The destructive nature of the
The subhumid Athabasca oil sands region (AOSR) of the Western Boreal Plain (WBP) comprises a mosaic of small lakes, forested uplands, and wetlands primarily as peatlands (Devito et al., 2012). Bogs are defined as ombrogenous peatlands, receiving water exclusively from atmospheric sources (Ingram, 1983). Conversely, fens receive water from both atmospheric and surface water and/or groundwater sources. In the WBP, fens are distinguished into three primary types (poor, moderate-rich, and extreme-rich) based on differences in water chemistry, indicator plant species, and species richness (Vitt et al., 1995). In the AOSR, moderate-rich fens are the primary peat-forming wetland (Chee and Vitt, 1989). The hydrology of bog, poor fen (Ferone and Devito, 2004; Wells et al., 2017), and saline fen (Wells et al., 2015a, b) systems have been studied in the WBP; however, the hydrology of moderate-rich fen systems in the AOSR remains largely unexplored.
Water availability in the WBP is constrained by annual precipitation rates
that are typically less than potential evapotranspiration demands (Marshall et al., 1999; Bothe and Abraham, 1993). Consequently, the timing,
frequency, and magnitude of wildfires are dictated by variability in the
hydrometeorological conditions over the growing season (Abatzoglou and
Kolden, 2011; Flannigan and Harrington, 1988), where moisture deficits
accumulate in upland duff (Keith et al., 2010) and near-surface peat horizons
(Lukenbach et al., 2015) over extended dry periods. Consumption of surface
and ground fuels in flaming and smouldering combustion during wildfires in
the WBP can total 3 kg m
The majority of summer wildfires are ignited by lightning (Tymstra et al., 2005), when wildfire behaviour can be predicted by drying signals in shallow forest duff horizons with relatively simple drying mechanisms (Wotton et al., 2005). Unlike summer fires, spring wildfires usually have human-caused ignition sources (e.g. recreational vehicle exhausts or unextinguished cigarettes) and are harder to predict given that widespread fires occur regardless of the presence of moisture deficits (Amiro et al., 2009). These spring fires therefore possess less obvious antecedent moisture signals, given that they occur post-snowmelt, an important rewetting period for wetlands and forested uplands in the region (Smerdon et al., 2008; Redding and Devito, 2011).
In Canada, early spring fire susceptibility is typically predicted with the Canadian Forest Fire Weather Index (FWI) System, a component of the Canadian Forest Fire Danger Rating System (CFFDRS) (Lawson and Armitage, 2008). The Drought Code (DC) is a component of the FWI which applies to slow-drying deep forest organic layers often found throughout the WBP, which are layers that can enhance wildfire intensity (Van Wagner, 1987). The DC is a semi-physical model which uses precipitation inputs and predicts water loss (as a function of daily noon temperature and day length) to estimate the moisture content of deep organic layers that typically dry logarithmically based on an estimated 53-day period required to lose two-thirds of held moisture (Lawson and Armitage, 2008). Values of the DC range from 0 (saturated soils at surface) to over 800 (residual soil moisture only), representing the origins of the index as representing the slow drying of stored water in Pacific coastal slash fuels (Turner, 1972). DC values have been related to the peatland water table (Waddington et al., 2012) as well as the extent of the peatland burned area (Turetsky et al., 2004). These DC calculations, although based on typical wetting and drying rates of relatively deep upland fuels (Lawson and Armitage, 2008) and regarded as general estimates, can be important predictors for fire managers immediately following snowmelt, especially when additional soil moisture information is lacking. Given the large moisture deficits that can develop in deeper upland organic layers, the DC is overwintered to incorporate the effect of fall DC and winter precipitation into the next year's starting value. Overwintering calculations generally include estimates of total winter precipitation from nearby climate stations, along with two estimated coefficients, which include a carry-over effect to adjust for antecedent (fall) moisture conditions, and the wetting-efficiency fraction of the snowpack to the specific soil layer (Lawson and Armitage, 2008). These coefficients, however, can be ignored if direct measurements of recharge into forest soils are available.
During the spring of 2016, the
Map of the Poplar Fen watershed (
We capitalize on an opportunity to explore pre-fire hydrometeorological data obtained from “Poplar Fen” from 2011 to 2016, which is an instrumented moderate-rich fen watershed that burned on 17 May 2016. The specific objectives of this research are (1) to use a combination of historical climate and field hydrological data to characterize the hydrometeorological conditions preceding the burning of a moderate-rich fen watershed to determine whether these conditions were outside the range of natural WBP climate cycles, (2) to use these hydrological data to explain the observed patterns in burn severity across the watershed, and (3) to identify whether hydrological data and hydrogeological setting parameters of the watershed can serve as indicators of deep smouldering and combustion risk.
Situated within the Athabasca region of the Boreal Plains Ecozone (Ecoregions
Working Group, 1989), Poplar Fen (56
The fen areas at Poplar Fen are underlain by thin (
Tamarack (
A 20-year record of meteorological data were obtained from Alberta
Agriculture and Forestry through the Alberta Climate and Information Service
(Alberta Agriculture and Forestry, 2017). This included daily values of
precipitation (rainfall and snowfall) and air temperature, which were
estimated for the Poplar Fen area (township: T092R10W4) using an
inverse-distance weighting interpolation procedure (IDW). Data from
2 to 7 stations were used for the IDW over the 20-year period with the nearest
station (Mildred Lake; Fig. 1) located
Hydrological data were collected between 2011 and 2016. Initial
instrumentation included a water table monitoring well in a fen area (NW
fen), located in the northwest section of the watershed, and a well in the adjacent
margin area (NW margin), located
Moisture probe profiles in upland duff and fen margin peat.
Precipitation was measured in an open area of the site with a logging Onset
RG3-M tipping bucket rain gauge; missing daily totals (October–May) were
supplemented with interpolated rainfall data for the Poplar Fen area (Alberta
Agriculture and Forestry, 2017). Between 21 March and 19 April 2016, snow
surveys were conducted using a Meteorological Service of Canada (MSC) snow
tube. Measurements of snow depth were taken at 178 locations,
Volumetric water content (VWC) was recorded half-hourly from June 2015 to May 2016 in upland duff and margin peat soils with arrays of Stevens Hydra Probe II (Figs. 1, 2). Two weeks of data (2–17 May 2016) could not be salvaged due to fire damage to the logger. The probes were calibrated in the laboratory to the respective soil types.
The Drought Code (DC) was calculated using the “cffdrs” package in the R
statistical program (R Core Team, 2016) for the 2015 growing season using
data obtained from the Mildred Lake climate station (Alberta Agriculture and
Forestry, 2017). This included noon measurements of air temperature and
cumulative precipitation from the previous 24 h. The DC was started on
12 April 2015, following 3 days of noon temperatures of 10
Measured accumulation and ablation of SWE at Gordon Lake snow pillow
Summary of scenarios used for calculating a starting DC for 19 April 2016.
n/a
Startup and overwinter upland duff DC were calculated four different ways (Table 1), each reflecting specific information of the hydrometeorological environment. For scenario 1, startup DC was estimated for the upland duff from a linear regression between DC and measured duff VWC from 27 June to 31 October 2015 and calculated based on the starting VWC for 19 April. Scenarios 2–4 were then calculated with the overwintering procedure (Eqs. 1, 2, and 3). For scenario 2, the startup DC was calculated using total winter precipitation values obtained from the Fort McMurray airport climate station and default carry-over and wetting-efficiency values (0.75) from the cffdrs package (Lawson and Armitage, 2008). For scenario 3, the startup DC was calculated from peak SWE data from snow survey data of Poplar Fen and carry-over (0.5) and wetting-efficiency (1.0) values used by Alberta Agriculture and Forestry. Scenario 4 applied the directly measured duff recharge (a mm value input, inferred from the upland duff site moisture probe) to the overwintering procedure, which eliminated the need for a precipitation value as well as estimates of carry-over and wetting efficiency. Following these methods, four differing startup DC values were generated for the upland duff. The DC was then calculated four times, corresponding to each startup DC value, starting on 19 April and were ran until 17 May 2016.
Measurements of burn consumption of organic soils were made in fen, margin, and upland areas that burned using differential GPS (Leica GS14 GNSS) survey data obtained pre- (October 2015) and post-fire (October 2016) from well-inferred surface (elevation of PVC top minus distance to ground surface) elevations; the difference between soil surface elevations at piezometer nests were compared between pre- and post-fire. This included nests from Poplar Fen additional (5 fen, 5 margin, and 10 upland nests) to those identified in Fig. 1 (not shown). Average vertical elevation (surface) change was calculated for each nest location. Depth changes were averaged for burned fen, margin, and upland organic soils, and these depths were multiplied by previously measured average bulk density values for each soil type to estimate terrestrial fuel loss.
Total hydrological year rainfall and snowfall from 1996 to 2016, interpolated for the Poplar Fen area.
Precipitation observations from 1996 to 2016 interpolated for Poplar Fen
averaged 380
Over the 2015–2016 winter (mid-October–mid-April), average air temperatures
were
The NW fen (Fig. 1) water table range was
The 2015–2016 hydrological year began with water levels that were among the lowest in the 6-year record (Fig. 5). By the end of winter, all manually surveyed fen monitoring wells were empty of water (water tables > 0.8 m b.g.s.). The comparison of fall 2015 logged water levels to manual winter 2016 observations (before snowmelt recharge and before pressure transducers were installed for the 2016 field season) evidenced an additional 0.12–0.26 m water table decrease, demonstrating mid-winter water table decline and drying of overlying peat substrate. Ground thawing at 0.1–0.2 m depth occurred in mid-April (earlier than 2013–2015) toward the end of snowmelt, and at this time (16 April 2016) the NW fen water table had increased to 0.67 m b.g.s. The remaining snowmelt period initiated a water table rise of 0.46 to 0.21 m b.g.s. on 3 May which then decreased in the total absence of rainfall to 31 cm b.g.s on 17 May, the day that the Poplar Fen area burned over (Fig. 5).
Daily records of
Logged (lines) and manually (“x” symbols) recorded water table position at NW fen (black) and NW margin (grey) (see Fig. 1), from 2011 to 2016, with field-measured rainfall (P), and total winter precipitation (WP) interpolated for the Poplar Fen area.
Average (SE) water table (black circles) and vertical hydraulic gradient (grey circle) between the water table and underlying mineral for lower and upper fen, and margin areas, along with logged (line) and manually recorded (“x” symbol) water table (blue) and hydraulic head (red) for lower and upper fen areas in 2015. A negative hydraulic represents a loss of water from the fen to the underlying mineral substrates. Rainfall is also illustrated.
The 19 April 2016 startup and final 17 May DCs for Poplar Fen using four different scenarios.
n/a
To examine how fen areas of varying topographic position were wetting and
drying over the 2015 growing season, water table and hydraulic gradients were
compared between the contrasting upper and lower fen areas (Fig. 6). Average
water table depth below surface differed by 0.05 m between upper
(0.22
Between June and October 2015, duff and margin peat VWC (both at
Following the first month of startup in 2015, the DC illustrated an inverse
relationship with upland duff VWC (
Average (
The greatest depth of burn was measured in the margins (0.13
Within the Boreal Plain region of northeastern Alberta, average precipitation is less than potential evapotranspiration in most years (Bothe and Abraham, 1993). Consequently, water deficits are common in the WBP, relying on infrequent wet periods every 10–15 years to replenish storage deficits (Marshall et al., 1999; Devito et al., 2005). Historical precipitation data illustrate that rain and snow patterns are variable in the WPB (Table 2; Fig. 3). Total snowfall was near or below average in years during which spring wildfires burned large areas. Although modest snowfalls are a recurring influence, they do not necessarily dictate fire magnitude; a total of 5 years with spring wildfires of low burn area were identified, possessing similar (or lower) total snowfall values than large spring burn area years (Table 2). Earlier snowmelt can extend the dry WBP spring and drying of organic soils, which could therefore extend the period over which spring fires can be generated (Westerling et al., 2006). However, the timing of snowmelt in the WBP does not appear expedited in years of large burned area in the spring, with no significant patterns in the timing of snow-free conditions observed in the 7-year Gordon Lake snow pillow record (Fig. 3). However, years of high total annual snowfall all align with years with low burned area in the spring (Table 2). This suggests that large SWE can contribute to decreasing the total annual area burned in the spring. Low and infrequent early precipitation events occurred in 3 of 4 years with high burned area in the spring. However, due a large proportion of rainfall in continental western Canada generally occurring in summer (Smerdon et al., 2005), dry early spring is not exceptional and not restricted to years of high burned area in the spring. The year with the lowest early spring cumulative rainfall in the 20-year record was 2008; however, above-average snowfall and late snow-free conditions decreased wildfire susceptibility in the spring, further demonstrating the importance of a large snowmelt for reducing wildfire vulnerability (CFRC, 2001).
Volumetric water content (m
The 2015–2016 hydrological year experienced the second warmest winter temperatures over the past 20 years. Periodic rises in air temperature above freezing conditions throughout the winter (Fig. 4a) supplied energy for mid-winter snowmelt and sublimation (Pomeroy et al., 1998), potentially decreasing available peak SWE for the spring snowmelt period. The modest snowpack melted over a 31-day period. Immediately following snowmelt, high air temperatures, low relative humidity, and high wind speeds (Fig. 4b, c) created weather conditions optimal for the spread of wildfire (Van Wagner, 1977). Similar mild winter temperatures and warm, dry spring conditions were present in previous years of high spring time burned area in 1968, 1998, 2002, and 2011. These years produced fires of a similar magnitude and total area burned to the Horse river wildfire of 2016 (Hirsch and de Groot, 1999; Tymstra et al., 2005; FTCWRC, 2012).
A 5-year (2011–2016) water table record illustrated the susceptibility of
Poplar Fen to extended drying periods, with years of high spring (2011 and
2016) and summer (2015) burned area corresponding with low water table
position (Fig. 5). At Poplar Fen, water tables also decreased over winter
periods in the absence of precipitation-driven recharge. These prolonged
periods of water table decline were evidenced by logged water table and
mineral piezometer observations from the lower and upper fen areas (Fig. 6).
In these areas, the hydraulic head in the underlying mineral substratum
(
Spring NW fen water table position was also related to the persistence of a
frozen upper saturated zone. For example, near-surface water tables in fall
of 2012 and 2013 (Fig. 5) allowed for relatively homogenous overwinter
freezing of the upper saturated zone (Price, 1983), which reduced the
permeability of the peat (Roulet and Woo, 1986; Quinton et al., 2009) and
helped store subsurface water over the winter periods (Price and FitzGibbon,
1987). Ground ice persisted into mid-late May in 2013 and 2014, thus limiting
snowmelt water infiltration (Roulet and Woo, 1986) and subsurface water loss
to the underlying silty sand and outwash layers (Price and FitzGibbon, 1987).
Conversely, the shallow (0–0.2 m) peat had reached above freezing
temperatures by the end of snowmelt (mid-April) in 2016, suggesting that low
(
Post-snowmelt 2016, the NW fen water table (0.3 m b.g.s.) was
Soil moisture in upland duff and margin peat followed a drying trend throughout 2015. Following snowmelt in 2016, water content in the upland duff and margin peat were not sufficiently higher than values observed in fall of 2015 (Fig. 7). These data suggest that there was no net wetting to the organic near-surface soils in the upland or margins at Poplar Fen from snowmelt infiltration. This soil moisture deficit was further enhanced by the lack of spring precipitation and increased evaporative demand (Hayward and Clymo, 1983) driven by the low humidity, high temperatures, and winds at the time of the Horse river wildfire (Fig. 4). This deficit would have increased the available fuels for the wildfire allowing for significant combustion of these organic layers (Table 4).
The historical meteorological and field hydrological data illustrate the susceptibility of regionally abundant WBP peatland watersheds to wildfire during extended dry periods. Results suggest that the wildfire at Poplar Fen, and the greater Horse river wildfire, was not simply a consequence of anomalous drought climate conditions, but rather interconnected hydrometeorological factors not uncommon to the Western Boreal Plain, occurring at least twice in the 5-year instrument record. These factors included low autumn soil moisture and water tables, modest snowfall, overwinter drainage, insufficient spring rainfall, and high spring air temperatures and winds. The synchronicity of these factors, occurring in the same hydrological year, combined with mature tree stands with high accumulated fuels ubiquitous to the region, likely contributed to the large magnitude Horse river wildfire. The similarities of the hydrometeorological events preceding the Horse river wildfire with previous years (1968, 1998, 2002, and 2011) of similar burned area in the spring (Hirsch and de Groot, 1999; Tymstra et al., 2005; FTCWRC, 2012) suggest that the mild and/or dry fall, winter, and spring conditions conducive for spring fire occur frequently in the region with a recent recurrence interval of 5 years. Moreover, conditions favouring spring wildfire may be enhanced by climate change, given the responsiveness of forest fuel moisture to changes in temperature and precipitation (Weber and Flannigan, 1997; Flannigan et al., 2016).
During summer 2015, vertical hydraulic gradients decreased in all fen and margin wells over periods of low precipitation. In lower fen these remained positive throughout the 2015 sampling period (Fig. 6), indicating upward groundwater discharge into the basal peat (water gain to peatland) from the underlying silty sand and outwash layers. In upper fen regions, these values were always lower and eventually became negative over time in the absence of rainfall, suggesting a flow reversal (downward) and loss of water from the basal peat to the underlying silt layer. Margin areas, located at a higher topographic position between fen and upland, exhibited the strongest negative hydraulic gradients, suggesting that these areas were recharging the underlying mineral layers throughout the entire year. These subtle differences in topographic position therefore played a large role in the observed differences in burn severity between these areas (Table 4). Hence, treed headwater moderate-rich fens and fen margin tracts in the WBP may be particularly vulnerable to wildfire.
The 2015 moisture conditions observed in the upland duff of Poplar Fen were illustrated reasonably well with the DC. The DC was overwintered for 2016 using a range of startup values from different methods (Table 3). Scenarios 2 and 3 produced DCs that were lower than the expected DC (scenario 1), since carry-over and wetting-efficiency coefficients overestimated the recharge to the duff layer by 15–21 %. These default coefficients may not have accounted for the high sublimation rates caused by low relative humidity and high solar radiation, common to the western boreal forests of Canada (Burles and Boon, 2011). The lower recharge values measured at Poplar Fen (35 % of melt water) may also be due to moisture deficits that accumulated since the summer of 2015, as a high proportion of the available meltwater went towards recharging the unsaturated mineral soil underlying the duff. The startup DC that was calculated using the directly measured duff recharge (scenario 4) was much closer to the expected DC, suggesting that the overwintering calculation is suitable for the duff layer at Poplar Fen when VWC is directly measured.
Due to differences in soil bulk density and depth of burn, average duff fuel
consumption was
This study applies a combination of pre-fire and historical hydrometeorological data from a moderate-rich fen watershed to contextualize the conditions preceding the 2016 Horse river wildfire. The fire was manifested by dry hydrometeorological conditions extending back to summer 2015. This included low fall soil moisture, modest snowfall, and no spring rainfall, with above-average spring air temperatures and high winds also prevalent – conditions not uncommon in the subhumid WBP. It was ultimately the less frequent synchronization of these factors that led to a wildfire of this size and observed depth of burn in boreal forests and wetlands as well as the associated fuel losses. These coinciding hydrometeorological conditions share stark similarities with previous years with large burned areas from spring fires, namely 1968, 1998, 2002, and 2011, which may support the notion that fires of this magnitude are a function of WBP climate cycles. However, as natural as these factors may be, spring conditions conducive to wildfire could be enhanced by climate change, given the responsiveness of these boreal watersheds to changes in temperature and precipitation.
Field data from Poplar Fen confirmed that moisture deficits accumulated between summer 2015 and the Horse river wildfire the following spring. Following a relatively mild winter, the modest 2016 snowmelt did not raise upland duff and margin peat moisture above fall 2015 values. This was in part due to the hydrogeological setting of Poplar Fen, as water tables and hydraulic head decreased in the absence of localized precipitation-driven recharge from adjacent uplands, with no evidence of a regional groundwater connection to supplement discharge during extended dry periods. We propose that headwater peatlands in this region fed by localized flow systems will be particularly susceptible to water table fluctuations under a drying climate, rendering them more vulnerable to burning from wildfire.
The dry conditions and subsequent duff fuel consumption observed at Poplar Fen in the spring of 2016 were difficult to illustrate with the Drought Code when carry-over and wetting-efficiency coefficients were applied to the overwintering procedure. Closer agreement was found when directly measured duff soil moisture recharge was applied to the overwintering procedure in place of the coefficients. In order to better gauge the susceptibility of WBP headwater systems to wildfire in the spring, management strategies could therefore benefit from monitoring soil moisture at different land classes and watersheds. These data would allow for more accurate overwintering DC calculations and would provide managers more time to prepare for a fire season by considering additional indicators that can be detected earlier.
The historical meteorological data are freely available
from the Alberta Climate and Information Service (ACIS) through Alberta
Agriculture and Forestry (available at
The authors declare that they have no conflict of interest.
The authors wish to thank Corey Wells, George Sutherland, Dylan Price, Eric Kessel, Julia Asten, and Sarah Irvine for their assistance in the field. We gratefully acknowledge funding from a grant to Jonathan S. Price from the National Science and Engineering Research Council (NSERC) of the Canada Collaborative Research and Development Program, co-funded by Suncor Energy Inc., Imperial Oil Resources Limited, and Shell Canada Energy. The authors would additionally like to thank Ralph Wright at Alberta Agriculture and Forestry for help with obtaining historical data as well as Tom Schiks for comments on an earlier version of the manuscript. Edited by: Mario Parise Reviewed by: two anonymous referees