[1] To investigate the effects of bedrock permeability on the linkage between hillslope and riparian groundwater in a weathered granite headwater catchment, the groundwater dynamics were studied using intensive hydrometric and tracer observations. Water flow from the hillslope, through the hillslope/riparian interface, and into the riparian zone consists of two components: saturated through flow on the soil-bedrock interface during storms and groundwater flow within the permeable bedrock occurring year-round, except during the driest season. Most of the water, which will contribute to the stream, infiltrates the largest part of the catchment and the hillslope area and recharges the deeper groundwater body. Therefore bedrock permeability is an important factor in determining the hillslope-riparian linkage.Citation: Katsuyama, M., N. Ohte, and N. Kabeya (2005), Effects of bedrock permeability on hillslope and riparian groundwater dynamics in a weathered granite catchment, Water Resour. Res., 41, W01010,

[1] Recent studies have suggested that bedrock groundwater can exert considerable influence on runoff generation, water chemistry, and the occurrence of landslides in headwater catchments. To clarify water infiltration and redistribution processes between soil and shallow bedrock and their effect on storm and base flow discharge processes in a small headwater catchment underlain by weathered granite, we conducted hydrometric observations using soil and bedrock tensiometers combined with hydrochemical measurements and water budget analyses at three different spatial scales. Results showed that in an unchanneled 0.024-ha headwater catchment, saturated and unsaturated infiltration from soil to bedrock is a dominant hydrological process at the soil-bedrock interface. Annual bedrock infiltration ranged from 35 to 55% of annual precipitation and increased as precipitation increased, suggesting a high level of potential bedrock infiltration, partly explained by the high buffering capacity of the soil layer overlying the bedrock. This physical property of the soil layer was an important factor in controlling the generation of bedrock infiltration and saturated lateral flow over the bedrock. In a 0.086-ha watershed including the unchanneled headwater catchment, exfiltration from the bedrock toward the soil layer composed more than half the annual discharge.

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Abstract:We measured deuterium excess (d D υD 8υ18 O) in throughfall, groundwater, soil water, spring water, and stream water for 3 years in a small headwater catchment (Matsuzawa, 0Ð68 ha) in the Kiryu Experimental Watershed in Japan. The d value represents a kinetic effect produced when water evaporates. The d value of the throughfall showed a sinusoidal change (amplitude: 6Ð9‰ relative to Vienna standard mean ocean water (V-SMOW)) derived from seasonal changes in the source of water vapour. The amplitude of this sinusoidal change was attenuated to 1Ð3-6Ð9‰ V-SMOW in soil water, groundwater, spring water, and stream water. It is thought that these attenuations derive from hydrodynamic transport processes in the subsurface and mixing processes at an outflow point (stream or spring) or a well. The mean residence time (MRT) of water was estimated from d value variations using an exponential-piston flow model and a dispersion model. MRTs for soil water were 0-5 months and were not necessarily proportional to the depth. This may imply the existence of bypass flow in the soil. Groundwater in the hillslope zone had short residence times, similar to those of the soil water. For groundwater in the saturated zone near the spring outflow point, the MRTs differed between shallow and deeper groundwater; shallow groundwater had a shorter residence time (5-8 months) than deeper groundwater (more than 9 months). The MRT of stream water (8-9 months) was between that of shallow groundwater near the spring and deeper groundwater near the spring. The seasonal variation in the d value of precipitation arises from changes in isotopic water vapour composition associated with seasonal activity of the Asian monsoon mechanism. The d value is probably an effective tracer for estimating the MRT of subsurface water not only in Japan, but also in other East Asian countries influenced by the Asian monsoon.