Abstract
Oxygen and hydrogen isotopic ratios and Cl- and SO42- concentrations of rain and river water were measured for the purpose of understanding the discharge process in a river source region, and the following observation results were obtained. The isotopic ratios and the chemical concentrations are higher in forest than outof forest, implying evaporation on leaves and wash out the chemicals sticking to leaves. River water keeps fairly constant isotopic ratios and chemical concentrations. This means that a reservoir of groundwater discharging the river is large and homogeneous. Exceptionally, during snow melting as well as continuous heavy rainfall, the river water is affected, especially in the chemical concentration, by discharge of the snow melt water, or those rain water. In estimation of the direct runoff rate of rain water based on variations in the isotopic ratios and the chemical concentration of river water, the estimate rates are different between bases of the isotopic ratio and the chemical concentration, and beside between Cl- and SO42- concentration during a decreasing of the rate after the highest rate. These imply that the isotopic ratios and the chemical concentration of base flow groundwater or direct runoff rain water are not always the same as the values of river water before the beginning or the average value of rain water outof forest.
