In order to estimate a long term chemical change of groundwater and streamwater by acid deposi-tion, it is necessary to know both the pathway and concurrent chemical change of precipitation in a watershed. We observed temporal variations in the chemistry and isotopic composition of the ground-water and streamwater in a mountainous watershed during a storm. Based on these observations, we divided groundwater and streamwater into 3 components; precipitation, soilwater, and groundwater. By reproduction of streamwater and groundwater chemistry using this result, this separation method is proved to be effective for making a prediction model of streamwater and groundwater caused by acid deposition. Key words : Acid Deposition, Isotopic Composition, Streamwater and Groundwater Chemistry, Runoff components