HR: 1340h
AN: H23C-1437    [Abstracts]
TI: Shallow Groundwater Response to Rainfall in a small Forested Watershed in Northwestern California
AU: * Dhakal, A S
EM: adhakal@scopac.com
AF: Scotia Pacific Company, LLC, Main Street, Scotia, CA 95565 United States
AU: Sullivan, K
EM: ksullivan@scopac.com
AF: Scotia Pacific Company, LLC, Main Street, Scotia, CA 95565 United States
AB: Shallow groundwater dynamics and heterogeneity related to rainfall and streamflow are examined to investigate influence of rainfall characteristics, location, and hydrogeomorphic properties within a watershed on shallow ground water tables. Findings are presented from one season (2004-2005) monitoring of shallow groundwater responses at 95 locations (soil depth vary from 0.7 to 1.8-m) in a complex topography underlain by erosive and unstable Wildcat geological group. Hillslope with different geomorphic significance in the 16-ha headwater sub-watershed of Elk River basin (135 km2) located in Northern California and dominated by redwood forest (stand age 60-years) was instrumented with 71 stand pipe piezometers and pore pressure head were manually measured following each rainstorm event; an additional 12 locations measured groundwater table in wells. Furthermore, a continuous pore pressure head was observed automatically at a temporal resolution of 15-minutes at 12 locations. Streamflow and rainfall were also monitored throughout the season. A vector Digital Elevation Model (DEM) with 1-m contour interval was derived for the entire sub-watershed through exhaustive field topographic survey undertaken with a total station. Basic soil characteristics were determined in the laboratory from 26 soil samples collected at the field. No Hortonian or saturated overland flows were observed in the study site throughout the monitoring period (November - June.) during which about 900 mm of rainfall was recorded. Positive pore pressure occurred at 72 out of 73 manual piezometric sites; all 12 automatic piezometers depict positive pore pressure. Interestingly, none of the 95 locations exhibit saturated condition throughout the season including during the high intensity (up to 9mmhr-1) rainstorm events that occurred in the wettest period (March and April). The ratio of pressure head to soil depth was > 0.5 only at five and three manual and automatic piezometric sites, respectively. Absolute water table height up to 0.9 m was observed at a few locations. Timing and magnitude of piezometric response depended on rainfall characteristics including the antecedent rainfall. Streamflow exhibited larger peaks during the late winter coinciding with larger rainstorm events. The data collected will be used to test the internal structure and applicability of existing distributed hydrological models in the region.
DE: 1829 Groundwater hydrology
DE: 1830 Groundwater/surface water interaction
DE: 1848 Monitoring networks
DE: 1854 Precipitation (3354)
DE: 1860 Streamflow
SC: Hydrology [H]
MN: Fall Meeting 2005