Variation of water balance of Lake Mashu — A closed volcanic lake in Japan

Summary

The small annual change of lake level is due to the smallness of the intake area and the relatively high leakage. A rapid rise and two types of decrease were characteristic in the change of water level of this lake.

One is a slow seepage, 1–5mm/day which increases at higher stage and decreases at the lower stage having 351.15 m as the critical level. This is an evidence that the seepage occurs mainly around that level, where the boundary of lava and tuff is found. The other is the rapid seepage right after rain having averagely 4 mm/h of rate. This type is explained by the increase of partial hydraulic gradient caused by the rapid rise of water surface. Thus the water balance of this lake is mainly influenced by geological conditions, rainfall and water level. And as a result, a lowering of water level in these years was observed presumably caused by deficit of rainfall in 1984.