Landslides are triggered in many cases by heavy rainfall. Here the forest vegetation of steep slopes play a significant role in causing landslides in an area. After 1945, Japan`s forests changed dramatically from natural forests to plantation forests.
The afforestation of the steep slopes induced frequent landslides, because in comparision with natural forests, afforestation like Cryptomeria japonica spread there thin roots shallow into the soil and support unstable slopes. And different vegetation covers on slopes have an effect on the water infiltration, generally the covered surface has a lower infiltration rate compared with the no cover surface.
Therefore, this paper investigated which rain infiltrates on steep slopes under a coniferous forest and a deforested area. Unsaturated seepage and rainfall infiltration in to slopes is an essential prerequisite to predict any landslide triggering.
The impact of rain penetration into soils, which was evaluated by comparing the budgets between afforestation and deforestation areas. Particularly, this study measured penetration depth on steep slopes using cone penetration test apparatus and the profiles of soil moisture content at depths from till 100cm.
The observed fluctuations of water content and along its profile owing to seasonal variations depended on rainfall duration and intensity, and forest types .
The uppermost layer of the profile is affected by incipient infiltration during low-intensity rainfalls. As the depth increases, the changes in moisture are controlled by the rate of infiltration and by the fluctuations in the position of the water table,thus enabling rapid changes from unsaturated to saturated conditions.