Present and Ancient Water Balance of Lake Hangetsu

Lake Hangetsu, situated at the western foot of Mt. Yotei in the western part of Hokkaido, is a closed crater lake with no outlets or inlets. The lake area is 0.045 km2 and that of the surrounding basin is 0.085 km2. There are fossil trees in the lake to about 10 m offshore from the present shoreline and these are 20 cm or 30 cm in diameter. The stock above the lake-level has disappeared through weathering, but the stumps remain taking root into the boulders and pebbles on the lake floor. Presumably, there was a dry epoch when these trees flourished (some 1700 A.D. according to C14 dating) and the lake-level was 0.5 m lower than the present one.The precipitation through the dry epoch in which the lake-level had been in equilibrium at the height of 0.5 m below the present lake-level, can be estimated by the ancient water budget. Assuming that evaporation and evapotranspiration do not noticeably differ between the present and the dry epoch, it is estimated that the annual precipitation of the dry epoch was approximately 1000 mm against 1600 mm for the present.