| Volcanic eruptions can affect key components of the hydrological cycle. Explosive volcanic eruptions often emit enough volcanic ash to completely cover the land surface near the volcano. The previousstudies have reported the dramatic increase in the sediment and water discharge from such a volcanically disturbed watersheds. In order to clarify the effect of volcanic ash deposits on hydrologic and erosional processes, the authors conducted field hydrologic measurements at the slope of the Miyakejima volcano having erupted in 2000 and the laboratory hydrologic tests of the volcanic ash. As aresult of the survey and tests, the following were clarified: 1) The volcanic ash has dramatically reduced the hydrological conductivities. Hortonian overland flow is a dominant runoff processes of ash-mantled hillslopes. Less surface runoff is generated at slopes mantled by coarse ash, mantled less thickly, or covered with a litter layer or grasses; 2) Erosion rates measured at volcanically disturbed slopes are considerably high and depend on the water discharge from the slope; 3) In recovering processes, sediment yield from the revegetated slope has decreased substantially while that from the unvegetated slopes has not. |
