Effect of volcanic ash on water quality of rainfall, stream and spring water

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Sakurajima, a volcano standing amidst the Kagoshima Bay in southern Kyushu, Japan, has been characterized by the explosive volcanic activity. Sakurajima Volcano has erupted continuously with emission of ash since 1955. As the result, the forested watersheds around Sakurajima Volcano are covered with volcanic ash. The deposits of volcanic ash in the forested watersheds cause lowering of infiltration capacity, increase of surface runoff and rapid progress of erosion. The surface runoff on ash-covered hillslope was launched simultaneously with the rainfall intensity exceeding 1.5-2.0mm in ten minutes. Moreover, the deposits of volcanic ash effect on water quality of stream and spring water.We have carried out some hydrological observations in the forested watershed covered with volcanic ash in the Takakuma experimental forest, Kagoshima University. The purpose of these observations is to understand effect of volcanic ash on water quality of stream. The results obtained by the analyzes of the hydrologic data observed in 1994 and 1995 are summarized as follows :1. The pH in the stream water was within the value-range from 6.8 to 6.9 during the fewer volcanic eruptions. On the other hard, the pH during the more frequent volcanic eruptions was within the value-range from 6.6 to 6.8.2. The pH in the stream water depend on the volcanic activity.