Experiments on Characteristics of Soil Water Movement during Drainage

It has been pointed out that the capillary fringe plays an important role in the infiltration process. In the drainage process, characteristics of the capillary fringe has not been elucidated yet, notwithstanding it significantly influences the flood recession curve.
The purpose of this study is to make clear characteristics of soil water movement during drainage and the influence of groundwater recharge upon the flood recession curve.
Experiments were carried out using a soil column with a water table maintained at 140 cm below the surface. The soils used in the present study are two kinds of glass beads. Artificial steady rainfall were supplied to the top of the soil column for six hours. The experiments on the drainage process started when the steady rainfall stopped. Discharge, matric suctions and water flux were measured continuously.
The results and conclusions of this study are summarized as follows ;
1. After the steady rainfall, the drainage front began to move faster downward than the wetting front in the infiltration process. When the drainage front reached the upper boundary of the saturated capillary water zone, it disappeared and the recession of discharge began instantaneously.
2. The soil water zone above the water table is divided into two zones during drainage. One is the water zone that the gradient of the total head maintains unity, and the other is that the gradient approaches zero. A break point in the profiles of total head moved upward with advancing of the drainage process from the upper boundary of the saturated capillary water zone to the upper boundary of the unsaturated capillary water zone.
3. It was found that the flood recession curve can be estimated from the equation of soil water movement considering gravitational term without diffusion term, if we determine hydraulic conductivity, water content in the suspended water zone and the depth of a break point in the profile of total head changed with time. 本稿は1983年度日本地理学会春季学術大会にて発表した内容に加筆 ・修正をしたものである. 筑波大学大学院地球科学研究科 千葉大学理学部地学教室 Graduate Student, Institute of Geoscience, University of Tsukuba Department of Earth Sciences, Faculty of Science, Chiba University