Volcanic drainage basins are generally the regions with the distinguished hydrological characteristics. This paper, proposes a hydrological model which sets recharging zone and discharging zone in the groundwater basin, taking the Aso caldera basin as research field. First, the characteristics of recharging zone which is the main field of infiltration and recharging is described, and it is pointed out that the continuous processes from infiltration in recharging zone to discharge of groundwater in discharging zone are main processes in hydrologic cycle of volcanic drainage basin. Then, the fact that in hydrologic cycle the chemical composition of groundwater is indicative of its origin and history gives some idea of a fact. For example, the calcium ion, which is in nature minor dissolved constituent in the groundwater of volcanic zone, increases as the materials for agriculture are supplied to the drainage area artificially. The variation of the concentration represents the rate of recharge, and the result is useful to know the actual condition of recharging in field. Since the concentration of anion species SO42-shows also increasing trend with groundwater movement, the fact helps to classify the boundary of groundwater zone. Next, the variation of groundwater storage is simulated by Sugawara's tank model for simulating daily discharge through the channel. And it is proved that the calculated results of groundwater level is comparable to the variation of real groundwater level. On hydrological balance of the basin, volumetric rate of flow through groundwater zone is estimated, as a results the actual condition of groundwater flow in hydrologic cycle of volcanic drainage basin is clarified.
The complex spatial distributions of hydraulic conductivity within a natural aquifer represented as a statistical homogeneous random process which is characterized by a new function, called the variogram. This variogram is related to the stationary covariance of the hydraulic conductivity difference between two points separated by the distance d. Using variogram and actual field data at the sample points, a statistical linearization technique is developed to estimate the spatial distributions of hydraulic conductivity. A case study is presented in order to illustrate the statistical methodology. Once the variogram is determined throughout an aquifer, this estimation method has proven to be adapted solving groundwater system's behavior.
Water level of Lake Kawaguchi in Yamanashi Prefecture had risen more than 3 m above the standard level after the heavy rainfalls in summer of 1983. On that opportunity the authors started to study the hydrogeological conditions around the lake with the use of long-term observation data of the lake water level, groundwater level, precipitation and others (Figs.4 and 5). The area is composed mainly of the Tertiary Misaka Group and the Quaternary volcanic rocks extruded from Fuji Volcano (Fig.1). The Misaka Group and the Kofuji Mud-flow Deposits, one of the effusive rocks of the volcano, constitute the hydrogeological impermeable bed rocks. Although Lake Kawaguchi has no natural mouth for surface discharge, the water is drained off through the man-made tunnels. On the other hand, it is likely that the water permeates through the volcanic rocks into the underground valley about 5 km south of the lake, judging from the contour line of the groundwater level (Fig.2). Figures 2 and 3 indicate that the hydrogeological watershed of the lake occupies only the northern part of the topographic watershed which extends southwards to the top of Mt. Fuji. The lake water is recharged from the surrounding mountains of the Misaka Group. The lake water remarkably rises after the 3 days rainfall reaches more than 200 mm (Figs.6 and 7). When the 3 days rainfall is less than 100 mm, it is invisible. Fluctuations of the lake water coincide with the rainfall pattern (Fig.4). The graph of the accumulation value of the rainfall variation is similar to the fluctuation of the lake water level.