In hydrological cycle investigations in large area which has severe' river basins, first of all we must define a general boundary condition considering ground water flow inter basins, before the application of detailed numerical models such as for ground water resource management.
Thus, in this paper, we take up as an applied problem of water balance method in the western piedmont of Aso volcano, because in such volcanic basin lava flows may be found as widely spread plains over some river basins and the lava beds include highly developed fractures and are highly permeable along the fractures, so ground water artery tend to spread widely beyond the divide.
Then in this study, we can estimate E (τ) with water balance equation ( 1 ) in a closed basin. Because P(τ) (precipitation), O(τ) (depth of run-off) are known quantities obtained by observation and ds (change of basin storage) can be neglected to first approximation by taking to an annual period of hydrological balance. Standard value of an annual loss E(τ) is equal to an annual evapo-transpiration in basin, so we assume first a standard value of an annual evapo-transpiration, next we estimate E(τ) each basin and compare estimated E(τ) with a standard value. The difference of the two represents unknown ground water flow rates inter basin. we investigated the actual circumstances to be based on the results. In this way a result which supports the assumption was obtained, that is, the following became clear, in the case of annual evapo-transpiration in basins = (700mm), ground water flows into the Kase River basin from the Shira River basin, the Gohshi River basin, the Midori River basin, their respective rates of ground water movement are 1.8×108m3/year, 0.6×108m3/year, 0.7×108m3/year. On the other hand from the Gohshi River basin ground water flows out to other basins rate of 1.2×108m3/year.