Runoff Generating Processes in Boreal Forest Environments with Glacial Tills

Abstract

This review describes the processes by which runoff is generated from rain or snowmelt in a boreal forest landscape with a shallow layer of glacial tills overlying a relatively impermeable bedrock. Glacial tills are the main surface deposits in the Nordic countries, and are also of frequent occurrence in other boreal forest environments in the Northern Hemisphere. Soil moisture and groundwater conditions control the partitioning of rain or snowmelt into evapotranspiration, runoff and temporary storage. Groundwater is viewed as the major determinant of runoff generation, both because it is the most important contributor to event flow, and because of its influence on saturation overland flow. A characteristic feature of till deposits is a saturated hydraulic conductivity which decreases rapidly with depth. As a consequence, topography has a significant impact on runoff by controlling movement and storage of water through convergence and divergence of flow. Modeling and field experience point to the idea of a continuum in both spatial and temporal occurrence of saturation overland flow and subsurface flow within individual catchments under different rainfall and snowmelt events and antecedent groundwater and soil moisture conditions. Results from studies using hydrological models with a structure which may be suitable for landscapes with glacial tills are presented.