Snowmelt runoff pathways in a boreal forest hillslope, the role of pipe throughflow

Abstract

On an instrumented slope of the Lac Laflamme basin, we observed that, during snowmelt season: (1) the unsaturated matrix flow seems to be a negligible component of the downslope flow through the soil horizons; (2) the groundwater flow from an aquifer in till, a few meters thick, is the major pathway for rain-melt inputs; and (3) when groundwater levels are high, a turbulent pipe throughflow occurs at the mineral and organic soils interface. Gauging one of the subsurface streams showed that, although its discharge is correlated and synchronized with nearby groundwater levels, its hydrograph often reflected that of rain-melt input. According to our calculations, pipe throughflow does not carry more than one fifth of the seasonal rain-melt input. During short periods (hourly basis) however, it may become the main source of water input to the lake. The water delivered by pipe throughflow forms a distinct 0.5–1 m thick acidic layer (pH: 4.4–4.8) that invades the lake just beneath the lake ice cover and is susceptible to affect the fish spawning ground.