Abstract
Yakushima Island is facing to the East China Sea, 800 km east of Shanghai and has been exposed to acid rain of which the annual mean pH is 4.65 and the annual mean precipitation is about 4, 300 mm. Because of its steep landform and thin soil layer, the effect of acid rain on plants, soil layer and freshwater should be estimated by classifying it into the direct one in a short term and the other in a long term. Especially, the long term effect is important in terms of the critical load of acid rain to a watershed. The effect of acid rain on freshwater should be treated by separating it hydrologically between the one due to surface and prompt subsurface runoff components during stormy days and the other due to delayed subsurface and groundwater runoff components during dry days. The observations of river water quality in the island were made at lower reach points in all major rivers and from the upper reach to lower reach in two large mountainous rivers in summer 1992 and winter 1994. Most river waters were low pH (5 to 7), alkalinity (less than 0.1 meq ? 1-1), and electric conductivity (less than 40 μS cm-1). The linear regression relations among these three indices were obtained with high correlation coefficients. The storm runoff observation of a large mountainous river in winter 1994 indicated that the low values in pH (min. 5.56), alkalinity (min. 0.0194 meq?1-1) and ion concentrations were mainly caused by surface runoff during the short time at peak flow. Therefore, the long term effect of acid rain should be subtracted from that due to the storm runoff component.
