RESPONSE OF GROUNDWATER TABLE TO RAINFALL IN KUMAMOTO PLAIN

Response of groundwater table to rainfall in a diluvial upland is analyzed by the weighted mean method developed by Tsuboi (1941). This method employs the Fourier series based on the assumption of periodic waves of the input and output. The groundwater level has been used as an output for analysis in previous studies (e. g. Hirata, 1971). The authors employ the amount of groundwater :recharge as :the output because it is the amount of groundwater recharge that directly causes changes in groundwater level. The analysis is also carried out using the groundwater level as the output and the results are compared with that using the amount of groundwater recharge. Results obtained are as follows: 1. The time lag between the rainfall and the amount of groundwater recharge is about 10 days for wells whose groundwater levels are about 70 to 80 meters below the surface. 2. The groundwater level is affected by rainfall with a time lag of about 3 months. This coincides with a theoretically derived phase difference of π/2. 3. It may be said that results mentioned above give support to the pressure transmission theory as one of the important groundwater recharge mechanisms.