealed soils, the relationship between slope angle and infiltration rate is negative. An exception to this can occur when the seal is breached by rills if the extent of rilling is positively correlated with slope angle. The change in hydraulic conductivity with runoff or ponding depth has implications for modelling hydrologic processes on hillslopes as well as for interpreting changes in infiltration rate with varying rainfall intensities.