比良山系大谷川流域源流部における侵食過程とそれに関連する微地形

Abstract

Discharge of two springs draining a zero-order valley at the headwaters of the Otani River in the Hira Mountains, Shiga Prefecture, was analyzed in relation to the hydrogeomorphological processes producing an interesting microtopography. The hydrographs of the two springs obviously showed three runoff components, I.e., direct runoff, throughflow and baseflow. Direct runoff was analyzed using a time variant function representing the contributing area of a concentration time within a given value, with assumption that the saturation overland flow feeds the direct runoff. Two modes of time variation in the contributing area thus calculated suggest different modes of additional direct flow superposed on the saturation overland flow on the seepage surface of the springs. One is subsurface matrix or pipe flow from extended seepage area at the time of increased groundwater discharge to the springs. The other is a surface flow induced by the stemflow, which represents a kind of Horton overland flow when the rainfall intensity is extremely large. This mode of direct runoff was witnessed at the time of Typhoon No.20 on 30 September, 1990.