Groundwater-surface water interactions in headwater forested wetlands of the Canadian Shield

Journal

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Journal of Hydrology 181 (1996) 127-147

Groundwater-surface water interactions in headwater forested wetlands of the Canadian Shield K.J. DevitoaT*, A.R. Hillb, N. RouletC aDepartment of Geography, University of Toronto in Mississauga, Erindale College, 3359 Mississauga Road North, Mississauga, Ont. L5L IC6, Canada bDepartment of Geography, York University, 4700 Keele Street, North York, Ont. M3J IP3, Canada ‘Department of Geography, McGill University, 805 Sherbrooke Street West, Montreal, Que. H3A 2K6. Canada

Received 16 November 1994; revision accepted 20 August 1995

Abstract

Groundwater and surface water interaction in two conifer swamps located in headwater catchments with contrasting till depth, typical of the southern Canadian Shield, were studied from June 1990 to August 1992. Both swamps had little influence on the regulation or attenuation of seasonal runoff response in the catchment. The two valley bottom swamps were connected to local aquifers but the upland-wetland connection was continuous in the catchment with deeper till and ephemeral in the catchment with thin till-rock ridges. Groundwater movement through the wetlands was restricted mainly to the surface peat layer in both wetlands, because a large portion of inputs from shallow soil layers and stream inflows enter near the peat surface. However, differences in upland-wetland connections resulted in contrasting hydrologic regimes in the two swamps. During seasons with larger inputs, both swamps were hydrologically connected to uplands and had a similar hydrology characterized by a high water table, rapid storm response, and predominance of saturated overland flow. In summer, upland inputs were absent in the catchment with thin till-rock ridges, resulting in cessation of baseflow and a lower water table that varied in response to variations in rainfall. Continuous upland inputs throughout the summer in the catchment with deeper tills (l-3 m) sustained baseflow and kept the water table near the peat surface. This study demonstrates the control of morphology and shallow subsurface geology on the hydrology of valley bottom swamps influenced by local aquifers.

* Corresponding author. 0022-1694/96/$15.000 1996 - Elsevier Science B.V. All rights reserved SSDI 0022-1694(95)02912-5

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1. Introduction

Valley bottom wetlands are a common feature of headwater basins in humid glaciated lowland regions and are also areas where groundwater and surface water hydrology are inseparable (Roulet, 1990a). Groundwater-surface water interactions in wetlands have received increased attention in recent years (Siegel, 1988a). These interactions influence runoff production (Verry and Boelter, 1978; Waddington et al., 1993) and water chemistry (Hill, 1990; Devito and Dillon, 1993) in wetland systems. Groundwater-surface water interactions in wetlands are complex and interpreting the different hydrologic functions of wetlands requires knowledge of the hydrogeologic setting of the particular wetland within the landscape (Verry and Boelter, 1978; Siegel, 1988b; Winter and Woo, 1990). Some studies have emphasized that the important control on the hydrologic behaviour of wetlands is the interaction of intermediate or regional scale groundwater with smaller scale local groundwater flows (Verry and Boelter, 1978; Siegel, 1988a,b; Roulet, 1991). In wetlands which receive continuous groundwater discharge, such as in fens and swamps, water table position and surface hydrology are relatively uniform as groundwater sources buffer episodic precipitation events. Studies on wetlands isolated from large-scale groundwater discharge, as in many raised bogs and poor fens, suggest that water interacts primarily with the surface layer of peat, and water table fluctuations and the surface hydrology of these wetlands are influenced by the seasonality of local scale groundwater links and precipitation (Verry and Boelter, 1978; Taylor and Pierson, 1985; Whiteley and Irwin, 1986). Field studies and numerical simulations of hillslope and lake hydrology have shown that ephemeral or continuous groundwater flow can occur in transient local flow regimes, depending on the unsaturated zone thickness and permeability (Winter, 1983; Hinton et al., 1993). Consequently, wetlands connected to local aquifers may show a range of groundwater connections and contrasting seasonal patterns of surface saturation and water levels resulting in different runoff patterns and water balance. It may be possible to generalize the hydrologic characteristics of valley bottom wetlands with respect to physiographic setting in regions restricted to local flow systems because of their simpler hydrogeology. However, the range of hydrologic behaviour of wetlands in these hydrogeologic settings is not well documented. To examine this behaviour a hydrometric approach was used to analyse the temporal and spatial variation in upland-wetland links, water table fluctuations and flow path in valley bottom forested swamps of headwater catchments of the Canadian Shield. This is a region of shallow, but varying, depth of glacial till over impermeable bedrock which restricts catchment runoff to local flow regimes; thus, groundwater sources are considered ephemeral and insignificant to the water balance of these wetlands (Winter and Woo, 1990). The two study sites are in catchments which represent the ends of the physiographic continuum, characteristic of the shallow till-rock ridges physiographic regions of the southern Canadian Shield, with till depths of less than 1 m and up to 10 m depth, respectively. In comparing the hydrology of sites in catchments with contrasting till depth we ask several questions about how the seasonality and complexity of the

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hydrology may differ in valley bottom forested swamps, the dominant wetland type in this landscape (Riley, 1988). Specifically, with an increase in till depth: (1) does the upland-wetland linkage shift from ephemeral to continuous? (2) Does an increase in input and depth of interaction of groundwater result in a larger proportion of subsurface flow (SSF) through deep peat? In headwater catchments, increased groundwater interaction moderates water table fluctuations and maintains wetland surface saturation (Roulet, 1990a), and the relative contribution of saturated overland flow (SOF) and SSF varies seasonally with water table response (Devito and Dillon, 1993). Therefore, (3) do differences in temporal and spatial upland-wetland connections result in differences in the amplitude and periodicity of water table variations and duration of surface saturation? (4) Do these differences influence the seasonal runoff response and proportion of SOF vs. SSF, particularly during dry periods?

2. Study sites The two wetlands are in headwater catchments of Harp Lake (45’23’N, 79”08’W) and Plastic Lake (45”1l’N, 78”5O’W),which are situated near the southern limit of the Precambrian Shield in south-central Ontario (Fig. 1). Annual precipitation in the area is 900-l 100 mm with 240-300 mm falling as snow between December and April. The mean January and July air temperatures are -10°C and 17.7”C, respectively. Annual runoff is similar in both catchments, varying between years from 400 to 600 mm. The physiography, geology and some hydrological and geochemical studies of Harp and Plastic Lake catchments have been reported by Devito and Dillon (1993), Hinton et al. (1993), and Devito (1994). Both catchments are underlain by impermeable Precambrian metamorphic silicate bedrock covered with thin basal till. Overburden in Plastic catchment is classified as thin till-rock ridges (less than 1 m depth), with a small area (10%) of sandy till l- 1.5 m depth. Most of the catchment of Harp swamp is classified as minor till plain (63%), with deposits of sand (29%) on the north side of the stream. Plastic is forested primarily with stands of white pine (Pinus strobus), hemlock (Tsuga canadensis) and balsam fir (&es balsumea). Vegetation in Harp is a deciduous forest of primarily maple (Acer spp.) and birch (Bet& spp.) on the dry uplands and a coniferous forest (white cedar (Thu$z occidentalis), hemlock and balsam fir) in low-lying areas. Plastic conifer swamp (2.2 ha) occupies a central bedrock depression and represents about 10% of the total basin area of 21.1 ha (Figs. 1 and 2). The swamp is forested primarily with white cedar and black spruce (Piceu muriunu) with some birch and maple. There is an understorey of Alnus spp. and Ilex verticillutu, and a welldeveloped layer of Sphagnum. A hummock-hollow micro-topography has developed throughout the swamp. Peaty humic mesisols up to 6 m depth (average 2-3 m) overlie regions of gyttja and deposits of silt, clay, sand and gravel up to 1 m depth in the bedrock basin (Devito, 1994). The soils of the adjacent hillslope are a combination of orthic humo-ferric and orthic ferro-humic podzols (LoZano et al., 1987). Soil and sandy basal till depth are generally less than 0.5 m with small areas near l-l.5 m. Three ephemeral inflows drain the upland forests.

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Fig. 2. The topography of Plastic swamp (upper) and Harp swamp (lower) showing the streams and location of weirs, groundwater wells and piezometers. Contour elevations are in metres above the outlet stream weir.

Harp conifer swamp (1.2 ha) occupies the valley bottom and represents 5% of the total basin area of 22.7 ha (Figs. 1 and 2). The swamp forest is dominated by white cedar with beech (Ezgxr grundzjbliu) and maples. It has a poorly developed shrub and bryophyte mat. Average swamp soil depth is 2-3 m of peaty cumula humisols, overlying layers of gyttja and pockets of silt, clay, and sand and gravel (Devito, 1994). Upland soils are orthic, humo-ferric podzols (LoZano et al., 1987). Depth of till and sand on the north hillslope is around 1 m and till on the south hillslope ranges from

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1 m to more than 2 m. One channelized inflow draining forested uplands and swamp lowland (14.2 ha) enters the swamp at the highest elevation. Subsurface inputs drain adjacent, steep forested upland slopes (7.3 ha).

3. Methods 3.1. Hydrometeorology Measurements of the hydrological variables and parameters were made from June 1990 to October 1992. The hydrologic year used in this study is from 1 June to 31 May. Precipitation and snow depths, relative humidity, air temperature and net radiation were obtained from an Ontario Ministry of Environment and Energy (OMEE) meterological station located within 1 km of each wetland (Fig. 1). These measurements were used to compute swamp evapotranspiration using a simplified form of the Penman-Monteith combination model (Munro, 1986). A canopy resistance of 100 s m-l and aerodynamic resistance of 5 s m-l were assumed for both swamps (Munro, 1987). Throughfall amounts were estimated from 10 m x.0.05 m ABS troughs (surface area 0.5 m2) at two locations with differing vegetation within each swamp (Fig. 2). Snow on the ground was measured two to four times each winter at the throughfall trough locations. Snow storage was estimated from the change in water equivalent between measurements. Snow depth and water equivalent were supplemented by additional measurements taken within each catchment by the OMEE. Stream stage at the catchment outflow. and the outflow and main inflow of both swamps were continuously monitored at 90” V-notch weirs (Figs. 1 and 2) as described in Devito (1995). Instantaneous discharge at three inflow streams (42, 43 and Q4) to Plastic swamp was measured on an event basis. Groundwater wells and piezometers were installed along a longitudinal and transverse transects in both swamps (Fig. 2). Groundwater wells were made from 5 or 10 cm diameter perforated ABS pipe and installed up to 1 m below the ground or peat surface. The piezometers were made from 1.25 cm diameter PVC pipe with a slotted point of 20 cm length. Piezometers. were installed in nests ranging from 0.5 to 5 m depth. The location and elevation of all weirs, wells, and piezometers were surveyed in October 1990 and May 1991 and 1992. Water table level and piezometric head in both swamps were monitored at least once a month at all locations, often more frequently, but only twice from December to March owing to problems with ice. At selected sites within the swamps, water table elevation was continuously monitored throughout the year with a float and potentiometer or strip chart recorder (Fig. 2). Groundwater wells in the hillslope and swamp perimeter were measured to estimate hillslope runoff on an event basis or at least once a week during lower flows. Water level elevation in one hillslope well (P12 and H13) was continuously monitored from September 1991 to October 1992. The piezometers along the middle’transverse (Pll-P18 and H12-H21) and longitudinal (Pl-PI0 and HI-HI 1) transects and all groundwater wells in the hillslopes (see

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Fig. 2) were used for the measurement of hydraulic conductivity by Hvorslev water recovery method (Freeze and Cherry, 1979). The hydraulic conductivity and specific yield in the top 50 cm of peat was predicted by regression with estimated bulk density as developed by Boelter and Verry (1977).

4. Results 4.1. Precipitation and evapotranspiration There were no clear seasonal patterns in precipitation at both Plastic and Harp swamps (Figs. 3 and 4). Precipitation at Plastic swamp was 1025 mm and 930 mm, with 25% and 29% falling as snow from November to April in 1990-1991 and 19911992, respectively. Maximum snow accumulation of 185 mm and 2 16 mm (as average water equivalent) occurred at the end of March in 1991 and 1992, respectively. Total precipitation at Harp swamp was 1032 mm and 914 mm, with 29% and 38% falling as snow in 1990-1991 and 1991- 1992, respectively. A maximum snow accumulation of 110 mm and 181 mm average water equivalent occurred at the end of March in the two years. There was considerable variation in depth of summer rainfall during the study period (Figs. 3 and 4). Total summer precipitation (June-August) at Plastic swamp was 120 mm, 219 mm and 275 mm, whereas Harp swamp received 159 mm, 191 mm and 213 mm from 1990 to 1992, respectively. Evapotranspiration was in excess of 100 mm month-’ from May to August, with the exception of June 1992, and decreased through September and October in both swamps (Figs. 3 and 4). It was assumed that no evapotranspiration occurred between November and March. Monthly evapotranspiration was in excess of monthly precipitation during the summer except for July 1991 and August 1992. Calculated evapotranspiration in Plastic swamp was 542 mm for 1990-1991 and 576 mm for 1991-1992, and corresponding values for Harp swamp were 532 mm and 562 mm. 4.2. Stream Jlow

The mean daily discharge at the outlet stream of both sites showed seasonal trends (Figs. 3 and 4). Discharge peaks during the late autumn to spring were associated with rain and melting snow, whereas some autumn peaks and many smaller discharge peaks were associated with rain events. The mean daily discharge at the outlet stream of Plastic swamp was 4.7 1 s-i (range O-97.3 1 s-l) and 2.9 1 s-i (range O-68.3 1 s-l) during 1990-1991 and 1991-1992, respectively, and the corresponding total annual catchment runoff (including the swamp area) was 695 mm and 431 mm. Outlet stream discharge stopped for 132, 110, and 69 days during the summer and early autumn of 1990 to 1992. Sporadic discharge occurred during July 1991 and July and August 1992 (Fig. 3). There was a low flow period in January and February in both years where the discharge dropped below 0.2 1 s-l. Seasonal patterns of discharge for the three inflow streams (Q2, 43

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and 44) were similar to those for the outflow. All surface water inflow stopped for several months during the summer and 2-4 weeks during the winter (Fig. 3). The mean daily basin discharge for Harp swamp was 4.9 1 s-’ (range O-98.6 1 s-l) and 3.7 1 s-’ (range O-55.2 1 s-t) during 1990-1991 and 1991-1992, respectively (Fig. 4). The total annual catchment runoff was 677 mm and 554 mm for each year, respectively. In contrast to Plastic swamp, Harp basin discharge continued throughout the summer, with the exception of 2 days during early September in 1990 and 1991 (Fig. 4). Small discharge peaks related to rainfall occurred throughout the summer. The seasonal pattern of discharge for the inflow streams (12) was similar to that for the outflow (11; see Fig. 2). The mean daily discharge of the inflow stream during 1990-1991 and 1991-1992 was 3.0 1 s-i (range 0.1-49.5 1 s-l) and 2.4 1 s-i (range 0.1-39.4 1 s-l), respectively. During extreme summer low flow the inflow discharge exceeded that of the outflow in Harp swamp. 4.3. Groundwaterflow 4.3.1. Distribution of hydraulic conductivity The hydraulic conductivity of the surface peat in Plastic swamp decreased from 10e2 cm s-l at O-10 cm to 10-3-10-4 cm s-l at 20-30 cm depth, as determined from mean bulk density measurements of the surface peat at nine locations. The hydraulic conductivity of the deeper peat, below 100 cm depth, and the underlying substrate in the bedrock basin in Plastic swamp generally ranged between lop5 and 10e6 cm s-l. The exception was a layer at about 200 cm depth where the conductivity was between 10d4 and lop5 cm s-’ . The conductivity of the shallow soil and till as measured in ten upland wells was high, over 10m2cm s-l. The conductivity decreased from 10p2- 1O-3 cm s-l at 0- 10 cm to 10p3- 10m5cm s-’ at 30-40 cm depth in the surface peat of Harp swamp, as determined from mean bulk density measurements of the surface peat at five locations. The hydraulic conductivity of the deeper peat, below 100 cm depth, generally ranged between 10e4 and lop5 cm s-i, and was one to two orders of magnitude lower in the silt clay layer that confines much of the organics in Harp swamp. The conductivity of shallow soils and till (O-50 cm) of the adjacent uplands was high, 10-2-10-3 cm s-l. Conductivities in deeper mineral substrate (10-4-10-5 cm s-l) were similar to conductivities in the organic peat. The conductivities in areas predominated by sand and gravel were Fig. 3. Plastic swamp, 1 June 1990 to 31 August 1992. (a) Daily depth of precipitation. (b) Monthly evapotranspiration, rainfall, snowfall and snow accumulation (mm water equivalent (WE)). (c) Swamp outflow discharge (l s-r) and water table elevation (cm above weir). The straight horizontal lines show periods when the upland is hydrologically connected to the swamp. Arrows show dates of groundwater flow nets in Fig. 6. Fig. 4. Harp swamp, 1 June 1990 to 31 August 1992. (a) Daily depth of precipitation. (b) Monthly evapotranspiration, rainfall, snowfall and snow accumulation (mm water equivalent (WE)). (c) Swamp outflow discharge (1 SK’) and water table elevation (cm above weir). The straight horizontal lines show periods when the upland is hydrologically connected to the swamp. Arrows show dates of groundwater flow nets in Fig. 7.

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Fig. 5. (a) Plastic swamp; (b) Harp swamp. The water table elevation relative to outtlow weir for the two hillslope wells and three wetland wells along the mid-transverse transect (see Fig. 2 for location of wells), 1 June 1990 to 31 August 1992. Continuous records for hillslope wells (P12 and H13) from September 1991 to August 1992 only. Straight line segments for Wells Pll and P12 represent periods when no water was in the well. The straight horizontal lines show periods when upland is hydrologically connected to the Plastic swamp.

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high (more than 10e3 cm s-l). These areas were located below the organic and silt layer at the downstream and upstream boundary and in pockets along the lateral perimeter of Harp swamp. 4.3.2. GroundwaterJEowpatterns Fluctuations in water table elevations at Wells Pl 1 and P12, located just upslope of the wetland perimeter, are indicative of seasonal variations in shallow groundwater inputs to Plastic swamp from the uplands during the study period (Fig. 5). Groundwater inputs were ephemeral, occurring during the autumn and spring, with peak groundwater elevations resulting from rain storms and snow melt. For several months during the summer and several weeks during the winter the hillslopes were dry. Thus, when 44 inflow stream was dry, the wetland was hydrologically disconnected from the upland catchment. Groundwater elevations at Wells H12 and H13 at the base of the hillslope represent groundwater flow along the south hillslope in the downstream portion of Harp swamp (Fig. 5). Groundwater input from this hillslope was continuous throughout the study period. Annual low flow occurred during late August and early September. Peak groundwater elevations and flow occurred during late autumn and spring. Water table elevation at H19, H20 and H21, located at the base of the opposite hillslope, were all similar (see Fig. 2). Groundwater flow at this location represents seasonal variations observed along the north hillslope and the south hillslope at the upstream portion of the swamp. The water table elevation in the upland wells periodically fell below that at the wetland perimeter and flow reversals (i.e. flow from the wetland to the hillslope base) occurred. Flow nets during low or no flow periods and peak flow conditions, representing the full range in water table fluctuation and groundwater flow development over a year, are shown for transects longitudinal and transverse to downstream flow in both swamps (Figs. 6 and 7). The lateral gradients in hydraulic head in Plastic swamp were generally less than 15 cm over the length of the swamp (Fig. 6). After large inputs (e.g. April), groundwater and stream inflow occurred at the slope-wetland contact, with predominantly horizontal flow to the wetland centre. There was little groundwater discharge through deep peat. Surface water in the centre of the upstream portion of the swamp was recharged to mid-depths of the peat and discharged to the surface near the outflow. Water deep in the bedrock basin, probably originating from the wetland margins, converged near the centre of the swamp below 3 m depth, This resulted in recharge into the mid depth of peat both from the surface and from basal peat. When Plastic swamp was not connected to the upland (e.g. September, 1991), the flow of groundwater in the peatland reversed. Water discharged from middepths of peat to the surface, the slope-wetland contact and outflow, as well as to deeper peat and mineral deposits. The flow nets reveal that the Harp swamp is situated in a groundwater discharge zone and the hydraulic gradients were much greater than those of Plastic swamp (Fig. 7). The flow nets constructed for low flow and peak flow indicate that seasonal reversals in groundwater flow do not occur in Harp swamp. Diffuse vertical discharge occurred near the south swamp-slope contact and the downstream portion of the

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swamp. This is due to a break in slope and the presence of high permeability material overlain with low permeability gyttja and peat. The flow in these locations switched from more horizontal during low flow periods to more vertical during higher flow periods. Deflection or convergence of water through higher conductivity peat at the gap in gyttja and clay that confines most of the peat resulted in upwelling at Piezometer Nest H5. This one location is where most of the deep groundwater contribution (90-99%) to Harp swamp originates (Devito, 1994). During high flow periods groundwater flow from both the north and south slope converged at the lagg (H18). The convergent zone at the lagg breaks down during low flow and water from the south slope flows across the wetland into the north slope. The upstream portion of Harp swamp switched from a discharge region during higher flows to a recharge region in low flow conditions. 4.3.3. Flowvelocities in peat Groundwater flow through peat was computed using measured hydraulic conductivity and head gradients. Water table and horizontal head gradients varied little and the volume of lateral flow contributing to outflow at a given depth in saturated peat was similar throughout the year (Figs. 6 and 7). Lateral subsurface flow in the top 50 cm of peat was greater than flow in underlain peat owing to greater hydraulic conductivities. The lower gradients in Plastic were offset by greater conductivity of the surface peat compared with Harp (lo-’ vs. 10e3 cm s-l) and maximum velocities approached lop5 cm s-’ in both swamps. Estimated flows through the top 50 cm were in the range of 0.01-0.1 mm day-‘. Gradients within deep peat of Plastic swamp ranged from 0.002 to 0.005, producing velocities ranging from 10e9 to lo-* cm s-l. Gradients within deep peat of Harp swamp were an order of magnitude greater (about 0.01) and lateral velocities ranged from 10e6 to lop7 cm s-l. Maximum water velocities of lop6 cm s-l through a cross-section of peat of 100 m width by 3 m depth, excluding the surface 0.5 m, generates only 0.01 mm day-’ (0.003 1s-l) laterally to the swamp outflow. These calculations suggest that lateral subsurface water transport does not contribute appreciable volumes of water when compared with the average annual outflow discharge in either swamp. Appreciable lateral transport of water through both wetlands must occur overland or near the surface (O-10 cm depth) through living vegetation or peat which is poorly humified. The vertical hydraulic gradients were steeper than lateral gradients, ranging from 0.01 to 0.1 in both wetlands, and resulted in greater velocities through deep peat, ranging from 10e6 to lo-* cm s-’ and 10e5 to 10e7 cm s-l in Plastic and Harp swamps, respectively. Owing to the orders of magnitude increase in crosssectional area, estimates of vertical flux of water range from 0.01 to 1 mm day-’ and from 0.1 to 10 mm day-’ at various locations and seasons in Plastic and Harp swamp, respectively. Maximum estimates of vertical groundwater flux represents as much as 20% of the total annual runoff from Harp swamp basin. The net flow through Plastic swamp on an annual basis was near zero, owing to seasonal recharge and discharge.

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PercentQreaterThan Fig. 8. Mean daily water table (a) and outflow stream discharge (b) duration curves for Plastic and Harp swamp average for two years, 1990-1991 and 1991-1992.

4.4. Water table and discharge fluctuations

Mean daily water table depths at Site P15 in Plastic swamp showed similar seasonal trends to swamp discharge and the water table was generally above the surface in the autumn and spring and below the surface for much of the summer and winter (Fig. 3). Annual change in the mean daily water table was 98.1 cm and 62.4 cm, and the median water table elevation was -3.9 cm and -1.8 cm relative to the peat surface for 1990- 1991 and 1991- 1992, respectively. Changes in water table depth decreased over the three summers of the study in response to different amounts of rainfall. Maximum water table depths of 62.5 cm, 23.5 cm and 13.1 cm below the surface occurred in the summers from- 1990 to 1992, respectively. Mean daily water table elevations at H16 in Harp swamp showed seasonal trends similar to the outlet discharge and were more stable than the water table at Plastic swamp (Fig. 4). The annual change in mean daily water table was 49.4 cm and 36.3 cm, and the median water table elevation was 0.1 cm and - 0.2 cm relative to the peat surface for 1990- 1991 and 1991- 1992, respectively. Changes in water table depth decreased slightly between three summers of the study, with maximum depths of 14.1 cm, 6.1 cm and 1.9 cm below the surface for 1990 to 1992, respectively. The influence of the magnitude and duration of groundwater flow on water table elevation and outflow discharge are summarized in the water table and discharge duration curves (Fig. 8). The water table duration curve of Harp swamp was much

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flatter than that for Plastic swamp, and the water table in Harp swamp remained within 5 cm of the surface over 90% of the time. The water table was above the surface for less time in Plastic (35-40%) than Harp swamp (50%). However, the water table in Plastic swamp was more than 5 cm above the surface as well as more than 10 cm below for a longer time compared with Harp. Discharge duration curves of both swamps were similar for mean daily discharge greater than 5 1 s-i (Fig. 8). The duration curve for Plastic swamp was much steeper than that for Harp during lower flows, and no flow occurred about 25% of the time in Plastic swamp. There was more constancy in baseflow at Harp swamp and the discharge remained between 2 and 5 1 s-i approximately 50% of the time.

5. Discussion 5.1. Upland-wetland linkages and wetland surface hydrology

These data show that a small increase in till depth resulted in the upland-wetland linkage shifting from ephemeral to continuous in the two headwater catchments. The ephemeral groundwater connections observed in Plastic swamp have been reported in wetlands connected only to local flow systems in catchments elsewhere on the Canadian Shield (Buttle and Sami, 1992), other boreal regions (Bay, 1969), as well as in temperate (Taylor and Pierson, 1985; Whiteley and Irwin, 1986) and arctic landscapes (Roulet and Woo, 1968). However, the occurrence of both ephemeral and continuous upland-wetland connections in catchments where only local flow systems develop have not been emphasized previously. This study shows the importance of slowly moving groundwater in the deeper strata of moderate depths of till (2-3 m) in maintaining an upland-wetland linkage during low flow periods in Harp swamp. The volume of continuous groundwater is small, well within the budget uncertainties (see Devito (1995)), and owing to inherent measurement errors the absolute values should be treated with caution. Nevertheless, groundwater from the deeper till is critical to the surface hydrology of Harp swamp. These results also show that the relationship between catchment physiography and the maintenance of a groundwater connection during seasons of small inputs is very important in controlling the amplitude of water table drawdown in wetlands which interact only with local aquifers. The seasonal variability in the amplitude and duration of water table elevation in response to the frequency and magnitude of rainfall in Plastic swamp is the classic seasonal response in ephemeral or ephemerally connected wetlands (Verry and Boelter, 1978; Taylor and Pierson, 1985). There are large interannual variations in water table elevation with variations in precipitation and these wetlands are susceptible to large water table drawdown during extended dry periods (Verry et al., 1988). In contrast, the surface hydrology of Harp swamp is similar to that of wetlands connected to larger-scale groundwater systems (Verry and Boelter, 1978; Roulet, 1990b). Although small relative, to total inputs, diffuse discharging groundwater is critical in sustaining saturation and reducing the temporal variability in saturation of the peat surface of Harp swamp. Maintenance of baseflow and the

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low antecedent water storage capacity in the swamp results in saturated overland flow (SOF) and storm runoff responses during larger rain events throughout the summer. Regardless of local physiography, all valley bottom wetlands will have high water tables during seasons of large inputs as a result of connections with the uplands. Saturation of peat and low antecedent water storage in the wetland results in storm runoff responses and a large portion of seasonal runoff as SOF. High water table and the predominance of SOF during autumn and spring have been demonstrated in a variety of wetland types (Bay, 1969; Taylor and Pierson, 1985; Buttle and Sami, 1992) characteristic of valley bottom wetlands in this landscape. The water table fluctuations in both study wetlands are large relative to annual changes of 10 cm in regional groundwater connected headwater wetlands (Roulet, 1990b). Annual changes in the water table of the study swamps reflect their connection to only local aquifers and are a little greater than changes of 40-50 cm in perched water table wetlands (Taylor and Pierson, 1985; Verry et al., 1988) and similar to those in a small headwater wetland (Buttle and Sami, 1992). Similar to other wetlands, the results of this comparative study demonstrate that despite the range in upland-wetland connections to local aquifers, valley wetlands have little influence on regulation or attenuation of seasonal and annual runoff response (Bay, 1969; Roulet, 1990b). Storage in Plastic swamp did not appear to be significantly greater than storage in the uplands during the summer, owing to its small area relative to the catchment area (lo%), low water retention capacity and rapid water table rise in compacted peat. Evapotranspiration appears to have little effect on the magnitude of stream discharge from the catchment during the summer because cessation of outflow from Plastic swamp occurred shortly after disconnection of upland inputs. It is unlikely that the evapotranspiration loss by the upland forests would be much different from that for the wetland forests (Munro, 1987). In Harp swamp integrated daily evapotranspiration for the summer would remove about 4 mm of water per day (120 mm month-‘) or equivalent to 0.5 1 s-l. This is similar to the reduction in discharge from the inflow to outflow during late summer in the swamp. However, reduction in baseflow at Harp swamp was associated with reduced stream inputs and reduced hydraulic gradients at the base of the hillslopes. Recharge into the underlying substrate and the hillslopes could also account for reduction in stream flow during the summer. 5.2. Subsurface flow through peat These results indicate that there is, in general, an increased probability of discharge beneath the peat throughout the centre of the wetland with an increase in till depth. The dominance of vertical movement of water through the peat is sign&ant because upon discharging to the surface the water interacts with the surface environment and is transported laterally as surface or shallow subsurface flow down gradient. The water table location relative to the peat surface and vertical variations in hydraulic conductivity controls the type and magnitude of lateral water transport through the wetland.

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Hydraulic conductivities, which vary vertically but little laterally, resulting in the subsurface movement of water in peat observed in this study, are similar to those observed in many continental wetlands (Boelter and Verry, 1977; Hammer and Kadlec, 1986; Roulet and Woo, 1986). Boelter (1972) reported lateral flows of less than 0.03 1 s-i per 100 m of ditch in deep peat from the Marcel1 Forest wetland with a K,, of 10e5 cm s-l. However, macropores associated with root and coarse woody debris can greatly increase the flow in deeper peat (Siegel and Glaser, 1987; Waddington et al., 1993). Such pores were observed occasionally when digging well holes in the study swamps (K. Devito, unpublished observation, 1990). However, as is typical of wetlands in continental climates (FitzGibbon, 1982; Verry et al., 1988), outflow discharge ceased when the water table dropped below 10 cm depth in Plastic and Harp swamps (Devito, 1995) suggesting no direct lateral link of macropores to the outflow below this depth. The larger vertical component of subsurface flow in the peat of Plastic and Harp swamps supports recent studies that show water can actively move through deep peat in a range of wetland types (Siegel, 1988a; Roulet, 1990b). Siegel and Glaser (1987) reported vertical velocities of 10-5-10-7 cm s-l and fluxes ranging from 22 mm year-’ (0.06 mm day-‘) to 1260 mm year-’ (3.5 mm day-‘) from a bog and fen complex connected to regional groundwater. These vertical fluxes are similar to estimates for Plastic and Harp swamps which represent the range of wetlandgroundwater interactions in valley bottom wetlands in catchments where only local flow systems develop. Although the seasonal reversal in Plastic swamp results in a net annual flux near zero, the reversals impart measurable subsurface flow through deep peat confined within a bedrock basin. The setting of Plastic swamp is similar to that of kettle wetlands and raised bogs (Verry et al., 1988) and represents a hydrogeologic setting where groundwater movement is assumed to be absent (Verry and Boelter, 1978; Ingram, 1983). Subsurface flow in Plastic swamp shows that pore water in decomposed peat may not be stagnant in any hydrogeologic setting.

6. Summary and conclusions Roulet (199Qa) has developed the idea of groundwater connectivity as a framework in understanding peatland hydrology. The present study is the first to show a wide range of hydrologic behaviour owing to ephemeral and continuous upland connections to wetlands located in settings where only local flow systems develop. In comparing two wetland sites located in catchments which represent the ends of the continuum of local physiography (till depth) this study documents a large proportion of the range of hydrologic response of wetlands in the Shield landscape. The comparison between Plastic and Harp swamp suggests that a modest increase in till depth from less than 1 m to l-3 m is suElcient to create a change in groundwater flow from ephemeral to continuous which results in contrasting seasonal patterns of water table fluctuation and surface hydrology. Considerable areas throughout the Canadian Shield landscape have till depths of more than 1 m (Chapman, 1975), suggesting

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that continuous groundwater connection to valley bottom wetlands may be common in this landscape. These contrasts in hydrologic behaviour have important implications for biogeochemistry as water table position influences both elemental export and transformations within wetlands (Devito, 1994, 1995). This comparison of the hydrology of two conifer swamps demonstrates the range in hydrologic function which can occur in wetlands with similar vegetation and classification. The current classification of wetland types (e.g. bog, fen, swamp and marsh; National Wetland Working Group, 1988) is not effective in evaluating groundwatersurface water interactions and cannot be used to generalize hydrologic function. Existing classifications of depth and type of surficial geology may provide a promising approach to generalizing the ephemeral or continuous nature of upland connections, and seasonal surface saturation of wetlands in the Canadian Shield and similar landscapes. Further evaluation of these classes of surficial geology in predicting contrasts in the hydrologic function is an important topic of study.

Acknowledgements

We thank David Cruickshank and Stan Sutey for invaluable assistance both in the field and in the laboratory, and three anonymous reviewers for their helpful comments on earlier versions of the manuscript. The authors wish to acknowledge the co-operation and logistic support provided by the staff at Dorset Research Centre. B. Warner and R. Aravena, University of Waterloo, provided peat substrate composition data. We thank C. Randall, York University, for drafting figures. The work for this study was conducted while K.J.D. was supported by NSERC and OGS Postgraduate Scholarships, and the research was funded by an Ontario Ministry of the Environment Research Grant to K.J.D. and an NSERC Grant to A.R.H.

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