Runoff Production in a Forested, Shallow Soil, Canadian Shield Basin

Abstract

Storm flow in forested basins on the Canadian Shield is largely supplied by subsurface water; however, mechanisms by which this water reaches the stream remain unclear. Side slope contributions to storm flow were studied using throughflow trenches on slopes in a headwater basin near Dorset, Ontario. Discharge, soil water content, and chemical and isotopic signatures of subsurface water were monitored at each site. Four hypotheses were tested: (1) most flow occurs at the soil-bedrock interface on shield slopes with thin soil; (2) a significant fraction of event water moves vertically to bedrock via preferential flow pathways 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 side slopes via subsurface flow along the soil-bedrock interface. Hypothesis 1 was confirmed from hydrometric observations during spring and fall rainstorms. Hypotheses 2 and 3 were supported by temporal trends in dissolved organic carbon and •80 in flow at the soil-bedrock interface and by isotopic hydrograph separations (IHSs) of hillslope runoff. Comparison with the streamflow IHS indicated that event water flux from the basin in excess of that attributable to direct precipitation onto near-channel saturated areas could be supplied by flow along the bedrock surface (hypothesis 4). Flow at the soil-bedrock interface on side slopes also contributed ---25% of preevent water flux from the basin. Much of the event water component of basin storm flow may travel considerable distances via subsurface routes and is not necessarily contributed by surface runoff processes (Horton flow or saturation overland flow). Therefore the assumption that event water undergoes little interaction with the soil during its passage downslope may be unwarranted here. One concept which may account for rapid subsurface transfer of preevent water to stream channels is the groundwater ridging mechanism [Sklash and Farvolden, 1979]. This process is limited to the near-channel area and results from rapid conversion of the tension-saturated zone (capillary fringe) tophreatic water by infiltrating event water. The subsequent rise in the near-stream water table increases the hydraulic gradient to the stream and/or the size of seepage faces, resulting in pronounced groundwater discharge to the channel. This rapid Copyright 1995 by the American Geophysical Union. Paper number 94WR03286. 0043-1397/95/94WR-03286505.00 rise in the water table has been noted in laboratory [Abdul and Gillham, 1984] and field investigations [Novakowski and Gillham, 1988; Abdul and Gillham, 1989] and is supported by numerical simulation [SMash and Farvolden, 1979]. However, studies on the Canadian Shield have questioned the ability of groundwater ridging to serve as the dominant means of supplying storm flow to the stream during snowmelt. Thus Buttle and Sami [1992] found that gro...

Dye staining and excavation of a lateral preferential flow network

“…Some steep, forested hillslopes have been reported to have large preferential flow paths, but it was not known how far upslope they extended (Roberge and Plamondon, 1987;Tsukamoto and Ohta, 1988;Kitahara, 1993;Uchida et al, 1999). Even though preferential flow paths are usually short and discontinuous, hillslopes produce fast tracer velocities and rapid subsurface flow responses (Peters et al, 1995;Tani,936 A. E. Anderson et al: Excavation of a lateral preferential flow network and subsurface flow responses have led to the idea of a preferential flow network, which describes a series of hydraulically connected preferential flow paths that appear to be physically discontinuous. The exact mechanisms that allow water to exploit these preferential flow pathways are not known, but it is often assumed that increasing soil moisture provides the connection between preferential flow paths (Sidle et al, 2001).…”

Hydrologic assessment of an urban variable source watershed in the northeast United States

“…Since the soil permeability of these areas is generally greater than the precipitation rate, runoff generation is predominantly by saturation excess. A saturated area may form during extended rainfall events where subsurface flow converges with the soil surface [Frankenberger et al, 1999], the slope flattens [Peters et al, 1995], the depth to the impervious layer decreases [Ogden and Watts, 2000], or the soil otherwise has little additional storage capacity [Dunne, 1978]. However, the spatial-temporal extent of these areas, termed variable source areas (VSA), is dynamic and not easily predicted.…”

Effects of bedrock groundwater on spatial and temporal variations in soil mantle groundwater in a steep granitic headwater catchment

“…Especially since the 1960s, increasing attention has been paid to subsurface flow as a dominant process for runoff generation in humid, soilmantled, and vegetated catchments. Detailed observations have been carried out at various study sites, including the Maimai catchment, New Zealand [Mosley, 1979[Mosley, , 1982Sklash et al, 1986;McDonnell, 1990], LI1 catchment at Llyn Brianne, Wales [Soulsby, 1992], Plastic Lake basin 1-08, Canada [Peters et al, 1995], and Tatsunokuchi-yama Experimental Forest, Japan [Tani, 1997]. The water flow processes in headwater catchments can generally be described as follows: in humid, soil-mantled, and vegetated catchments, most of the rainwater infiltrates into and flows downward through the permeable soil layers, eventually reaching a less permeable layer and forming a transient perched groundwater table.…”