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Seepage area and rate of bedrock groundwater discharge at a granitic unchanneled hillslope
Author(s) -
Uchida Taro,
Asano Yuko,
Ohte Nobuhito,
Mizuyama Takahisa
Publication year - 2003
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1029/2002wr001298
Subject(s) - bedrock , hydrology (agriculture) , geology , hydrograph , groundwater , surface runoff , streamflow , drainage basin , subsurface flow , geomorphology , geotechnical engineering , ecology , cartography , geography , biology
Recent studies have demonstrated the importance of water movement through the bedrock in the rainfall‐runoff process on steep hillslopes. However, quantitative information on this process is still limited. The objective of this study was to address the following questions: (1) How large is the area where bedrock groundwater seeps into the soil layer, and (2) what is the rate of water flow out of the bedrock? To address these questions, detailed hydrological, hydrochemical, and thermal measurements were conducted at a forested steep unchanneled granitic concave slope in the Tanakami Mountains, central Japan. The relationship between the amplitude of annual soil temperature variation and the measurement depth showed that in a normal low‐flow period, the seepage area ranged between 14 and 21 m 2 and the ratio of this area to that of the whole catchment was about 2.0%. In a drought period the seepage area ranged between 3.5 and 5.5 m 2 , and the ratio to the whole catchment was around 0.5%. The variation in the area of seepage was controlled both by the short‐term precipitation pattern during the preceding several weeks and by the long‐term pattern over several preceding months. A two‐component geochemical hydrograph separation indicated that the ratio of bedrock groundwater to streamflow was about 0.82 for the normal low‐flow periods and 0.90 for the drought period. The rate of flow out of the bedrock into the soil layer ranged from 0.5 to 3.3 m 3 d −1 . That is, although the seepage area was small (0.5–2.0% of the catchment), the contribution of bedrock groundwater was considerable (50–95% of streamflow).