Implementation of a Peatland-Specific Water Budget Algorithm in ...

Implementation of a Peatland-Specific Water Budget Algorithm in HYDROTEL Sylvain Jutras, Alain N. Rousseau, and Clément Clerc

Abstract: HYDROTEL, a distributed hydrological model, was used to simulate streamflows of the Necopastic watershed (N53°40.6’; W78°09.8’), James Bay, Quebec. Because of the prevalence of peatlands in the studied environment, important issues regarding soil parameterization in the vertical, three-layer, water budget sub-model (BV3C) of HYDROTEL were raised. Since BV3C was originally developed for mineral soils, an alternative, peatland-specific, water budget sub-model (PHIM) was integrated into HYDROTEL. Basic data requirements included a description of the organic soil structure and a watertable/discharge relationship. PHIM was calibrated with data collected on a peatland complex located within the Necopastic watershed. Preliminary results of observed and simulated streamflows using both the original and the adapted versions of HYDROTEL compared well, therefore leading the way to future simulations of North-Boreal watersheds. Résumé : Le modèle hydrologique distribué HYDROTEL a été utilisé pour simuler les débits du bassin versant de la rivière Nécopastic (N53°40.6’; W78°09.8’), Baie de James, Québec. L’abondance des tourbières dans l’environnement étudié a permis d’identifier certains aspects contradictoires liés au paramétrage des sols dans le sous-modèle de bilan vertical trois couches (BV3C) d’HYDROTEL. Puisque ce sous-modèle était originalement conçu pour les sols minéraux, un sous-modèle spécifiquement adapté à l’écoulement des tourbières (PHIM) a été intégré dans HYDROTEL. Les données nécessaires au fonctionnement de PHIM incluent une description de la structure verticale des sols organiques et une relation liant le débit à la profondeur de la nappe phréatique. PHIM a été calé à l’aide de données provenant d’un complexe de tourbières localisé à l’intérieur du bassin versant de la Nécopastic. Les résultats préliminaires montrent une bonne concordance entre les débits observés et simulés, qu’ils aient été obtenus à l’aide de la version originale ou adaptée d’HYDROTEL. Ces résultats préliminaires démontrent les potentiels d’applications de différentes versions d’HYDROTEL dans le contexte spécifique des basins versants Nord-boréaux.

Sylvain Jutras1,2, Alain N. Rousseau1, Clément Clerc1 Centre Eau, Terre et Environnement, Institut national de la recherche scientifique (INRS-ETE), Quebec, QC G1K 9A9 2 Now at: Faculté des sciences de l’agriculture et de l’alimentation, Pavillon Envirotron, Université Laval, Québec, QC G1V 0A6 1

Submitted October 2008; accepted February 2009. Written comments on this paper will be accepted until June 2010. Canadian Water Resources Journal Revue canadienne des ressources hydriques

Vol. 34(4): 349–364 (2009)

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Introduction Since the second half of the 1980-1990 decade, hydroelectric reservoirs of Quebec’s North-Boreal region have shown unexpected periods of low water levels (Roy, 2004). During these low-level periods, it was also noted that inflows to reservoirs following important rainfall events were sometimes delayed and characterized by smaller amplitudes than expected. These damped responses could have been related to the substantial water stocking capacities of peatlands which occupy an important proportion of the NorthBoreal landscape. For example, peatlands cover about 20% of the La Grande Riviere watershed (Tarnocaï et al., 2000). Thus, finding an appropriate way to account for the rainfall-runoff behaviour of peatland represents a key issue for hydrological modelling of the NorthBoreal watersheds. The objectives of this study were to: (i) integrate a peatland-specific, rainfall-runoff, sub-model (PHIM) into HYDROTEL; (ii) evaluate the sub-model performance on a small (< 1 km²) peatland-dominated pilot sub-watershed; and (iii) simulate streamflows of the 188-km² Necopastic watershed, James Bay, Quebec, using original and adapted versions of HYDROTEL. This study also highlights the challenges encountered when setting up a hydrological model in remote and/or ungauged watersheds. Nonetheless, this study investigates the suitability of the proposed modelling approaches under these conditions. HYDROTEL

HYDROTEL is a distributed hydrological model (Fortin et al., 2001a,b; Turcotte et al., 2003) that has been successfully applied on various southern Canada watersheds (Turcotte et al., 2004; Lavigne, 2007; Ricard, 2008; Rousseau et al., 2008a,b). However, it has never been applied to North-Boreal environments (> 50th parallel). Its ability to take advantage of any geographic information, whether it comes from traditional governmental maps, recent geospatial acquisitions, remote sensing interpretations or any other sources, enables applications to remote watersheds where cartographic data are rare or under constant development. HYDROTEL has a good balance between physically-based algorithms and ease of use of empirical equations that have resulted in

successful applications on both small (Lavigne, 2007, 9 km²) and large (Ricard, 2008, 2500 km²; Fortin et al., 2001b, 6800 km²) watersheds. The spatial distributions of the hydrological processes simulated by HYDROTEL, such as meteorological data interpolation, evapotranspiration, vertical water budget, snow cover evolution, landsurface runoff and channel flow are done for each computational units, that is relatively homogeneous hydrological units (RHHUs), corresponding to small sub-basins drained by a section of the hydrographic network. The sub-model that accounts for the physical properties of soils is BV3C (Bilan Vertical Trois Couches), a three-layer, vertical water budget. BV3C simulates the redistribution of water into the soil using several physical equations, sometimes simplified. These equations require numerous parameters describing the hydrodynamic properties of the soil matrix. They include parameters such as the saturated hydraulic conductivity (Ks), the water content at saturation, field capacity and wilting point (θs, θfc and θwp, respectively), and the bubbling pressure entry (ψs). Water flux within the simulated soil column of a RHHU is estimated using Darcy’s equation and Campbell’s unsaturated hydraulic conductivity equation (Campbell, 1974). While still widely used in conceptual models because of simple parameterization requirements, the Campbell equation works reasonably well for mineral soils but is problematic when applied to organic soils. Indeed, it is well known that Campell’s equation does not mimic well the hydrodynamic behaviour of organic soils, especially close to saturation (Clapp and Hornberger, 1978; Weiss et al., 1998). Moreover, for organic soils, many hydraulic properties needed for this equation are difficult to obtain with reasonable accuracy.Organic soils are also known to vary a lot spatially (Letts et al., 2000). Therefore, it seems unrealistic to describe adequately water movement in organic soils by describing them with a pedo-transfer function specific to mineral soils and by using mineral soil water movement equations (Letts et al., 2000). In the absence of adapted and validated peat-specific equations, the use of simple models for peatland streamflow simulations represents a pragmatic approach (Verry et al., 1988; Letts et al., 2000). For these reasons, the peatland specific rainfallrunoff sub-model (PHIM) included in the Peatland Hydrologic Impact Model (Guertin et al., 1987; Brooks et al., 1995), was selected for this study. © 2009 Canadian Water Resources Association

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PHIM

Data Acquisition

The hydrological behaviour of peatlands can be compared to what is happening when water is added to a sponge (Bay, 1969; Ingram, 1983). The input and output of water will be completely different depending on the initial water content. If the sponge is dry, additional water will be absorbed and no water, or very little, will flow out. This situation will remain until the maximum water holding capacity of the sponge is nearly reached. As more water is added, water will begin to flow out of the sponge and, as this process continues, the outflow will reach a steady state. PHIM considers this dual behaviour by calculating the outflow of peatlands using two logarithmic equations relating discharge and water-table level. The Peatland Hydrologic Impact Model is a deterministic hydrological model for undisturbed or harvested upland and peatland watersheds (Guertin et al., 1987; Brooks et al., 1995). In this study, only the sub-model that simulates peatland discharge and water-table level (PHIM) was used. PHIM is based on two equations mimicking the behaviour of water within a peat column: (i) water-table elevation (WTE) and discharge (Q), and (ii) cumulative water storage (CWS) and water-table elevation (WTE). Both equations are established using field observations.

The empirical equations required by PHIM were fitted using local data gathered on a peatland site (N53°40.4’; W78°09.5’) located within the Necopastic watershed (Clerc et al., 2008). This site drains 0.9 km² and is largely occupied by organic soils, most of these being ombrotrophic peatland (63%). This easily accessible site has the ecological and hydrological characteristics of a representative peatland-dominated watershed. Furthermore, one of the key features of this watershed is a unique and easily identifiable outlet which facilitates outflow monitoring. Both discharge and water-table levels were monitored throughout the 2007 growing season. Discharge was measured at a 15-minute interval using a submersible pressure-transducer (Levelogger™ M5 Gold, Solinst, Georgetown, ON, Canada) suspended in a stilling well attached to a 12-in SRCRC trapezoidal flume (Accura-Flo™, Tempe, AZ, USA ) ( Jutras et al., 2007). Water-table level relative to the soil surface was also measured at a 15-minute interval using a submersible pressure-transducer suspended in a well installed in the middle of the peatland. The WTE vs Q relationship was numerically described after fitting two regression equations on the logarithmic transformations of the variables (Figure 2a). The CWS vs WTE relationship is a representation of the amount of water needed to fill the void in each centimetre of the soil matrix within the acrotelm. In this study, the acrotelm, defined as the hydrologically active layer of the peat soil (Verry, 1984), was fixed at 50 cm. The water storage is defined as the difference between the water contents at saturation and field capacity (θs - θfc). It has been derived from observations of numerous peat soil profiles on the peatland site. A quadratic relationship, as required by PHIM (Brooks et al., 1995), describes the relationship between the WTE in the acrotelm (the bottom of the acrotelm being the datum and the average soil surface being the top) and the CWS (Figure 2b). The meteorological data was acquired by a network of three weather stations (Figure 1). From each of these, the maximum and minimum daily air temperatures (°C) and the daily precipitation (mm) were gathered and later used for hydrological simulations.

Material and Methods Study Areas

The Necopastic watershed, James Bay, QC, Canada, covers 242 km² and its outlet discharges directly into the La Grande Riviere (Figure 1). Virtually unaffected by human activities, this watershed represents well the eastern part of the La Grande Riviere basin. A gauging station, that drains 188 km², was set up in 2004 and equipped with an Argonaut-SW™ acoustic velocity sensor (SonTek/YSI, San Diego, CA, USA) (N53°40.6’; W78°09.8’). The altitude of the watershed varies from 100 to 150 m, with numerous flat surfaces. The watershed was part of the Tyrrell post-glacial sea from 8000 to 6500 years ago, resulting in the deposition of fine material in lowland areas (Vincent, 1977). Peatlands now occupy these poorly drained lowlands, following a long paludification process.

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Figure 1. Study site.

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Figure 2a. Relationship between water-table elevation (WTE) and discharge/streamflow (Q) of a 0.9-km² peatland-dominated watershed.

Figure 2b. Relationship between cumulative water storage (CWS) and water-table elevation (WTE) of a 0.9-km² peatland-dominated watershed. © 2009 Canadian Water Resources Association

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Integration of PHIM into HYDROTEL

Integration of PHIM into HYDROTEL was done in a way that both BV3C and PHIM could be used during the same simulation, HYDROTEL calling either one or the other as a function of the RHHU dominant soil type (i.e., mineral or organic). Peatland Site Streamflow Simulation

In a first step, the original HYDROTEL version using BV3C as the rainfall-runoff sub-model was employed to simulate daily streamflow of the peatland site over a continuous three-year period beginning in October 2005. The watershed was modelled using a single 0.9-km² RHHU dominated by an ombrotrophic peatland environment in order to: (i) simplify the computational effort and (ii) mimic the average size of a standard RHHU for a mid-size watershed streamflow simulation. Initial values for calibration parameters (Table 1) and soil hydraulic properties (Table 2) were based on literature review and past simulation experiences. These values were then adjusted, varying within acceptable limits, until simulated streamflows corroborated with observed values ( June to November 2007 and June to October 2008). Then, a second HYDROTEL simulation was performed, this time using the adapted version (PHIM). All parameters not linked to the BV3C or the PHIM sub-models kept precisely the same values in order to enable a robust comparison between the sub-models. The parameter values required by the PHIM sub-model (Table 3) were determined by field observations and measurements and, once again, some were adjusted until the best fit was obtained between simulated and observed streamflows. Necopastic Watershed Streamflow Simulation

The daily streamflow at the gauging station of the Necopastic watershed was simulated over a continuous three-year period, beginning in October 2004, using both the original and the adapted versions of HYDROTEL. The studied 188-km² watershed was discretized into 203 RHHUs (mean area = 0.92 km²) for which the most dominant soil type was assessed by a remote-sensing procedure. Land cover was

determined after classifying a LANDSAT 7-ETM+ image (August 20, 2000) using an object-oriented software (Definiens V5.0) and numerous control sites identified with the help of aerial photographs (overall classification accuracy = 83%; see Table 4). The dominant soil type of each RHHU was later determined on the basis of the resulting land cover (i.e., organic soils being associated with wetland and peatland vegetation). Observed streamflows were unavailable during winter since the flow monitoring device could not provide precise measurements under ice cover. Occurrences of early spring ice-jams have been documented for this river (Boucher et al., accepted) and there is a strong probability that observed streamflows during this period were affected by this natural phenomenon. Thus, both winter and early spring periods were omitted from the figures and from further analysis. The simulation approach was the same as that used for the peatland site, where in a first step; streamflows were calculated using the original version of HYDROTEL while the adapted version of HYDROTEL was used for the second simulation. In both cases, values for calibration parameters and soil hydraulic properties were taken from the previous simulations executed on the peatland site.

Results and Discussion Peatland Site Calibration

The use of the original version of HYDROTEL to simulate streamflow at the outlet of the peatland site required several adjustments of the parameter values, but especially those related to the soil hydraulic properties (Table 2). Data obtained from field observations and soil parameter values reported by Letts et al. (2000) completed the specific description of organic soils. Finding mean values for the parameters describing the top 70 cm of organic soils was complicated by the fact that these soils vary a lot spatially, both vertically within soil profile and horizontally between profiles scattered throughout the whole peatland area. For example, the measured values of hydraulic conductivity at saturation for the top 10 cm of peat for different locations on the peatland site varied from 0.023 to 18 m/h. Therefore, the last series of parameters found in Table 2 correspond to both the best physically determined values and the best simulation results. Then, the depths of the three © 2009 Canadian Water Resources Association

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Table 1. Selected sub-models (in italic) and values of selected parameters for HYDROTEL simulations. Parameters

Unit

Value Peatland BV3C

Necopastic

PHIM

BV3C

BV3C-PHIM

Meteorological data interpolation : Thiessen polygons Vertical precipitation gradient Vertical temperature gradient Rain-snow temperature threshold

mm/100m °C/100m °C

0 -0.5 0

0 -0.5 0

0 -0.5 0

0 -0.5 0

Snow-pack evolution: Mixed approach Snow-soil interface melt rate coefficient Maximal snow cover density Compaction coefficient Snow-air interface melt rate coefficient - Forest Melting temperature threshold - Forest Snow-air interface melt rate coefficient - Open area Melting temperature threshold - Open area

mm/d kg/m³ d-1 mm/d °C °C mm/d °C °C

0.78 466 0.10 8.1 -1 6 0

0.78 466 0.10 8.1 -1 6 0

0.78 466 0.10 8.1 -1 6 0

0.78 466 0.10 8.1 -1 6 0

1

1

1

1

0.3 0.5 0.7 0.00001

n.a. n.a. n.a. n.a.

0.05 0.3 0.5 0.00001

0.05 0.3 0.5 0.00001

0.1 0.03 0.05 0.001

0.1 0.03 0.05 0.001

0.1 0.03 0.05 0.001

0.1 0.03 0.05 0.001

Potential evapotranspiration: Thornthwaite Multiplicative coefficient Vertical water budget : BV3C Depth of the bottom of layer 1 Depth of the bottom of layer 2 Depth of the bottom of layer 3 Recession coefficient Channel flow: Kinematic wave Manning’s overland roughness - Forested area Manning’s overland roughness - Water Manning’s overland roughness – Organic soils Geomorphological hydrograph reference runoff height

cm cm cm m/h

dimensionless dimensionless dimensionless m

Note: For further explanation of the equation linked to these parameters, refer to Fortin et al. (2001a) and Turcotte et al. (2007).

soil layers were adjusted to fine-tune the streamflow response following rainfall events. The second simulation performed on the peatland site was done using the adapted version of HYDROTEL. The PHIM sub-model parameters related to the soil properties of the acrotelm and the catotelm (Table 3) showed little influence on simulated streamflows, so they were fixed to their mean values estimated from field observations. The value of the groundwater inflow parameter was found to have an important impact on streamflows. It was taken from Brooks et al. (1995) and subsequently adjusted

during the calibration process. The CWS vs WTE relationship coefficients were also calibrated within the theoretical limits determined by soil observations (Figure 2b) until the best fit was obtained between simulated and observed streamflows. Finally, since the determination of the WTE vs Q relationship coefficients was done from precise field observations (Figure 2a), no variation was allowed for them during the calibration procedure. The simulated streamflow using the original and the adapted versions of HYDROTEL showed a reasonable corroboration with those observed © 2009 Canadian Water Resources Association

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Table 2. Soil hydraulic properties of various soil textures Soil Texture

Sand Loamy Sand Sandy Loam Loam Silt Loam Sandy Clay Loam Clay Loam Silty Clay Loam Sandy Clay Silty Clay Clay Fibric Peat Hemic Peat Sapric Peat Peat (Necopastic)

θs

0.417 0.401 0.412 0.434 0.486 0.330 0.390 0.432 0.321 0.423 0.385 0.930 0.880 0.830 0.900

θfc

0.091 0.125 0.207 0.270 0.330 0.255 0.318 0.366 0.339 0.387 0.396 0.275 0.620 0.705 0.600

θwp

0.033 0.055 0.095 0.117 0.133 0.148 0.197 0.208 0.239 0.250 0.272 0.050 0.150 0.250 0.150

Ks

Ψs

0.2100 0.0611 0.0259 0.0132 0.0068 0.0043 0.0023 0.0015 0.0012 0.0009 0.0006 1.0080 0.0072 0.0004 0.1000

0.1598 0.2058 0.3020 0.4012 0.5087 0.5941 0.5643 0.7033 0.7948 0.7654 0.8560 0.0103 0.0102 0.0101 0.0103

λ

0.694 0.553 0.378 0.252 0.234 0.319 0.242 0.177 0.223 0.150 0.165 0.370 0.164 0.083 0.350

Note: θs : Water content at saturation (cm³/cm³); θfc: Water content at field capacity (cm³/cm³); θwp: Water content at wilting point (cm³/cm³); Ks: Hydraulic conductivity at saturation (m/h); Ψs: Soil water suction at saturation (cm); λ: Pore size distribution parameter (dimensionless). Sources: Mineral soil: Rawls and Brakensiek (1989); Peat: Letts et al. (2000).

(Figures 3 and 4; Table 5). The use of the BV3C submodel resulted into slightly more biased streamflow values than those simulated by the PHIM sub-model (Table 5). The suitable streamflow simulation for both wet (late-summer of 2007; Figure 3) and dry episodes (late-summer of 2008; Figure 4) during a continuous three-year simulation demonstrate the effectiveness of both modelling approaches. An autumn bias was observed for the PHIM sub-model simulation in 2007 (Figure 3), where simulated streamflows remained high during October and beyond (data not shown). This behaviour occurred at the beginning of the soil freezing period, suggesting a possible change in water flow properties of organic material under cold weather conditions. Field observations used for the establishment of the WTE vs Q relationship also showed a slightly different behaviour in early winter. The use of a different WTE vs Q relationship for soil freezing periods could possibly improve the simulation in early winter, although it would be of little help once the peatland outflow freezes during cold winter temperatures.

Necopastic Watershed Calibration

Using the original version of HYDROTEL and the parameter values determined during the peatland site simulations, only the soil depth parameters had to be adjusted in order to find the best fit between simulated and observed streamflows. These depths are common for all RHHUs, however, each RHHU has the soil hydraulic properties (Table 2) of their dominant soil texture. In this case, all peatland-dominated RHHUs were described using the parameter values for the last organic soil reported in Table 2. The simulation using the PHIM sub-model used the same calibrated soil depths for non-peatland dominated RHHUs while peatland-dominated RHHUs used parameter values previously determined during the peatland site simulations. Once again, very little calibration was necessary in order to obtain acceptable simulation results (Figures 5, 6 and 7; Table 6); only a diminution of the value of the groundwater inflow parameter was necessary (Table 3). It should be noted that both simulations were performed using a unique set of parameter values for all peatland-dominated RHHUs. Although © 2009 Canadian Water Resources Association

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Table 3. PHIM sub-model parameters values. Parameters

Soil properties of the acrotelm Depth Water content at saturation Water content at field capacity Water content at wilting point Soil properties of the catotelm Depth Water content at saturation Water content at field capacity Water content at wilting point Limiting saturated hydraulic conductivity Groundwater inflow CWS vs WTE relationship Parameter A Parameter B Parameter C WTE vs Q relationship Parameter A (High WT) Parameter B (High WT) Parameter A (Low WT) Parameter B (Low WT)

Unit

Value Peatland

Necopastic

cm cm³/cm³ cm³/cm³ cm³/cm³

50 0.9 0.6 0.15

50 0.9 0.6 0.15

cm cm³/cm³ cm³/cm³ cm³/cm³ cm/h cm/h WTE = A•(CWS)² + B• (CWS) + C

70 0.9 0.6 0.15 0.002 0.011

70 0.9 0.6 0.15 0.002 0.005

-0.02 3 0

-0.02 3 0

log(Q) = A•log(WTE) + B

11.6 -20.7 1.5 -4.7

11.6 -20.7 1.5 -4.7

Note: CWS = Cumulative water storage (cm); WTE = Water-table elevation above the bottom of the acrotelm (cm); Q = Discharge (m³/s).

Table 4. Object-oriented classified LANDSAT 7-ETM+ coverage of the Necopastic watershed. Soil Occupancy

Peatlands (bog + fen) Forest on organic soil Forest on mineral soil Channel vegetation Bare rock Water Non classified Total

Coverage (km²)

62.5 44.4 39.7 23.4 13.3 4.8 0.2 188.2

Coverage (%)

33.2 23.6 21.1 12.4 7.1 2.5 0.1 100.0

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Figure 3. Observed and simulated summer 2007 streamflows of a 0.9-km² peatland-dominated watershed using the original (BV3C) and adapted versions (PHIM) of HYDROTEL.

Figure 4. Observed and simulated summer 2008 streamflows of a 0.9-km² peatland-dominated watershed using the original (BV3C) and adapted versions (PHIM) of HYDROTEL. © 2009 Canadian Water Resources Association

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Table 5. Comparative analysis of summer (June 1 to October 31) streamflow simulation of the peatland site. 2007

Observed mean summer streamflows (m³/s) Sub-model used for the simulated RHHU Predicted mean summer streamflows (m³/s) Mean Relative Error (%) Correlation coefficient Nash-Sutcliffe coefficient

BV3C 0.0251 -17.1 0.94 0.82

0.0302

2008

PHIM 0.0301 -0.6 0.89 0.80

BV3C 0.0110 3.5 0.83 0.69

0.0104

PHIM 0.0102 -2.1 0.90 0.79

Figure 5. Observed and simulated summer 2005 streamflows at the monitoring station of the Necopastic watershed using the original (BV3C) and adapted versions (PHIM) of HYDROTEL.

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Figure 6. Observed and simulated summer 2006 streamflows at the monitoring station of the Necopastic watershed using the original (BV3C) and adapted versions (PHIM) of HYDROTEL.

Figure 7. Observed and simulated summer 2007 streamflows at the monitoring station of the Necopastic watershed using the original (BV3C) and adapted versions (PHIM) of HYDROTEL. © 2009 Canadian Water Resources Association

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Table 6. Comparative analysis of summer (June 1 to October 31) streamflow simulation of the Necopastic watershed. 2005

Observed mean summer streamflows (m³/s) Sub-model used for non-peatland RHHUs Sub-model used for peatland RHHUs Predicted mean summer streamflows (m³/s) Mean Relative Error (%) Correlation coefficient Nash-Sutcliffe coefficient

BV3C BV3C 3.82 2.6 0.91 0.82

3.72

major differences in soils properties and hydrological behaviour might occur between the numerous peatlands found within the Necopastic watershed, the selected approach for this study has shown to be promising for northern and remote areas. The lack of precise soil description over broad surfaces or either precise WTE and Q relationships could be the most important limitation for large-scale simulation on ungauged basins. Therefore, using a set of parameter values obtained on a single small-scale gauged site represents an obvious generalization of the great complexity of these ecosystems although a successful approach to simulate streamflow. Comparative performances of summer streamflow simulations showed the very similar fit of the adapted version of HYDROTEL when compared to the original version, which uses BV3C (Table 6). Overall, performances of both versions were satisfying for 2005 and 2007, but they were not as accurate for 2006. Total summer precipitations ( June 1 to September 30) were 523, 345 and 510 mm for 2005, 2006, and 2007, respectively while the 30year mean summer precipitation was 336 mm (La Grande Riviere meteorological station). Both summer 2005 and 2007 were characterized by above average rainfall while summer 2006 was close to normal climatic values. When results for 2006 are examined in details, it seems that both versions overestimated streamflows following rainfall events occurring after a dry period. During these simulations, the streamflows of nearly half of the RHHUs was calculated using two completely different modelling approaches and, ultimately, very similar results were obtained. This shows the appropriate and comparable value of both computational methods. Hence, the selection of one

2006

BV3C PHIM 3.65 1.8 0.91 0.80

BV3C BV3C 2.11 42.8 0.63 -0.84

1.48

2007

BV3C PHIM 2.07 39.7 0.71 -0.74

BV3C BV3C 4.81 -15.3 0.93 0.85

5.68

BV3C PHIM 4.78 -15.7 0.93 0.84

approach over the other should consider other factors than simulation performance in this case. Considering the effort given to find the most appropriate parameters to achieve these results, the adapted version of HYDROTEL seems more appropriate than the original version. The PHIM submodel required the adjustment of fewer parameters than the BV3C sub-model and they were easily determined with precision using field observation data. However, the determination of the WTE vs Q relationship, required by the PHIM sub-model, needed extensive field monitoring. Hopefully, preliminary evaluation of other peatland sites are showing similar patterns (S. Tardif and C. Clerc, pers. comm.) which would lead the way to a demonstration of the suitability to extrapolate, within certain limits, the coefficients of the WTE and Q equations from one site to another. With the help of appropriate parameter values and acceptable ranges for different types of peatlands, it could therefore be possible to simulate streamflow with minimal calibration in North-Boreal environment.

Conclusion The adapted version of HYDROTEL, which uses PHIM to simulate streamflows of peatland-dominated RHHUs, offers a reasonable alternative to the previous water budget sub-model, namely BV3C. Indeed, it provides a way to deal with organic soils and obtains similar simulation results when compared to those obtained with the original version. Moreover, the little need of parameter calibration required by PHIM represents an advantage for future simulations on similar ungauged basins. It is safe to say that there is a need © 2009 Canadian Water Resources Association

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for continuous monitoring of small-scale, peatlanddominated sites in the North-Boreal environment in order to obtain a larger variety of curves required for peatland streamflow simulation. Of particular concern here is the hydrological behaviour of minerotrophic peatlands. Finally, this study showed that it is possible to calibrate both modelling approaches using very short streamflow records and that more monitoring is required (for at least another three-year period) to further validate the calibration exercises.

Acknowledgements The authors would like to thank Alain Royer, Simon Ricard, Martin-Pierre Lavigne, Stephane Savary, and Yves Gauthier of INRS-ETE, and Michel Nadeau, Luc Martell, and Isabelle Chartier of IREQ for their technical support and suggestions. The authors would also like to thank Kenneth Brooks of the University of Minnesota for supplying the computational code and the user manual of the Peatland Hydrologic Impact Model (PHIM) as well as giving permission to conduct our research using the software. This study was accomplished with the financial and technical support of Ouranos and Hydro-Québec through a NSERCCRD grant awarded to A. N. Rousseau.

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