The Shira, the Gohshi and the Kase River basin which are located on the western slope of Mt. Aso-Somma are the area for ground water recharge, and in this area it had been clear that ground water flow tend to spread widely beyond the divide of ground surface. And here 700,000 of people live in Kumamoto city and surrounding country.
Then, this study aims at making clear that since 1975 actual state of hydrological cycle in the middle area of the Shira River basin have changed due to being on the decrease in volumes of irrigating intake. Now, the middle area of the Shira River basin is unclosed drainage, so water balance equation on the unclosed drainage basin is the following (1).
Σ (P-Q-E) · Δt=Σ (ΔS)- Σ (GI-Go) · Δt (1)
P (precipitation) and Q (depth of run-off) are known quantities obtained by observation. As the standard value of an annual E (evapo-transpii-ation) in this basin 700 750mm have been estimated, and S (variation of basin storage) can be reduced to zero by taking for a long term. But on the other hand GI, Go (depth of ground water inflow or outflow) cannot be neglected. In brief, river water which is taken in irrigating the paddy fields in the middle area of the Shira River basin turns into ground water in neighboring the Kase River basin, and run off to channel of the Kase.
Accordingly, by estimating the second term of right side in Eq (1), that is, rechrging volumes of ground water, we can show the change of hydrological cycle mechanism.
The results indicated that in the last 10 years the decrease in recha r ging volumes in the middle area of the Shira River basin is at a remarkable rate.

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For clarifying the movement of seepage and groundwater at the rock fill dam, water quality analysis was carried out before and after filling the reservoir. Based on the results, the groundwater was classified by multivariate analysis, the relationship among the seepage, surface water and its geology was investigated through the water quality changing with time.
The results were clear as shown below:
(1) The groundwater could be clas sified satisfactory by the principal componenta nalysis and cluster analysis based on the water quality such as Cl^-, SO₄^2-, Na⁺, k⁺, Ca²⁺, Mg²⁺.
(2) It was estimated that the right abutment, the left foundation and the left abutment of the dam are the flow routes of seepage.
(3) The flow routes shown by the above were prover by the concentration of radon in water.
(4) The characteristics of the groundwater quality by the rock property through wh ich it passed were clearly shown.

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