山地小流域に関する陸水学的研究(V) ―山腹斜面の土壌水分収支と溪流の基底流出の関係―

Abstract

It had been suggested by the former reports that the base flow from the Takigataniexperimental basin in Tanakami mountain range originates from unsaturated soil moisturein the hillalopes. In this report, the water balance of soil moisture in the hillalopes wasexamined in more detail so as to elucidate its relation with the base flow.First, the characteristics of the soil moisture in the hillslopes with different vegetativecover was compared in qantitative terms. It turned out that diffusivity of soil moisture ina bare-soil land in drying process was larger than that in a forest land. Further analysisrevealed that it is due to the fact that the top soil of the bare-soil land has much smallervalue of the capacity of soil moisture retention than that of the forest land whereas theunsaturated permeability is lsrger in the forest land.Next, the water balance of the soil moisture was examined for the point which seemedto be representative of prevailing vegetative cover in a subbasin of the experimental basin, according to the result of the observation of soil moisture profile and the soil moisture potentialat different depths. It was revealed that the recharge and the discharge of soil moisture occurmainly at the top layer of the depth of shout 1. 5 meters. Both the surface gradient ofsuction and that of soil moisture content indicated the existance of an uninterupted upwardflux of soil moisture except for the time of the infiltration of rain water. At the depthsgreater than 1. 5 meters, the soil moisture is almost in a steady state, where the flux isoriented downward.The calculated value of the volume of stream discharge based on the water balance ofthe soil moisture was reasonably consistent with the observational value.