STREAMFLOW IN A BOREAL CATCHMENT IS PRIMARILY VIA SUBSURFACE FLOW PATHS, BUT RAPID SUBSURFACE FLOW THROUGH SHALLOW MOSS AND ORGANIC HORIZONS CONTRIBUTES NEW WATER TO STREAM DISCHARGE

Climate-sensitive boreal forests can have shallow root systems and thick understory mosses that can influence soil permeability and hydrological processes. We examined these hydrological processes in a maritime boreal forest at both the hillslope scale, where interactions among precipitation, evapotranspiration (ET), soil characteristics, tree rooting depths, infiltration, and groundwater were monitored and the catchment scale, where precipitation, ET calculations, stream flow, and stream chemistry data were used to evaluate streamflow sources and water balances. Groundwater head, hydraulic conductivity, and gradient data were used determine hillslope groundwater fluxes; chemical hydrograph separation analysis was used to determine ‘old’ (presumably groundwater) and ‘new’ water contributions to streamflow. During spring snowmelt events, ‘old’ water initially contributed to the hydrograph, with progressively more ‘new’ water added towards the hydrograph peak. Comparison with hillslope groundwater data indicate that near-stream hillslopes that were nearly saturated prior to snowmelt provided initial contribution to streamflow during snowmelt. Infiltrating snowmelt that moved rapidly through permeable moss and organic horizons contributed the 50-70% ‘new’ water that constituted snowmelt hydrograph peaks. During summer months, stream discharge declined as significant precipitated water was used for forest transpiration. Stream chemistry became progressively more solute-rich as evapotranspiration returned much of the precipitation to the atmosphere and increased the solute load of the shallow groundwater. Hydrograph separation analysis indicate that nearly all summer discharge was ‘old’ water. Only a little ‘new’ water, which was added during storms, took rapid, shallow subsurface paths to the stream. Although almost all flow to the stream took subsurface flow paths, newly infiltrated water that saturated moss and organic horizons during low ET periods moved rapidly to streams to arrive at the stream as chemically ‘new’ water.