Throughflow variability during snowmelt in a forested mountain catchment, coastal British Columbia, Canada

This study documented the spatial and temporal variability of outflow from a forested hillslope segment during snowmelt at a small mountain catchment in south coastal British Columbia, Canada. A pit 5 m wide was established just upslope from the stream channel. Outflow from the organic horizon was intercepted and measured by a single trough, and outflow from the mineral horizons was measured separately for three adjacent sections. Throughflow exhibited non-steady-state behaviour involving shifting allocations of flow amongst different sections of the outflow pit, as well as threshold effects and hysteresis in the relationship between pit outflow and water table elevation. Most of the pit outflow originated from the mineral horizons, indicating that throughflow was the dominant pathway by which water was delivered to the stream channel. Direct precipitation and snowmelt onto near-stream saturated areas can account for less than 20% of the total outflow from the hillslope segment. Throughflow from the mineral sections consistently peaked either at the same time as or earlier than stream flow from the 150-ha catchment during diurnal snowmelt cycles, indicating that throughflow appears to respond rapidly enough to contribute to snowmelt-induced peak stream flow. Pit outflow cannot be extrapolated reliably to the catchment scale on the basis of simple length- or area-based ratios. Copyright (C) 2004 John Wiley Sons, Ltd.