ステップ状降雨入力のもとでの降雨浸透現象

Rain infiltration into the ground is basic phenomena related to plant growth, ground stability, groundwater recharging, etc. The infiltration phenomena, though, have been studied mainly from the viewpoint of ponded infiltration on which the phenomena of rain infiltration are not well explained. It may have been considered that varying rain intensity is averaged by the ground surface, and only the total amount of rain has an influence on infiltration.<BR>However, such ground hazards as landslides or slope failures which are caused by rain often occur when the rain intensity increases after a long duration of weak rain, or several hours after rain. These observations indicate that the behavior of soil water in the ground should be related to the varying rain intensity on the ground.<BR>The present study is focussed on dynamic behavior of the soil water content change responding to varying rain intensity. Considering the phenomena of rain infiltration from the viewpoint of the dynamic system, the phenomena are characterized by:(1) Infiltration is performed under an unsaturated condition.(2) Ponding does not always exist on the ground surface through which rainwater should infiltrate.(3) Rain water is supplied to the surface of the ground not as a succession of water, but as raindrops from a microscopic viewpoint, and as a varying rain intensity with intermissions from the macroscopic viewpoint.(4) It is considered that the soil water content changes responding to the varying patterns of rain intensity.<BR>To investigate the phenomenon of rain infiltration, a series of experiments were planned and conducted under the conditions of different soils, different pattern of rain intensity, and different ground depths. A new measuring method of soil water content was developed theoretically by use of the apparent electrical specific resistance of soil to follow the varying rain intensity with enough speed, and realized in these experiments.<BR>The results of the experiment under the step-type (continuous) intensity are shown and discussed. Selected phenomena of the results are as follows: (1) Velocity of movement of the wetting front is higher under a heavy rain intensity than under weak rain intensity. (2) The wetting front moves downward at a constant velosity under the constant rain intensity. (3) Soil water content above the wetting front reaches its constant value under the constant rain intensity, and equilibrium infiltration is established. To explain the phenomena of rain infiltration, “Open Unsaturated Capillary Perocolation” model is proposed.