Investigation of Soil Erosion from Bare Steep Slopes of the Humid Tropic Philippines
At the Visayas State College of Agriculture (ViSCA) on the island of Leyte in the Philippines, hydrologic and soil-loss measurements were recorded for 32 erosion events over 3 yr on three 12-m-long bare soil plots with slopes of approximately 50%, 60%, and 70%. Measurements included rainfall and runoff rates at 1-min intervals, total soil lost per event from the plot, rill details when observed after an erosion event, and soil settling-velocity characteristics. Storm events are characterized by high rainfall rates but quite low rates of runoff, because of the consistently high infiltration rate of the stable clay soil (an Oxic Dystropept). Both observation and modeling indicated that overland flow is commonly so shallow that much of the soil surface is likely to be unsubmerged. For the 70% slope plot, half the events recorded mean sediment concentrations from 100 to 570 kg m−3. A somewhat constant hydrologic lag between rainfall and runoff is used to estimate a Manning’s roughness coefficient n of about 0.1 m−1/3 s, a value used to estimate velocity of overland flow. Possible effects of shallow flows and high sediment concentrations on existing erosion theory are investigated theoretically but are found to have only minor effects for the ViSCA dataset. A soil erodibility parameter β was evaluated for the data whenever rilling was recorded following an erosion event. The values of β indicate that, except for events with higher stream powers, other erosion processes in addition to overland flow could have contributed to soil loss from erosion plots in a significant number of events.