INFLUENCE OF RAIN ON THE GROUND WATER LEVEL

Abstract

It is true that the rainfall causes to rise the level of ground water, but the rate of rising is dependent on the characteristics of rainfall, antecedent rain and evapotranspiration etc. The author carried out an investigation on the rainfall effects which influenced the change in the ground water level on the basis of rainfall characteristics and the nature of soil layer of the sand dune in natural condition. In selecting an observation well, the author examined several conditions such as the depth to ground water table, the effect of lateral ground water flow and the effect of tide. The southern part of Kashima (Fig. 1) was selected as the study field where no appreciable influences mentioned above were found. To obtain basic data, precipitation was measured by a self-recording gauge, water table level by a self-recording water level gauge, infiltration by an infiltration gauge (made by myself) and soil moisture by the oven-dry method (Fig. 3). The investigation was made during three months from June 27 th to November 25th in 1967 (Fig. 4). The results obtained were summarized as follow: (1) Soil moisture increase caused by a single rain amounting to 155.0mm depth was calculated to be 132.7mm, while 22.3mm was lost by evapotranspiration. (2) Deficit of soil moisture before rainfall is an important factor influencing the rate of rising of ground water level. (3) The following point was made clear by measuring the effective porosity of the shallower (sand) and the deeper part (clayey sand under 80cm from the surface) of the soil column at the observation site that the smaller effective porosity in the deeper part was caused by capillary rise of ground water.