RUNOFF PRODUCTION IN A FORESTED, SHALLOW SOIL, CANADIAN SHIELD BASIN

Storm flow in forested basins on the Canadian Shield is largely suppli ed by subsurface water; however, mechanisms by which this water reache s the stream remain unclear. Side slope contributions to storm flow we re studied using throughflow trenches on slopes in a headwater basin n ear Dorset, Ontario. Discharge, soil water content, and chemical and i sotopic signatures of subsurface water were monitored at each site. Fo ur hypotheses were tested: (1) most flow occurs at the soil-bedrock in terface on shield slopes with thin soil; (2) a significant fraction of event water moves vertically to bedrock via preferential flow pathway s and laterally over the bedrock surface; (3) relative preevent water contribution to subsurface flow on shield slopes is a function of soil thickness; and (4) a significant portion of event water flux in storm flow from forested basins with shallow soil cover is supplied from si de slopes via subsurface flow along the soil-bedrock interface. Hypoth esis 1 was confirmed from hydrometric observations during spring and f all rainstorms. Hypotheses 2 and 3 were supported by temporal trends i n dissolved organic carbon and O-18 in flow at the soil-bedrock interf ace and by isotopic hydrograph separations (IHSs) of hillslope runoff. Comparison with the streamflow IHS indicated that event water flux fr om the basin in excess of that attributable to direct precipitation on to near-channel saturated areas could be supplied by flow along the be drock surface (hypothesis 4). Flow at the soil-bedrock interface on si de slopes also contributed similar to 25% of preevent water flux from the basin. Much of the event water component of basin storm flow may t ravel considerable distances via subsurface routes and is not necessar ily contributed by surface runoff processes (Horton flow or saturation overland flow). Therefore the assumption that event water undergoes l ittle interaction with the soil during its passage downslope may be un warranted here.