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Hydrologic contributions of subsurface flow during snowmelt and rainfall in a forest catchment, coastal British Columbia Kim, Hyeong Jeong


This research investigated hillslope subsurface flow processes and the mechanisms of runoff generation in a forested catchment in British Columbia in response to rainfall and snowmelt events. Observations at a hillslope segment included subsurface outflow from a soil pit, ground water levels, hydraulic conductivities, and snow water equivalent. Stream discharge and meteorological data were monitored in the catchment. Subsurface flow at the hillslope segment responded rapidly enough to inputs of rain and snowmelt to contribute to stormflow at the catchment scale. On a contributing area basis, outflow from the pit at the hillslope segment exceeded peak stream discharge and total runoff, indicating that subsurface flow is able to contribute significantly to peak stormflow and event runoff at the catchment scale. Estimates of catchment-wide subsurface flow cannot be reliably estimated by scaling up pit outflow using the ratio of pit length to the length of stream bank seepage faces in the watershed. Estimates of effective contributing area to pit drainage at the hillslope segment that were derived from runoff ratios, surface topography, and water-balances varied. Contributing area estimates based on runoff ratio were much higher than estimates based on topographic survey and water balance method. This lack of agreement indicates that it is problematic measured pit outflow to extrapolate to the catchment scale using contributing area ratios. Peak rainfall intensity explained 79% of the variation in total subsurface flow volume from mineral section 2 of the pit for 7 rainfall events in 1998-99 and total precipitation and 7-day precipitation prior to storm were not significant. However, drier antecedent soil moisture conditions have a major role in generating subsurface flow. During the snowmelt season, outflow from the organic horizon was generated as saturated throughflow and overland flow as a result of the rising water table. However, during the autumn storms, outflow from the organic horizon occurred as a lateral subsurface flow despite the fact that the mineral soil was unsaturated, possibly due to the existence of hydrophobicity at the boundary between the organic horizon and mineral soil. The results of this study contradict assumptions of quasi-steady state, topographically driven flow in models such as TOPMODEL and TOPOG. For the wet soil moisture conditions, the fraction of outflow from the individual mineral sections varied with time and with changes in total mineral horizon outflow throughout the melt season. For dry soil moisture conditions, the fraction of outflow from the organic horizon and individual mineral sections varied with changes in pit outflow. In addition, relations between hillslope discharge and water table elevation measured from the well throughout the melt season and during autumn storms showed marked hysteresis. Saturated hydraulic conductivity back-calculated from Darcy's law was more than an order of magnitude higher than the saturated hydraulic conductivity values derived from slug tests. However, both methods generated values within the range of results of other studies in forested areas. The larger estimated hydraulic conductivities are likely due to existence of root channels as preferred pathways in forested hillslope.

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