Role of capillary water zone in groundwater recharge. Observation of rain infiltration by lysimeter.

Abstract

Observations were carried out using a couple of monolith lysimeters with a water table at 145 cm below the ground surface. Measurements of rainfall, outflow and soil water movement were made in order to clarify a mechanism of groundwater recharge caused by a heavy rainfall. The lysimeters were packed with sand 0.38 mm in median grain diameter and Kanto loam originating from volcanic ash soil whose heights of capillary rise are about 45 cm and larger than 125 cm, respectively. Groundwater recharge in the Kanto loam soil begins usually to increase and decrease earlier than in sand. The time required until the beginning of the increase in outflow after the peak of a rainfall intensity is an hour or two in Kanto loam soil, and more than two hours in sand. During the observation period of about one year, outflow from the Kanto loam soil stopped for 34 days and that from the sand for 3 days. These data indicate that groundwater recharge occurs as soon as the wetting front reaches the upper boundary of the capillary water zone, soil water in the suspended water zone is drained slowly and that in the capillary water zone quickly until a water equilibrium is established.