HR: 1340h
AN: H53G-1504    [Abstracts]
TI: Asymptotic Response of Shallow Groundwater and Interactions Among Precipitation, Throughfall, and Stem Flows
AU: * Dhakal, A S
EM: adhakal@scopac.com
AF: Scotia Pacific Company, 125 Main Street, Scotia, CA 95565,
AU: Sullivan, K
EM: ksullivan@scopac.com
AF: Scotia Pacific Company, 125 Main Street, Scotia, CA 95565,
AB: Pore pressure head dynamics and heterogeneity were examined monitoring piezometric responses at 83- locations during three seasons (2004 ¨C 2007) in a rain dominated Doe Creek headwater watershed (13.4 ha) in Northern California. Also, during the hydrological year 2006-2007, throughfall and stemflows were monitored at 20 and 4 locations, respectively, to examine the dynamics of net rainfall amount reaching the forest floor and subsequently their potential influence on dynamics of piezometric responses. Soil depth in the study area varies from 0.7 m \¨C\ 1.8 m, and the soil is derived from rocks belonging to Wildcat geological group. No Hortonian or saturated overland flows were observed in the study area throughout the study period (2004- 2007) during which about 1,150 (return period 1.4-yr), 1780 (return period 20-year), and 1200- mm (return period 1.5-yr) annual rainfalls were recorded in 2004-5, 2005-6, and 2006-7, respectively. Positive pressure head occurred at all but four sites located in the divergent hillslope. Interestingly, none of the 83 locations exhibit full soil column saturation during the period. The ratio of pressure head to soil depth was > 0.5 only at five (6% of total sites), eleven (13% of total sites), and six (7% of total sites) piezometric locations in 2004-5, 2005-6, and 2006-7, respectively. The maximum piezometric response varied from site to site ranging from P/D values of 0.1 to 0.72. The peak pressure head during a storm was dependent on the short-term characteristics of rainstorm events and short-period antecedent conditions (¡Ü 2-weeks). The striking difference in the seasonal piezometric responses among hydrological years was on the base levels of piezometric responses than on the peaks. Due to continuous input of rainfall, positive pressure head persisted in most piezometers almost throughout the season during the wet year 2005-6. Despite incessant positive pressure head throughout the mid-winter in 2005-6 in most piezometers, many large rainstorms, occurred at the time when large positive pressure head persisted at the onset of a storm, did not generate dramatic peak pressure heads. The observed scenarios highlight the maxima related to upper limit of increase in pressure head with increasing rainfall. Such a peak threshold may be related to the increased efficiency of preferential flow pathways at higher soil water contents. This kind of asymptotic response for highly reactive piezometers was observed for pressure head return period > 8.4- months. Moderately and least responding piezometers, however, depict asymptotic pressure head return period threshold of 2.4-months. During large storms, the difference in net rainfall amount reaching the ground surface between open space and under the canopy was not significant. Therefore, owing to asymptotic nature of piezometric responses, interception was found less likely to affect the peak pressure head during a large storm.
DE: 1818 Evapotranspiration
DE: 1854 Precipitation (3354)
DE: 1860 Streamflow
DE: 1871 Surface water quality
DE: 1895 Instruments and techniques: monitoring
SC: Hydrology [H]
MN: 2007 Fall Meeting