J. M. Buttle

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

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 18O 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.

Coupled vertical and lateral preferential flow on a forested slope

[1] Coupling of vertical and lateral preferential flow paths was examined on a forested slope with thin soil cover during artificial irrigations. Point-scale infiltration was measured at sites with differing soil macroporosities using vertical profiles of time domain reflectometry probes and suction samplers. Lateral fluxes of water and solutes from the slope were determined at a through flow trench. Sites with greater macroporosities tended to exhibit vertical preferential flow, while infiltration at sites with relatively small macroporosities was largely by vertical propagation of a well defined wetting front through the soil profile. Generation of vertical preferential flow at sites with relatively large macroporosities increased with input intensity. Lateral macropores made a minor contribution to slope runoff. Instead, runoff largely occurred in a thin saturated layer at the soil-bedrock interface, both in a highly conductive zone at the bedrock surface and in the overlying saturated soil matrix. Some assumptions underlying the use of isotopic and geochemical tracers to study runoff generation are called into question by complex mixing of event and pre-event water in this saturated layer. Soil depth, bedrock topography, and antecedent soil wetness determine the thickness, connectivity, and upslope extent of the pre-event saturated layer above the bedrock surface. These, in turn, control whether vertical preferential and matrix flow reaching the bedrock surface participate in slope runoff.

INFERRING HYDROLOGICAL PROCESSES IN A TEMPERATE BASIN USING ISOTOPIC AND GEOCHEMICAL HYDROGRAPH SEPARATION: A RE‐EVALUATION

Simultaneous monitoring of conservative and non-conservative tracers in streamflow offers a valuable means of obtaining information on the age and flow paths of water reaching the basin outlet. Previous studies of stormflow generation in a small forested basin on the Canadian Shield used isotopic (IHS) and geochemical hydrograph separations (GHS) to infer that some event water during snowmelt reaches the stream via subsurface pathways, and that surface water runoff is generated by direct precipitation on to saturated areas (DPSA) in the stream valley. These hypotheses were tested for rainfall inputs using simultaneous IHS (18O) and GHS (dissolved silica) of basin stormflow, supplemented by hydrochemical and hydrometric data from throughflow troughs installed on basin slopes. Comparison of pre-event and subsurface water hydrographs did not provide conclusive evidence for subsurface movement of event water to the stream, owing to the appreciable uncertainty associated with the hydrograph separations. However, IHSs of runoff at the soil–bedrock interface on basin slopes indicated that event water comprised 25–50% of total runoff from areas with deep soil cover, and that these contributions supplied event water flux from the basin in excess of that attributable to DPSA. The surface water component of stormflow estimated from the GHS was also largely the result of DPSA. GHS assumes that dissolved silica is rapidly and uniformly taken up by water infiltrating the soil and that water moving via surface pathways retains the low dissolved silica level of rainfall; however, neither assumption was supported by the hillslope results. Instead, results suggest that the observed depression of silica levels in basin stormflow previously attributed to dilution by DPSA was partly a function of transport of dilute event water to the channel via preferential pathways. Implications of these results for the general use of simultaneous IHS and GHS to infer hydrological processes are discussed.

Impacts of clearcut harvesting on snow accumulation and melt in a northern hardwood forest

Abstract Snow accumulation and melt on north- and south-facing slopes in the Turkey lakes watershed (TLW) in central Ontario were compared for a mature hardwood maple stand and an adjacent clearcut in late winter and spring of 2000 and 2001. Snow accumulation in the clearcut exceeded that in the forest, although the degree of difference varied with slope aspect and year. Melt was significantly larger in the south-facing clearcut and forest relative to corresponding north-facing sites. The south-facing forest sites lost all snowcover 27 days before the north-facing clearcut in 2000, and 4 days before the north-facing clearcut in 2001. Daily melt in the clearcut was slightly greater and more spatially variable than in the adjacent forest, with the exception of the south-facing slope in 2000. Nevertheless, the effect of aspect on spatial variations in melt was larger than that due to clearcutting. There was a slight increase in melt rate with decreased canopy density in the south-facing clearcut and forest; however, variations in canopy density did not explain inter-point differences in daily melt within either the north-facing clearcut or the forest. The hydrological consequences of greater pre-melt snow water equivalent and larger daily melt in clearcuts include quicker delivery of meltwater to the soil surface and promotion of rapid near-surface runoff to receiving waters relative to undisturbed forest stands at TLW.

Spatial variability of saturated hydraulic conductivity in shallow macroporous soils in a forested basin

Field (single-ring infiltrometer) and laboratory (constant head parameter) measurements of saturated hydraulic conductivity (KH) were used in combination to examine macropore effects on KH in shallow forest soils in a small basin in southern Ontario, Canada. Soil matrix KH generally decreased with depth for hillslope podzols and stream valley gleysols. Differences in average maximum and minimum matrix KH were observed between soil types, with lowest values occurring in the valley gleysols. Bulk profile KH (assumed to include the effects of macropores) had no apparent spatial pattern within a given soil type, and mean bulk KH did not differ between soil types. KH of macroporous soil was similar to matrix hydraulic conductivities for the porous hillslope podzols; however, it could be several orders-of-magnitude greater than the minimum matrix KH observed at a given site for the gleysols. This difference increased with total profile thickness. Geostatistical analyses revealed anisotropic distributions of both bulk and matrix KH in the basin, with the maximum and minimum directions of spatial correlation being parallel and normal to the stream valley, respectively. Degree of anisotropy and scale of spatial dependence were lower for bulk KH relative to the minimum matrix KH values.

Testing the groundwater ridging hypothesis of streamflow generation during snowmelt in a forested catchment

Hydrometric and isotopic techniques were used to test the groundwater ridging hypothesis of stream-flow generation during snowmelt in a forested catchment on the Canadian Shield. The catchment contains a central wetland that drains to an ephemeral channel. Hydrograph separation using 2H as a tracer indicated that approximately 60% of total stream flow during snowmelt was supplied by pre-event water. This was supported by hydrometric results. However, contrary to the groundwater ridging hypothesis, the response of water-table levels in near-stream areas to initial melt inputs failed to promote a rapid flux of ground water to the wetland and stream. All stream flow originated as saturation overland flow from the wetland, and groundwater flow during much of the melt was directed down the catchments main axis and back under hillslopes surrounding the wetland and channel. Although the hydrograph separation indicated pronounced short-term variations in pre-event discharge from the catchment, groundwater fluxes to the wetland remained relatively invariant following initiation of surface saturation. Streamflow fluctuations appear to have been generated by displacement of water held in surface storage, induced by melt and rainwater inputs to the wetland. This stored water had an isotopic signature intermediate between that of event and pre-event water. Suggested oscillations in pre-event runoff were an artifact of the mixing model approach to hydrograph separation, rather than the result of rapid changes in groundwater fluxes to the wetland.

Semi-distributed water balance dynamics in a small boreal forest basin

Information on water balance dynamics is an essential component of studies of the role of the boreal forest in surface-atmosphere interactions and climate change. The water balance of a small boreal forest basin in northern Manitoba was examined using a semi-distributed approach to assess basin sensitivity to climate change, provide a framework for distributed hydrological modelling, and explore data aggregation and micro-to-meso scaling of hydroclimatological variables. Black spruce forest with a highly variable canopy density was the main land cover in the basin. Spring snowmelt dominated basin runoff, while summer outputs were largely via evaporation. Annual differences in spring runoff were controlled by variations in snow water equivalent, rainfall timing and magnitude, thaw depth, and antecedent water content in surface stores and upland soils. Water storage in small wetlands and ephemeral surface depressions in the open-canopy black spruce forest and its subsequent loss via evaporation was a fundamental component of the basin water balance. However, its role could be overlooked by inappropriate spatial lumping of landscape units when scaling-up variables or in the production of depressionless digital terrain models. Hydrological consequences of climate warming in this part of the boreal forest include: (i) increased evaporation following spring snowmelt in open black spruce areas; (ii) decreased surface and soil water storage on basin slopes; and (iii) reduced streamflow response to spring runoff and summer and fall rainstorms.

ROAD SALT ACCUMULATION IN HIGHWAY SNOW BANKS AND TRANSPORT THROUGH THE UNSATURATED ZONE OF THE OAK RIDGES MORAINE, SOUTHERN ONTARIO

Pathways and fate of road deicing salt (NaCl) applied during the 1994-1995 winter were studied for a 14-km section of a major highway that crosses the Oak Ridges Moraine in southern Ontario. Total salt applications over the winter ranged from 29 to 74 kg m -1 of highway, and NaCl concentrations in snow banks adjacent to the roadway reached 9400 mgl -1 during the later stages of snow cover development. This salt was released to the ground surface during snowmelt. Sodium chloride (NaCl) loadings to soil from snow cover during the final melt phase were relatively uniform along the study section (3-5kg kg NaCl m -1 of highway). However, the snowpack at all transects retained 2m exceeded 500 mgl -1 and 1000 mgl -1 , respectively. Approximately 75% of the net flux of NaCl below the upper soil was retained in the 0-2.8m depth interval at this site, and results from more permeable soils traversed by the highway indicate an even greater penetration of the annual NaCl application into the unsaturated zone along the moraine. This saline water likely recharges groundwater in this portion of the Oak Ridges Moraine.

A statistical model of spatially distributed snowmelt rates in a boreal forest basin

Spatial variation in snowmelt rates in the boreal forest can be explained by diAerences in canopy density. Canopy density, represented as gap fractions (GF), controls both the amount of short-wave radiation reaching the snowpack surface and wind speed over the snow surface, which in turn regulates sensible and latent heat fluxes. Reductions in shortwave, sensible and latent heatfluxes outweigh any increased contributions from longwave radiation from the canopy. DiAerences in the total energy available for melt do not translate to equally proportional changes in melt rates under diAerent canopy densities. As available energy increases, the melt rate increases with decreasing canopy density and the form of the relationship can vary depending on climatic conditions. A good relationship between ground-based GF measurements and a canopy closure index derived from Landsat TM provides the spatial fabric for the distribution of snowmelt rates that show comparable patterns of snow ablation during years of very diAerent climatological conditions. This physically meaningful method of determining the spatial variability of snow ablation and subsequent meltwater delivery to the soil interface is particularly useful for providing insight to the heterogeneous active layer development in permafrost regions of the boreal forest and the implications for runoA processes. #1998 John Wiley & Sons, Ltd.

The Processes, Patterns and Impacts of Low Flows Across Canada

This paper provides an overview of low flow characteristics for six regions of Canada: the Arctic; the Mountains; the Prairies; southern Ontario; the Canadian Shield and the Atlantic. Processes that influence low flows are contrasted between the six regions examined. Data from a common analysis period for 51 gauging stations are used to evaluate flow duration curves and to explore the relationship between low flows and drainage area. The results reveal a diversity of processes influencing low flows and illustrate important regional differences in low flow characteristics and the impacts associated with low flows.