Water Balance of the Groundwater Basin in Ichihara Region, Chiba Prefecture

Abstract

On the basis of the concept of the groundwater basin (Freeze and Witherspoon,1967), which is the three dimensional closed system that contains the entire flow paths followed by all water recharging the system, the evaluation of the natural basin yield (Freeze and Witherspoon,1967) is attempted in Ichihara region, Chiba Prefecture.
Ichihara region is situated to the east of Tokyo Bay and covers an area of about 200 square kilometers. Topography of the area mainly consists of diluvial uplands called Shimosa Upland and alluvial lowlands. The basin is bounded on the bottom by the kasamori Formation which is composed of very muddy material and is considered as the impermeable basement, on the top by the water table which is the replica of the topography, and all sides by imaginary impermeable boundaries which simulate the groundwater divides.
Amount of recharge to the groundwater basin is estimated from the following surface water balance equations,
R= (1 -f) P-EtPEt where R is the recharge to the groundwater basin, f is the direct runoff coefficient, P is the precipitation and Et is the evapotranspiration. Meteorological data at Ushiku and f=20% lead to the annual recharge to the basin 606mm/year. About 600mm/year of recharge can be expected in the investigated area.
As the development of the Keiyo industrial zone in the coastal area, the amount of withdrawal of groundwater had been increased. At the same time, groundwater level had declined and many undesired phenomena had been arisen.