WPGM (Western Pacific Geophysics Meeting) is one of regional assembly of AGU. This year's WPGM was held at Yoyogi, Tokyo, on the end of June 2000. I made a poster presentation there. Following is the summary submitted.
The on-line abstract can also be seen via WWW from AGU's WWW server.
Also, you can browse all Posters (18 Sheets of A4 paper) used in my presentation via WWW.
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Hydrologists have stated that the one of the most serious problem to understand the hydrological characteristics of volcanic area is the extraordinary spatial complexity of geological and topological condition. All volcanoes in Japan, however, are simply classified into only 3 categories from the viewpoint of the histories of volcanic activities. Moreover, some geomorphic analyses showed that many stratovolcanoes have experienced the quite similar erosion history. These facts mean that apparent complexness of hydrogeological conditions of volcanoes could be explained by means of the concept of "stage of the landform development sequence " of volcano body.
The author calculated the total(q) discharge from groundwater springs for 5 stratovolcanoes in Japan. Then the runoff ratio values(r) of springs were obtained as the ratio of q to water input amount (precipitation minus evapotranspiration). The values of r for volcanoes studied were various, and have strong relationships between the baseflow runoff amounts of neighboring rivers.
Among some topographical factors investigated, the degree (Dd) of dissection of volcanoes that had strongest correlation to the value of r. The value r will decreases as Dd increase.
The volcanoes studied are classified into 2 groups: less eroded "early stage" volcanoes (Volcano Fuji, Shiribeshi and Kampu), whose value of r is about 100%, and well dissected "late stage" ones (Volcano Rishiri and Yatsugatake), which have r values of much less than 20%.
The author classified the springs on the volcano bodies into 4 types: L1 type springs are located at the terminal slope of lava flow or pyroclastic flow. L2 type springs run off from the sidewall of lave flows. F type springs are located on the surface of piedmont fan. B type springs are located on the basement rock of each volcano.
For all springs on 4 of 5 studied volcanoes, the author determined their type and obtained the clear relationship between the erosional stage and the dominant spring type: in the early stage volcanoes, L1 was dominant and the discharge from spring are balanced to the water input, while in the late stage ones almost all springwater discharges from F type springs.
In early stage volcanoes, like Mt. Fuji, many lava flows mainly cover the volcano body. These lava flows reach to the mountain foot and the L1 type springs are found at the terminal point. From the previous study on Mt. Fuji, it can be said that L1 springs are mainly maintained by large groundwater flow system in some specific lava flow. Almost all precipitated water moves along this flow system and discharges at the terminal of lava flow, which make L1 type springs.
On the late stage volcanoes, however, no lava flow reaches to the foot. In volcano Yatsugatake, the main groundwater flow system is very deep and the age of spring water is known to be about 20-50yrs. This deep groundwater flow path seems to have nothing to do with the existing lava flow. And quite small ratio of water from atmosphere moves down along this type of flow path.
The hydrogeological conditions of Japanese stratovolcanoes are quite different to each other volcano bodies. These differences, however, can be explained by the erosional stage that each volcano is situated on, rather than mere complexness of volcanic geology and/or topography.