“Shirasu” is a popular name of non-or slightly welded pumice flow and fall deposits which are widely distributed in southern Kyushu. The deposits are rich in pore space and fissure, accordingly, specific discharge of the “Shirasu” area is generally a range of 0.02-0.04m³/sec/km² in low water season for. the drainage basin of about 100km² or more.
The author measured specific discharges at two hilly districts composed mainly of “Shirasu” in low-water season. Each districts are divided into small units of watersheds of 0.05-1.0km² according to topographic features. As results, contrary to expectation, specific discharges are widely dispersed within a range of 0.003-0.157m³/sec/km². The cause of above-mentioned dispersion considered as below: namely, the ground water of the “Shirasu” area pours out towards the streams not always equal quantity around the ridge but within some definite direction affected by the following factors.
1) unisotropy of permeability caused by unhomogenuity of “Shirasu” deposits.
2) geomorphologic feature of each watersheds.
3) existence of concealed hydrogeologic barrier.
So far as the area studied, the author concluded that dispersion of specific discharge is caused by the existence of concealed hydrogeologic barrier, based on relief of boundary between “Shirasu” deposits and underlying non-pyroclastic sediments presumed from geologic background.
And from the results, it is pionted out the way for presuming concealed geologic structure is clarified from hydro-geologic measurements of specific disharge in low-water season.