Observation of Soil Moisture Variation on a Hillslope and Numerical Analysis Using the Saturated-unsaturated Infiltration Model

Abstract

A sediment related disaster occurs frequently in rainy seasons in Japan. In this study, pressure heads in the soil were measured at 10 points on the hillslope consisting of weathered granite to get fundamental data to explain the behaviour of rainwater on a forested hillslope and the mechanism of hillside landslides. The observed suction for natural rainfalls was reproduced by a numerical model that estimates the process of infiltration as a vertical unsaturated seepage and a parallel saturated flow along the slope.
Following matters were found by this study.
(1) Drying and wetting processes of soils step forward to deeper layers from the surface.
(2) The arrival time of rainwater to the bedrock decreases with a decrease of the depth of soils.
(3) In the initial stage of rainfall, a large amounts of rainwater are temporally stored in the weathered surface layer that contains organic matters. And, it reaches the bedrock layer gradually as the time goes.
(4) Comparison of the observed and calculated soil-moisture content showed that this simulation model is adequate for reproducing the arrival time and the infiltration process, of rainwater.