Abstract
A simulated rainfall study was conducted to evaluate the effects of slope gradient on interrill erosion of Shirasu, a volcanic ash soil. A simulated 30-min., 62 mm/h rainfall was applied to the slopes of 6°, 9°,12°,15° and 20° to measure interrill erosion and runoff. Whether runoff or soil loss rates all showed rapid increase in first ten minutes from the start of experiment except for slope 6° and 9°. Runoff became almost constant for slopes of 12°,15° and 20°, but soil loss rates showed a different variance according to slope gradient at the latter half of 30 min run. A third -degree polynomial for the relation of gradient with runoff and soil loss rates can be established based on the result of regression analysis of data. But variance in gradient for runoff differed from that for soil loss. For runoff, under the conditions of the experiment, the maximum was 15°, and there was a decrease beyond this slope gradient. Soil loss rates increased with slope gradient within the range of steepness in the experimental conditions. But increment rates differed according to the slope gradient, rapid increase was noted for slopes from 9° to 15°, and soil loss rates flattened beyond 15°.
