Hillslope Runoff or Recharge? The Battle of the Boreal Plain

There is little known about hillslope runoff generation processes on the boreal plain region of Alberta, Canada. Hillslopes in this region are characterized by soils with thick unsaturated zones and no obvious confining layers to direct lateral subsurface flow. To quantify the precipitation and antecedent soil moisture conditions necessary to generate lateral flow (overland flow and interflow) from aspen forested hillslopes, we conducted rainfall simulation experiments on two 3 m2 plots. The WET plot was isolated from tree root uptake for much of the growing season. The DRY plot was not isolated from tree root uptake over the growing season. Runoff collection troughs were installed at the base of the forest floor (FF), Ae horizon, Bt horizon and at the base of the down slope pit face. The size and order of the simulated rainfall events were: 10, 10, 10, 20, 20, 40, 40 and 20 mm, each day, applied as rapidly and evenly as possible. The simulated event sizes correspond to relatively large and infrequent events in this region. At the initiation of the rainfall simulations, the mean profile (0-0.75 m) soil moisture content in the WET plot was 0.5 m3m-3 compared to 0.32 m3m-3 for the DRY plot. No overland flow occurred. Initiation of interflow on the WET and DRY plots occurred during the second 20 mm and first 40 mm events respectively. Interflow was produced during all subsequent events, occurring primarily in the clay-rich Bt horizon through preferential flow paths associated with roots. For all events combined, the WET and DRY plots had runoff coefficients of 22% and 5% respectively. For both plots, flow was predominantly vertical, filling available soil storage and recharging deeper sediments. When these results are considered in the context of the longer-term precipitation record, it apears that hillslopes only contribute lateral flow sporadically to receiving surface waters and wetlands. These results will have important implications for the way we model and manage boreal plain catchments.