Abstract
Hydrological measurements were performed during and after rainfall from April to November in 1990 in the Enokigawa experimental basin close to Hiroshima city. The Enokigawa experimental basin was divided into two parts which were the urbanized lower reaches and the forested upper reaches with headwaters. Water quality of stream did not change remarkably by rainfalls in the urbanized lower reaches. There was also no clear change of water quality of stream in the forested upper reaches, except the SO42- concentration gradually increased in this observation period. The ratios of pre-event water flow (groundwater runoff) to the discharge increment of two storm events were shown to be 73 and 87% by the runoff separation using the 180 stable isotope method. These facts imply that the water quality of stream in the upper reaches depends almost on groundwater corresponding to headwaters.
