EXPERIMENTS OF RAIN INFILTRATION ON CHARACTERISTICS OF SOIL WATER MOVEMENT USING A SOIL COLUMN

Abstract

Characteristics of soil water movement during infiltration are of great importance to study the groundwater recharge. It is also important to clarify the relationship between the type of retaining soil water in pore space and the mechanism of soil water movement. The purpose of this study is to make clear the characteristics of soil water movement during infiltration, from a kinetic point of view, by experiments on rain infiltration in the vadose zone. The experiments were carried out using a soil column (15cm×15cm×150cm) with a water table fixed at 140cm below the surface. Artificial steady rainfalls were supplied to the top of the soil prepared in field capacity. Groundwater discharge, matric suctions at the depths of 10cm, 33cm, 58cm, 83 cm, 108cm and 133cm and soil water flux at the depths of 30cm, 80cm and 130cm were measured continuously. To measure the matric suctions and the soil water flux, automatic tensiometers using water manometer and mi-croflowmeters were produced. The former converts a change of capacitance into matric suction and the latter converts a temperature difference into soil water flux. Fig. 1 shows a diagram of the experimental apparatus. Experimental soils used in this study were a sand (0.17mm in median grain size) and two kinds of glass beads (0.066mm and 0.13mm in median grain size) as shown in Fig. 2, whose physical properties were given in Table 1. The results of the experiments on the rainfall condition of 40mm/h×6 h were shown in Figs. 3 to 5. The results and conclusions of this study are summarized as follows; 1. Equilibrium water content distributions for each experimental soil are shown in Fig, 6. According to the figure, the vadose zone above the water table should be classified into zones of suspended water, unsaturated water and saturated water, although it is usually divided into unsaturated and saturated zones. 2. Soil water movement in the capillary water zone and groundwater discharge through the water table take place, whenever a wetting front reaches the upper boundary of the unsaturated capillary water zone, which is the maximum height of capillary rise above the water table as seen from Figs. 6 to 9. These phenomena can be explained by the destruction of static equilibrium between an upward capillary force and a downward gravitational force of soil water in the ca-pillary zone. 3. In the suspended water zone, the downward velocity of the wetting front dose not depend on the soil grain size, but depends on the rainfall intensity (Fig. 10). There-fore, after a beginning of rain, soil water starts to flow faster through the water table in the small grain size soil with high capillary rise than in the large grain size with low capillary rise. 4. The mechanisms of soil water movement depend upon a difference in the type of retaining soil water in pore space. Thus mechanism of soil water movement in the sus-pended water zone is specified by the water held at points of contact among soil particles, while that in .the capillary water zone is specified by the water filled in pores. Consequently, these flow mechanisms of soil water during rain infiltration are distin-guished between the suspended water zone and the unsaturated capillary water zone. 5. Soil water movement from the land surface to the water table clarified in this study seems to be an important factor to explain the mechanism of soil water movement re-lated to quick groundwater discharge into river on hill slope region after a beginning of rain.