Tuesday, November 14, 2006

Contribution of Weathered Granitic Bedrock to Hydrological Processes in Headwater Catchments.

Shin'ya Katsura, Ken’ichirou Kosugi, and Takahisa Mizuyama. Kyoto Univ., Kitashirakawa Oiwakecho, Sakyoku, Kyoto, Japan

To evaluate the contribution of bedrock to hydrological processes in headwater catchments, we carried out detailed hydrological observations in a granitic headwater catchment in central Japan. Using a coil-type TDR probe (made by the authors) together with commercially available profile soil moisture sensors, we monitored the water content profile from the soil surface through 11-cm depth from the bedrock surface for 1 year. Even rainfall events with relatively small cumulative rainfall of 15 mm increased the bedrock water contents, and the increments were comparable to those in the soil. After the end of the rainfall events, the bedrock water content displayed a more rapid drop than the soil, and varied little during the period of no rainfall. The water storage, calculated as the total water volume held, showed similar tendencies. We also monitored the groundwater level in three boreholes inside and outside the catchment (depth: 12, 15 and 20 m). The groundwater levels continued to decrease from the start of the observations (26 October 2005) through early February 2006, and then began to increase. Rainfall events had little impact on them. The groundwater table lies nearly horizontally at the level corresponding to the catchment outlet, where a perennial stream originates. The saturated hydraulic conductivity, measured for four intact bedrock core samples, ranged from 1.39×10-5 to 1.68×10-4 cm s-1, equivalent to the rainfall intensity of 0.5 to 6.0 mm hr-1. These results suggest that (1) the bedrock water content is controlled by relatively clearly distinguishable macropores and micropores within the bedrock, (2) base flow is sustained by the water discharged from the bedrock, and (3) bedrock, conventionally treated as impermeable, does conduct and hold substantial amounts of water, and therefore contribute greatly to hydrological processes in headwater catchments.