Groundwater Recharge Assessment in the Chateauguay River ...

Groundwater Recharge Assessment in the Chateauguay River Watershed Anne Croteau, Miroslav Nastev, and René Lefebvre

Abstract: The objective of this study was to evaluate groundwater recharge in the Chateauguay River watershed. The Hydrologic Evaluation of Landfill Performance (HELP) model was used to assess daily values of recharge, evapotranspiration and runoff. The study area was divided into a regular grid, 250 m × 250 m, for a total of 47,616 grid elements. The input parameters included soil physical properties, land use, vegetation and climate data. Calibration of HELP was carried out against runoff and baseflow estimates obtained from separation of five river hydrographs. Over a 39 year period, the mean annual recharge rate was estimated at 86 mm, or 9% of the total precipitation. Areas characterized by high water level elevations and unconfined flow conditions were identified as the main recharge areas. Daily estimates show that recharge takes place mainly in spring and fall. Over the observed period, the annual variations of evapotranspiration and runoff were directly related to changes in precipitation, whereas the annual recharge response was subdued, with much lower variations. HELP was also used to assess potential climate change scenarios using data for the driest and most humid years. The mean annual recharge was 51 mm for the driest year and 99 mm for the most humid year. Differences in the spatial distribution of recharge for the predictive scenarios indicate that the areas most sensitive to climate change correspond to the preferential recharge areas. Résumé : L’objectif de cette étude était d’évaluer la recharge des aquifères sur le territoire du bassin versant de la rivière Châteauguay. On a utilisé le modèle HELP (Hydrologic Evaluation of Landfill Performance) pour évaluer les valeurs journalières de recharge, d’évapotranspiration et de ruissellement. On a divisé la zone d’étude en mailles de 250 × 250 m pour un total de 47 616 éléments. Les données d’entrée incluent les propriétés physiques des sols, l’utilisation du territoire, le couvert végétal et les données climatiques. Le modèle HELP a été calé sur les estimations de débit de base et de ruissellement obtenues à partir de la séparation de cinq hydrogrammes de rivière. Sur une période de 39 ans, le taux de recharge annuel moyen a été évalué à 86 mm, ou 9% des précipitations totales. Dans le bassin versant, les zones ayant des niveaux piézométriques élevés et des conditions d’écoulement de nappe libre ont été identifiées comme étant les principales zones de recharge. Les résultats journaliers indiquent que la recharge se fait principalement au printemps et à l’automne. Durant la période d’observation, les variations annuelles d’évapotranspiration et de ruissellement sont directement corrélées aux précipitations, alors que les Anne Croteau, Miroslav Nastev, and René Lefebvre Genivar, Gatineau, Quebec J8T 7W3 Geological Survey of Canada, Natural Resources Canada, Quebec, Quebec G1K 9A9 3 Institut national de la recherche scientifique, Centre Eau Terre Environnement, Québec G1K 9A9 1 2

Submitted April 2010; accepted June 2010. Written comments on this paper will be accepted until June 2011. Canadian Water Resources Journal Vol. 35(4): 451–468 (2010) Revue canadienne des ressources hydriques

© 2010 Canadian Water Resources Association © Her Majesty in Right of Canada

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réponses annuelles de la recharge demeurent assez constantes. Le modèle HELP a aussi été utilisé pour évaluer des scénarios potentiels de changement climatique à partir des données des années les plus sèches et les plus humides. La recharge annuelle moyenne pour l’année la plus sèche est de 51 mm et pour l’année la plus humide, de 99 mm. Les différences dans la distribution spatiale de la recharge lors des scénarios indiquent que les zones les plus sensibles aux changements climatiques sont les zones de recharge préférentielles.

Introduction Estimating groundwater recharge is an essential step towards the quantitative assessment and sustainable use of groundwater resources and has been an active and challenging area of research in the past decades (Scanlon and Cook, 2002; National Research Council (NRC), 2004). While some studies have defined the most appropriate techniques for estimating recharge as a function of climate conditions and site characteristics (Stephens et al., 1996; Flint et al., 2002), others describe concepts related to recharge integration and interaction with other water budget components (Devlin and Sophocleous, 2005; de Vries and Simmers, 2002). Scanlon et al. (2002) present a comprehensive overview of physical, chemical and mathematical techniques for recharge estimation and water transfer during the infiltration process. These techniques are classified according to the zone to which they apply (surface, unsaturated or saturated zone) and to their application scale, both spatial (local or regional scale) and temporal (short or long term). Numerous regional studies carried out in the Chateauguay River watershed (Figure 1) have described the geological and geomorphical features as well as the hydrogeological settings in the watershed (Dufresne, 1979; Desmeules and Gélinas, 1981; McCormack, 1981; Choinière, 1984). Based on the observed hydraulic properties of the aquifers, Dufresne (1979) suggested an average recharge rate of approximately 263 mm/yr. A recharge analysis conducted by Hamel (2002) in the Mirabel region located north of Montreal with similar hydrogeological and climate conditions, inferred an overall recharge rate of 45 mm/yr. More recently, Pontlevoy et al. (2004) estimated a recharge rate of

210 mm/yr in the coarse gravelly sediments through calibration of a groundwater flow model at the Mercier esker (~100 km2). Although these previous studies in the watershed have a more or less regional character, systematic estimation of groundwater recharge over the entire aquifer system has not been conducted to date. The objective of the present study is to provide estimates of the spatial and temporal aquifer recharge variability in the Chateauguay River watershed. In this study, groundwater recharge is considered as the amount of water that percolates through various layers of unconsolidated sediments and which eventually reaches the regional aquifer consisting of fractured sedimentary rocks. In the case of unconfined flow conditions, groundwater recharge is the amount of water that reaches the regional water table. As the watershed is assumed to be a closed hydrogeological system bounded with natural no flow limits, precipitation represents the only source of incoming water. In this case, the interactions between climate conditions, topography, land cover and geology are the major factors influencing the recharge rate (Cherkauer and Ansari, 2005). Several direct and indirect techniques for recharge estimation were considered, such as recharge modelling, baseflow estimation, and analyses of water table fluctuations from well hydrographs (present study); numerical modelling of the groundwater flow system (Lavigne et al., 2010); and groundwater chemical analysis (Blanchette et al., 2010). This paper focuses on the application of the Hydrological Evaluation of Landfill Performance - HELP model (Schroeder et al., 1994) for the mathematical simulation of the recharge rates. The other assessment methods were mainly used to validate recharge results obtained from the HELP model (details are provided in Croteau, 2006).

Methodology Analytical Recharge Model and Conceptualization of Water Infiltration

Groundwater recharge to the regional aquifers was estimated using the infiltration model HELP (version 3.07; Schroeder et al., 1994), initially written to compute water balance components at landfill sites. HELP is a quasi two-dimensional (2D) deterministic model that simulates water percolation in a vertical soil © 2010 Canadian Water Resources Association © Her Majesty in Right of Canada

Croteau, Nastev, and Lefebvre

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column assuming the following basic water balance equation: Precipitation – Runoff – Recharge = Evapotranspiration ± DStorage

(1)

The model simulates a large number of hydrologic processes on a daily basis: surface accumulation of snow; snowmelt; runoff (curve-number method); evapotranspiration, consisting of surface water evaporation (simplified Pennman approach), plant transpiration (plant growth and decay model), and soil evaporation (extraction rate from the evaporative zone); infiltration; percolation (Darcy equation adapted for unsaturated conditions, unsaturated hydraulic conductivity is computed after Brooks and Corey, 1964); leakage (Darcy equation for saturated conditions); and lateral saturated drainage (Boussinesq equation). HELP has also been applied for groundwater recharge assessment in numerous hydrogeological studies ( Jyrkama et al., 2002; Gogolev, 2002; Scibek and Allen, 2006a; 2006b). But given the comprehensive set of modelled hydrologic processes, for given climate conditions, considerable over- or under-estimation of the water balance parameters were occasionally obtained. The reported principal setbacks of the model can be summarized as follows: for average daily temperatures equal or below zero, soil is considered as frozen with no snowmelt, runoff or spring infiltration taking place even if the maximum daily temperatures are well above zero; only gravitational forces are modelled as driving forces of water flow, which is unsuitable for unsaturated soils where capillary forces dominate water transport; vertical flow (percolation, leakage) and lateral flow (surface runoff, lateral drainage) are coupled for each soil column; however, lateral flow can not be transferred to adjacent soil columns; and the vegetation cover model is that of landfills (grass, shrub) and is not adapted for dense woodlands. On the other hand, the advantages of HELP are numerous: it is easy to use and adaptable for regional studies, the Fortran code is available for modifications, input data can be prepared in GIS format, etc. The various stratigraphic layers constituting a soil column in HELP can be modelled as one of the three pre-defined layer types: percolation layer, lateral drainage layer, or barrier soil layer. Figure 2 shows a conceptual model of groundwater recharge through a

typical stratigraphic sequence indicating the various flowpaths. Depending on the hydrogeological setting (Lamontagne and Nastev, 2010), the vertical sequence of layers varies from a single layer for simulating recharge where bedrock outcrops, to a maximum of six layers for simulating multiple soil horizons and the underlying Quaternary units when present. The top layers were defined using the regional soil map consisting of three sequential pedological horizons. The spatial distribution of Quaternary sediments overlying the regional rock aquifer units were used to define the lower set of layers (Lamontagne and Nastev, 2010; Tremblay et al., 2010). The water balance components are also shown in Figure 2. In the present study, groundwater recharge is defined as the vertical percolation rate at the base of the soil column; runoff is defined as the combination of surface runoff and subsurface runoff #1 (where it exists); whereas subsurface runoff is defined as subsurface runoff #2 in Figure 2. Essentially, subsurface runoff #1 is interflow.

Bedrock

Figure 2. Example of a vertical profile used for groundwater recharge estimation. Flowpaths are indicated with arrows. Numbers in brackets represent layer codes: percolation layer [#1], lateral drainage layer [#2] and barrier layer [#3]. © 2010 Canadian Water Resources Association © Her Majesty in Right of Canada


The input parameters required by HELP include climate data, land use, and vegetation, along with data describing the site drainage, soil column and soil properties. Climate Data

The climate data used in the simulations consist of daily sums of precipitation, mean daily air temperature, daily sums of solar radiation, average annual wind speed, and quarterly air humidity. The daily precipitation and temperature records were obtained from the Canadian Daily Climate Data (Environment Canada, 2004a) for 12 stations in the Canadian portion of the watershed, and from the National Climatic Data Center (NCDC, 2004) for seven stations in the U.S. portion. Average wind speed and daily solar radiation were obtained from the Canadian Weather Energy and Engineering Data Sets for the station located at Trudeau Intl. Airport, Montreal. The Canadian Climate Normals provided the quarterly relative humidity for the period 1971-2000 (Environment Canada, 2004b). To evaluate the observed differences in climate records, Thiessen polygons were used to define individual areas of influence around each of the climate stations. The grid elements belonging to the same polygon were assigned the climate data of the respective station.

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as a function of the hydrologic soil group, land use and slope for four major land use classes: agriculture, pasture, forest, and urban, defined from the simplified land use-land cover (LULC) map (Lamontagne and Nastev; 2010). The hydrologic classes A, B, C and D indicate respective drainage properties of soils, from lower to highest. The resulting curve number varies from 24 (low runoff ) for dense forested areas, to 90 (high runoff ) corresponding mainly to urban and intensive agricultural zones. In addition to the terrain slope, land use and vegetation cover also have obvious impacts on the runoff. For example, for a similar slope and drainage class, intensive agriculture induces higher runoff compared to forested areas. Finally, the drainage length, which represents the length of the downslope flow until a surface water collection system is reached, is used to simulate lateral drainage from the layers. The drainage distance was assessed for the four land use types given in Table 1. For agricultural land use, the drainage length is given by the distance to irrigation drains; in urban zones, it corresponds to the network of sewer systems and ditches, whereas for pasture and forest land use, it was assumed as the average distance to the nearest stream which first appears during a precipitation event. Table 1. Runoff Curve Number (CN) after Monfet (1979) and drainage length. Land Use

Drainage Parameters

Drainage parameters considered in the HELP model include slope, runoff curve number, and drainage distance. The terrain slope is used as the drainage inclination for runoff estimation. Elevation data from the National Aeronautics and Space Administration (NASA) 90 metre digital elevation model (NASA, date) were processed with ArcGIS to determine the distribution of slope magnitudes. The runoff curve number (CN) determines the amount of precipitation that ends up as runoff after a precipitation event. It was defined according to the United States Department of Agriculture (USDA) Soil Conservation Service curve number method (USDA, 1981; 1985). This approach was adapted by Monfet (1979) for soil types and climatic conditions in the province of Québec. Table 1 gives CN estimates

Slope

Hydrologic Class

Drainage

(%)

A

B

C

D

Length (m)

Agricultural land

8

62 64 70

72 76 80

79 84 87

82 88 90

10

Pasture

8

32 44 59

51 65 74

72 77 83

79 82 87

25

Forest

8

24 33 44

54 59 66

68 73 78

76 79 83

20

Urban

Dense

73

83

88

90

5


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Evapotranspiration Parameters

Potential evaporation in HELP is computed using the standard Penman approach for evaporation from an open water surface (Schroeder et al., 1994). The actual evapotranspiration corresponds to the sum of the available water evaporation, snow sublimation, soil water evaporation, and plant transpiration. In the HELP model, evapotranspiration is limited to the depth of the evaporative zone, which corresponds to the maximum depth from which water may be removed. For the simulations, the evaporative zone depth was assumed equivalent to the average plant root penetration, and the LULC map was used to identify the vegetation types (Lamontagne and Nastev, 2010). The corresponding rooting depth is given in Table 2. The density of vegetation cover was assessed in accordance with the maximum leaf area index (LAI). The algorithm used to define the LAI, developed from LANDSAT-5 satellite images and from ground measurements, is discussed in Latifovic et al. (2010). Figure 3 depicts the LAI distribution over the study area. The LAI values vary from 0 for bare soils to 8 for very dense canopy. In the mainly agricultural region to the north, the LAI is relatively low with values around 2. In the southern part of the watershed, which is mainly forested, LAI values are equal to or higher than 4. Definition of the vegetation growing season was correlated to temperature according to the guidelines Table 2. LULC classes and corresponding root depths. Root LULC class

depth (m)

Agriculture Mature to old tree canopy ( >60 y) Young tree canopy (30~40 y), mediumto low density High-low or herb-shrub dominated Wetlands Medium biomass Rock outcrops, low vegetation cover Low vegetation cover Urban and built-up Cropland Orchard

0.3 1.0 0.75 0.25 0.3 0.5 0.2 0.3 0.2 0.6 1.0

given in the HELP User’s manual (Schroeder et al., 1994). The average temperature weighted over the climate stations was used to set the growing season from May 1st to September 30th for a total duration of 152 days. Soil Properties

Each layer in the vertical soil profile was defined by its thickness and soil properties (total porosity, field capacity, wilting point, and saturated hydraulic conductivity). In this study, particular attention was given to the characterization of the topsoil units since these have a direct impact on runoff and evapotranspiration estimates. The soil types given by the Institut de Recherche et de Développement en Agroenvironnement (IRDA) (2004) and USDA (2004) databases and their corresponding textures were used. The existing soil types present in the study area were assigned a soil texture for each of the three horizons. Overall, 90 texture combinations were obtained (Croteau, 2006). Their physical properties, including physicochemical alteration and weathering, were compiled by Lamontagne (2005) based on pedological studies including field measurements and laboratory analysis. The vertical stratigraphy and corresponding thicknesses of the Quaternary sediments were defined from a 3D geological model (Tremblay et al., 2010). The 3D model was further simplified for a maximum of three stratigraphic units for each vertical profile. The initial saturated hydraulic conductivities (Ks) were assigned on the basis of values reported by previous studies carried out in the watershed (Dufresne, 1979; Hamel, 2002; Pontlevoy et al., 2004; Technorem, 1998) and from data collected from the literature (de Marsily, 1986; Fetter, 1980; Freeze and Cherry, 1979; Gerber and Howard, 2000). The Ks value was used as the main calibration parameter in the infiltration simulations. The other physical properties (total porosity, field capacity and wilting point) were defined with the default properties embedded in HELP that relate to Ks.



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Integration of Input Data

The complete set of input data needed for estimating the water balance components was prepared prior to the simulations. The study area was divided into a 250 m square grid for a total number of 47,616 grid elements. For each grid element, the input data were assigned and integrated into an MS Access database. Spatial referencing was managed with ArcGIS (Brodie, 1999), which was also used for visual representation of results. Furthermore, the HELP code had to be modified to support batch computation for integrating input data and generating outputs for all grid elements. The water balance components were computed neglecting fluxes, surface or subsurface runoff, between neighbouring grid elements. In this way, the lateral fluxes (runoff and subsurface runoff ) were automatically removed from the groundwater flow system. Sensitivity Analysis

A sensitivity analysis was conducted before attempting to calibrate the input parameters. The analysis was carried out for a soil column consisting of six layers with a total thickness of 20 m, which is representative of the average hydrogeological conditions within the Chateauguay River valley. Each input parameter was modified within the range of its probable values, while other data remained constant. The climate data, which constitute the most important factor controlling water availability for infiltration, were the only input data excluded from this analysis. Annual recharge at the end of the simulation period was compared as a percentage of the total precipitation for that period. Three parameters: Ks of the Quaternary sediments and bedrock, the CN, and the evaporative zone depth, were identified as having major impacts on the results, with the Ks value being the most important (Croteau, 2006). An example of the effect of the Ks value, CN, and evaporative zone depth on the recharge rate for a representative six-layer column is given in Figure 4. Note that these effects are valid only for this particular set of input parameters. Although the trends would be the same, the effects shown in Figure 4 can be of different magnitudes for other parameter combinations. In order to simplify the calibration process, the Ks value of the Quaternary sediments was retained as the sole calibration parameter, whereas the values of


the other input parameters were assumed fully known throughout the simulations. Calibration Procedure

The annual baseflow estimations obtained with the hydrograph separation technique were used as calibration targets (Croteau, 2006). In this way, the annual recharge rate for the area up-gradient of a given gauging station was assumed to be equal to the aquifer contribution to the stream flux measured at that station. The calibration process was conducted over the des Anglais River sub-watershed (Figure 1) where three gauging stations have suitable hydrographs (Environment Canada, 2004c). Hydrograph separation was carried out with the filter algorithm proposed by Lyne and Hollick (1979) including its later modification by Chapman (1999; Figure 5). The final soil physical properties are presented in Table 3. As expected, the fine marine sediments and compact glacial sediments have the lowest Ks values, whereas coarser sediments, such as reworked till, fluvio-glacial sediments, and alluvial sediments, are the most permeable. Table 3. Physical properties of Quaternary sediments and bedrock. Total Sediment type

Field

Porosity Capacity

Wilting

Ks

Point

(m/s)

(vol/vol) (vol/vol) (vol/vol)

Organic soil Alluvium Littoral and eolian sand and gravel Lacustrian clay, silt and sand Marine clay and fine silt Fluvio-glacial coarse sand and gravel Till Reworked till Bedrock Backfill

0.90 0.40

0.30 0.05

0.11 0.02

3.0×10-5 1.0×10-4

0.40

0.05

0.02

1.0×10-4

0.50

0.28

0.14

1.0×10-5

0.45

0.42

0.33

9.8×10-10

0.40 0.36 0.46 0.05 0.46

0.03 0.26 0.13 0.04 0.36

0.01 0.15 0.06 0.01 0.20

3.0×10-3 5.5×10-7 1.3×10-4 2.2×10-6 5.5×10-9



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10

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8 6 4 2 0 0

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150 200 250 Time (Julian day 1987)

300

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Figure 5. Example of hydrograph separation for the gauging station #02OA057 at the des Anglais River for 1987. The runoff contribution is obtained as the difference between the river flux and aquifer contribution. All values are normalized to the area upstream of the gauging station.


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Results Overall Water Budget Parameters

The daily output from the HELP simulations were obtained over a period of 42 years from 1960 to 2001. However, the first three years, 1960-1962, were only used to initialize the soil layer moisture profiles and were not considered in the computation of averages of the water balance components. Simulated averages for the entire study area are presented in Table 4. The highest proportion of the total precipitation, 52%, is accounted for by the evapotranspiration, 487 mm. The average recharge rate attains 86 mm/yr, which represents 9% of precipitation, whereas the combined runoff and subsurface runoff of 371 mm represent the remaining 39% of the precipitation. Table 4. Annual water balance components averaged over the 1963-2001 time period. Water Budget Component

Precipitation Evapotranspiration Runoff Subsurface runoff Recharge

Average ± s

%

(mm)

Precipitation

943 ± 116 487 ± 42 245 ± 61 126 ± 35 86 ± 10

100 52 26 13 9

Spatial Distribution of Recharge

The spatial distribution of the average annual recharge rate over the considered period is shown in Figure 6. The recharge rate varies considerably across the watershed and ranges from 0 mm/yr to a maximum value of 404 mm/yr. Such a significant range confirms the heterogeneous geological and hydrogeological setting. Recharge was not computed for grid elements representing surface water (i.e., streams, lakes, swamps) and for neighbouring discharge areas with upward flow gradients. For these grid elements, recharge was assumed nil (0 mm). Such discharge areas cover approximately 9% of the study area (Lavigne et al., 2010) and any vertical infiltration through these areas was converted to subsurface runoff.

The lowest recharge rates of approximately 20 mm/yr were computed for fine marine sediments along the Chateauguay River valley, where confined flow conditions prevail. The highest recharge rates were computed for the Covey Hill, Le Rocher and the Huntingdon Hill areas, which are mainly underlain by thin unconsolidated sediments and/or rock outcrops. The Beaver Crossing esker to the east, and the Mercier esker to the west, also appear to be major recharge areas (≥250 mm). These results were also corroborated with the groundwater chemistry data, which showed elevated tritium, attesting to an important influx of recent atmospheric water (Blanchette et al., 2010). Table 5 provides recharge averages for the four major types of Quaternary sediments in the region. The fine marine sediments, modelled generally as barrier layer [#3], show the lowest recharge rates. Although these sediments cover approximately one-third of the study area, only 7% of the recharge occurs when these sediments are present. Depending on their thickness and on the assumed hydrogeological setting, glacial sediments have intermediate recharge rates. Being almost ubiquitous in the study area, till controls most of the recharge in the watershed, estimated at 74%. The shallow reworked till and coarse sand and gravel show the highest recharge rates, and combined they account for 19% of the recharge. Transient Recharge

The annual variation of the computed water balance components throughout the considered time period is shown in Figure 7. Evapotranspiration and runoff seem highly correlated to precipitation fluctuations. Subsurface runoff shows a relatively subdued response with much smaller variations than the other two components. The annual recharge, however, seems only slightly correlated to precipitation. Annual recharge is found to be relatively constant and varies in a narrow range of ±10 mm/yr around an average value of 86 mm (Table 4). In addition, recharge response appears delayed in time relative to precipitation. No major trends can be observed in Figure 7, i.e., none of the water balance components shows a marked increasing or decreasing trend from 1963 to 2001. Over the same period, the mean annual temperature appears uniform and has no apparent impact on the computed water balance components. © 2010 Canadian Water Resources Association © Her Majesty in Right of Canada


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Table 5. Annual recharge rates for the four major classes of sediments in the study area. Sediment Class

Fine marine sediments Till Reworked till Sand and gravel

Annual Recharge

% of

% of Total

Assumed Flow

± s (mm)

Study Area

Recharge

Conditions

21 ± 7 112 ± 15 191 ± 39 251 ± 50

32 60 6 2

7 74 13 6

Confined Semi-confined Unconfined Unconfined

Figure 7. Annual variation of the water balance components for the Chateauguay River watershed (1963-2001).

The daily variation of temperature and water balance components averaged over the study area (47,616 grid elements) is depicted in Figure 8. Only the results for the year 1985 are shown as they represent typical climate conditions for the region (Croteau, 2006). On a daily basis, it appears that there is no direct relationship between the precipitation and the water balance components. The comparison with the temperature, however, shows close correlations. The major recharge period occurs in spring when the daily temperature stabilizes above 0oC. At the same time, the surface and subsurface runoffs attain their maximum values mainly due to the rapid snowmelt, and evapotranspiration starts to increase. The second recharge period occurs in the fall, coinciding with decreased temperature and evapotranspiration. During

this period, vegetation cover decreases so that water uptake by vegetation roots is minimal and more water becomes available for percolation and runoff. Climate Change Scenarios

Having calibrated the model for actual climate conditions, it was possible to predict the potential effects of climate change on the groundwater recharge for the Chateauguay River watershed. The potential impact of climate change on groundwater resources has become an important issue in the last decade (Intergovernmental Panel on Climate Change (IPCC), 2002). Two extreme climate scenarios were considered based on the driest and wettest recorded years in the region. These simulations are thus intended to © 2010 Canadian Water Resources Association © Her Majesty in Right of Canada


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Figure 8. Daily variation of weather and hydrologic parameters during year 1985 for the Chateauguay River watershed.

anticipate the response of recharge to extreme changes in climate. In order to simulate a drought scenario, the year 1964 was selected as it was the driest year between 1960 and 2002. The total precipitation for year 1964 was 683 mm, which is 260 mm below the average annual precipitation computed for the study area (943 mm). On the other hand, the year 1972 had the

highest annual precipitation, 1,243 mm, and was thus selected as climate input for the humid scenario. The simulations were carried out by repeating the same daily climate data over six consecutive years to assure adequate model spin-up. The results for the last year were considered representative for the given scenario. The resulting water balance components (averages ±1 s) for the drought and humid scenarios, respectively, © 2010 Canadian Water Resources Association © Her Majesty in Right of Canada

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are shown in Figure 9 together with those obtained for the average climate conditions discussed earlier. The standard deviation in precipitation for both climate scenarios is approximately ±30% of the average. However, the impact of changes in precipitation on the water balance components is not proportional. For the humid scenario, an increase of 15% (13 mm) in average annual recharge is estimated for a 300 mm increase in precipitation. Most of the water surplus during the humid climate conditions was accounted for by the increase of runoff (76%, 185 mm) and subsurface runoff (56%, 71 mm), i.e., it will rapidly discharge to surface waters. Thus, the capacity of soils to store surplus water and eventually transmit it as downward via percolation seems to be limited in this study area. The decrease in annual recharge rate, 41% (35 mm), during the drought scenario is considerably more significant. In this case, evapotranspiration shows the lowest proportional decrease, -16% (80 mm), and its proportion to the total precipitation increases to a maximum of 60%. During dry periods, most of the precipitation water is retained in place; it is then evaporated and/or consumed for plant transpiration. In addition, the impacts of the two climate change scenarios on the water balance components are not uniform over the study area. To emphasize the most sensitive areas, the decrease in the recharge rate was considered as the most critical factor for the drought scenario. The spatial decreases in recharge rates are

shown in Figure 10. The major recharge zones defined under average climate conditions are subject to the highest reduction in absolute magnitude. Areas such as Covey Hill, Huntingdon Hill, Le Rocher, and the Beaver Crossing and Mercier eskers appear to be the most sensitive to decreases in precipitation. The impact of the dry conditions on recharge rates in zones of confined groundwater flow characterized by low annual replenishment is much smaller.

Conclusions The HELP infiltration model was used to estimate spatial and transient variations of groundwater recharge to the fractured sedimentary rock aquifers in the Chateauguay River watershed. The input data consisted of typical climate, hydrological, and evapotranspiration properties. Representative soil profiles were used to simulate infiltration, which typically consist of topsoil and Quaternary sediments, mainly till and marine clays, overlying the regional rock aquifer units. Model input parameters were calibrated against the baseflow contribution to the streamflow obtained by hydrograph separation. The saturated hydraulic conductivity of the Quaternary sediments was used as the calibration parameter. Results from simulations indicate an overall annual recharge of 86 mm, or ~9% of the total precipitation. Humid scenario

600

Average climate conditions

+ 6%

+32%

943 mm

1250

1000

487

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Drought scenario

+76% -16%

750 -28%

500

300 245

150

+56%

-30%

Precipitation (mm)

Water balance components (mm)

750

250

126 86 -59%

+15% -41%

0

0

EvapoSurface Subsurface Recharge Precipitation transpiration runoff runoff Figure 9. Water balance components for average climate conditions and for the two extreme climate scenarios (% change compared to average climate conditions).



465 74°15’

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Figure 10. Annual decrease inrecharge rate for the drought scenario compared to average climate conditions.

Evapotranspiration accounts for most of the total precipitation, 52% on average, whereas approximately 39% of the precipitation discharges via overland flow and subsurface runoff. The recharge distribution varies from 0 mm near surface water bodies, ~20 mm in areas of fine marine sediments, ~110 mm in areas of glacial till, to more than 250 mm in areas of permeable coarse sediments (reworked till, sand and gravel). Hydrogeological settings thus exert an important control on recharge under the conditions found in the

study area. The highest recharge rates are observed for water table aquifers found mainly at topographic highs. The major recharge zones in the study area are the Covey Hill, Le Rocher, Huntington Hill regions, and the Beaver Crossing and Mercier eskers. The daily distribution of recharge rates indicates that spring and fall are the key recharge periods. Throughout the considered 1963-2001 period, annual evapotranspiration and surface runoff show a direct correlation with the total precipitation. The annual recharge rate shows a © 2010 Canadian Water Resources Association © Her Majesty in Right of Canada

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more subdued response, which is relatively stable with a low standard deviation of 10 mm. Extreme climate conditions were simulated considering the recorded driest (1964) and wettest (1972) years. The simulated average annual recharge rate was 51 mm and 99 mm for these dry and the humid scenarios, respectively. The spatial distribution of reduction in recharge for the drought scenario was used to define the zones most sensitive to potential extreme climate conditions. These zones coincide with the major recharge areas identified under average climate conditions. Efforts to protect the groundwater resource should be focused on these recharge zones, which are at the same time the most vulnerable to surface contamination. The use of a physically-based model allowed spatially varying recharge to be estimated based on the sequence and properties of soils and surficial sediments. After calibration using readily available river hydrographs, this approach provided recharge estimates that take into account spatially varying climate conditions, physiography and geological conditions.

Acknowledgements The authors thank the Ministère du Développement durable, de l’Environnement et des Parcs du Québec, the Geological Survey of Canada, and Institut national de la recherche scientifique (INRS) - Centre Eau Terre Environnement for their financial support. René Lefebvre acknowledges the support of an NSERC Discovery grant. The authors are grateful to Luc Lamontagne from Agriculture and Agri-Food Canada, and Richard Fernandes from the Canadian Centre for Remote Sensing (Ottawa). Finally, the authors thank Professor John Molson for reviewing an early draft of this manuscript.

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