Water flow processes in weathered granitic bedrock and their effects on runoff generation in a small headwater catchment

Abstract

[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.

Contributing sources to baseflow in pre‐alpine headwaters using spatial snapshot sampling

“…Here, hydrochemical patterns were attributed to differences in vegetation, geologic substrates and wetland areas. Other studies found soil depth (Buttle et al, 2004;Kosugi et al, 2006) or active zones of seeping deep groundwater (Asano et al, 2009;Zimmer et al, 2012) to be important factors for baseflow generation. However, the importance of different spatial controls varies with geographic settings and especially steep pre-alpine regions with high precipitation amounts (P > 2000 mm year -1 ) are not fully understood yet.…”

Estimation of Bedrock Infiltration on a Weathered Granitic Mountain Covered by Japanese Cypress Forest using Water-Budget and Eddy Covariance Methods

“…Waichler et al [2005] suggested that groundwater recharge as a loss term in water balance could account for about 12% of precipitation in three small catchments (0.06-0.1 ha) in H. J. Andrews Experimental Forest, Oregon, USA. Kosugi et al [2006] conducted hydrometric observations in different-scale weathered granitic catchments and suggested that annual bedrock infiltration ranged from 35% to 55% of annual precipitation in an unchanneled 0.024-ha headwater catchment underlain by weathered granite. In addition, in a 0.086-ha watershed including an unchanneled 0.024-ha headwater catchment, exfiltration from the bedrock toward the soil layers made up more than half of the annual discharge.…”

“…Waichler et al [2005] used observed values of E from eddy covariance or sap-flux measurements in 1999 and 2000 to simulate changes over a long period . Under the assumption that Q B is small for a relatively large watershed containing several headwater catchments, Kosugi et al [2006] applied the evapotranspiration rate estimated by short-term water-budget methods to larger watersheds containing a headwater catchment.…”

Effects of bedrock groundwater on spatial and temporal variations in soil mantle groundwater in a steep granitic headwater catchment

“…Although subsurface flow through bedrock has conventionally been ignored in modeling small headwater catchments, many recent studies have suggested the contribution of bedrock groundwater flow to hydrological processes and landslides in headwater catchments. Kosugi et al [2006] used pressure head measurements in shallow bedrock to demonstrate that unsaturated bedrock infiltration dominantly occurs in the middle-slope to upslope regions where a thick soil layer dampens rainfall intensity. The ratio of bedrock infiltration to annual precipitation in their small unchanneled catchment ranged from 35 to 55% .…”