HOW DO RIPARIAN ZONES AND GEOMORPHIC HOLLOWS AFFECT STORMFLOW GENERATION IN HEADWATER CATCHMENTS?

Abstract

Headwater catchments are important source areas for water, sediments, and nutrients in mountain streams, yet processes by which these materials are transported to channels remain in question. In mountainous terrain, these catchments are typically highly dissected with narrow riparian corridors and steep side slopes. Recently a new hydrogeomorphic paradigm for stormflow generation in headwater catchments has been proposed that examines three distinct subunits: (1) riparian corridors and channels; (2) linear hillslopes; and (3) zero-order basins (geomorphic hollows). Both the first and third of these subunits would be considered ‘wetlands’ depending on definitions. Studies in Japan and other temperate regions suggest that most of the total storm runoff is generated from narrow riparian corridors via saturated overland flow or channel interception during relatively dry conditions. For slightly wetter conditions, subsurface flow from the soil matrix contributes substantially to storm hydrographs. As wetness further increases, preferential flow in soils contributes to stormflow by expanding into self-organized systems that drain significant portions of the hillslope. Concave geomorphic hollows appear to accumulate subsurface water, and once a threshold is reached (depending on soil depth and other conditions), these hollows may begin to contribute significantly to stormflow generation. Thus, from dry to wet antecedent conditions in steep, incised headwater catchments, there is a large shift from primarily non-Hortonian overland flow from the riparian zone to a complex multi-component response that includes discharge from geomorphic hollows, subsurface matrix flow, and preferential flow. Most of these hydrologic responses during the wet season exhibit non-linear behavior. At larger catchment scales where the riparian corridor expands, the connectivity between hillslope hydrology and stormflow response diminishes and runoff from these gentler gradient regions is more controlled by processes within the wet riparian corridor.