山地源流域の湧水の形成過程に及ぼす岩盤地下水の影響

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  • Contribution of Bedrock Groundwater on Perennial Spring at a Forested Headwater Catchment
  • サンチ ゲンリュウイキ ノ ワキミズ ノ ケイセイ カテイ ニ オヨボス ガンバン チカスイ ノ エイキョウ

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<p>To evaluate the roles of bedrock groundwater on runoff generation in a granitic steep headwater catchment, runoff, soil pore water pressures, soil temperatures and water chemistry were simultaneously at Fudoji headwater catchment, Tanakami, central Japan. The catchment is covered with a closed natural forest, and is underlain by granite. Stream water at Fudoji headwater catchment is delivered from two perennial springs, one from the soil matrix and the other from a crack in the bedrock.</p><p>Tensiometers suggested that a saturated zone was existed near the spring throughout the observation period. Upslope tensiometers indicated that a saturated zone was not formed at the upper hollow during the period of baseflow, and that most of storms produced a transient saturated groundwater zone above the bedrock. Hydrometric observations also indicated that downward hydraulic head gradients existed during the baseflow period within both near the spring and the upslope area. However, results of soil temperature suggested that the thermal variation near the spring might reflect the effect of upward water flow from the bedrock zone. Hydrochemical observations supported to the emerging of bedrock groundwater near the spring. From these results, it is summarized that, during the baseflow period, two flowpaths (infiltration in soil layer and water emerging from bedrock) meet in a small area near by channel head and form a perennial saturated zone. These results also indicated that the bedrock groundwater had a small impact on the formation of transient groundwater at the upslope area.</p><p>The larger SiO2 concentration was found at the lower sampling site. The highest SiO2 concentrations were found in the bedrock spring. Unlike SiO2, however, there was no significant difference in the Cl- concentration among perennial groundwater, spring water, stream water, and bedrock spring water. The pattern of SiO2 concentration indicated that the ratio of bedrock groundwater was greater than that of perennial groundwater near the spring. Moreover, the SiO2 concentration of stream water decreased with the increase of streamflow, suggesting that the ratio of bedrock groundwater discharge to streamflow decreased with the increase of streamflow. During the baseflow period, the ratio of bedrock groundwater discharge ranged from 34 to 93% and the mean value was 78%.</p>

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