Hydrometeorological conditions preceding wildfire, and the subsequent burning of a fen watershed in Fort McMurray, Alberta, Canada

Abstract

Abstract. The destructive nature of the ~ 590,000 ha Horse River Wildfire in the Western Boreal Plain (WBP), northern Alberta in May of 2016 motivated the investigation of the hydrometeorological conditions that preceded the fire. Historical climate and field hydrometeorological data from a moderate-rich fen watershed were used to identify a) whether the spring 2016 conditions were outside the range of natural variability for WBP climate cycles; b) explain the observed patterns in burn severity across the watershed; and c) identify whether fall and winter moisture signals observed in peatlands and lowland forests in the region are indicative of fire susceptibility. Field hydrometeorological data from the fen watershed confirmed the presence of cumulative moisture deficits prior to the fire. Hydrogeological investigations highlighted the susceptibility of fen and upland areas to water table and soil moisture decline over rain-free periods (including winter), due to the watershed's reliance on supply from localized flow systems originating in topographic highs. Subtle changes in topographic position led to large changes in groundwater connectivity, leading to greater organic soil consumption in wetland margins and at high elevations. The 2016 spring moisture conditions measured prior to the ignition of the fen watershed were not illustrated well by the Drought Code (DC) when standard overwintering procedures were applied. However, close agreement was found when default assumptions were replaced with measured duff soil moisture recharge and incorporated into the overwintering DC procedure. We conclude that accumulated moisture deficits dating back to the summer of 2015 led to the dry conditions that preceded the fire. The infrequent coinciding of several hydrometeorological conditions, including low autumn soil moisture, a modest snowpack, lack of spring precipitation, and high spring air temperatures and winds, ultimately led to the Horse River wildfire spreading widely and causing observed burn patterns. Monitoring soil moisture at different land classes and watersheds would aid management strategies in the production of more accurate overwintered DC calculations, providing fire management agencies early warning signals ahead of severe spring wildfire seasons.

Hydrogeologic setting overrides any influence of wildfire on pore water dissolved organic carbon concentration and quality at a boreal fen

“…This fen is dominated by Larix laricina , Betula pumila , Equisetum fluviatile , Smilcina trifolia , Carex spp., and brown mosses, largely Tomenthypnum nitens . The average prefire peat depth ranged between 1.3 and 1.5 m. Fen areas experienced a modest and consistent depth of burn (DOB) of 2.0 ± 0.2 cm from the fire, with DOB higher in margins between fen and upland, averaging 13.0 ± 0.1 cm (Elmes, Thompson, Sherwood, & Price, ), within a similar range to another peatland also impacted by the Horse River wildfire (between 2.5 ± 3.5 cm and 16.0 ± 10.2 cm; Wilkinson et al, ). However, certain fen areas were left virtually unburned where DOB equalled 0 cm.…”

“…The hydrometeorological conditions preceding the burning of Poplar Fen were summarized by Elmes et al (). In short, the authors confirmed the accumulation of moisture deficits prior to the fire, between summer 2015 and early spring 2016.…”

Seasonal ground ice impacts on spring ecohydrological conditions in a western boreal plains peatland

“…Similar timing of SGI free conditions were reported by Smerdon and Mendoza (2010) in a riparian peatland by the end of April, attributed to water exchanges with an adjacent lake. A 3-year study by Elmes, Thompson, Sherwood, and Price (2018) on a rich fen reported SGI free conditions, which ranged from late April to mid-May, which they attribute to low water tables prior to freezing, which prevented the formation of concrete SGI. The faster melting at Pauciflora was not expected due to the cooler climate compared with other WBP peatlands.…”

“…Pauciflora is a wet site (Wells et al, 2017) compared with other peatlands in the area (Elmes et al, 2018;Wells & Price, 2015), and the potential range of soil moisture during spring conditions was likely not captured in 2017 explaining the weaker relationships found between SGI and VMC. Pauciflora is a wet site (Wells et al, 2017) compared with other peatlands in the area (Elmes et al, 2018;Wells & Price, 2015), and the potential range of soil moisture during spring conditions was likely not captured in 2017 explaining the weaker relationships found between SGI and VMC.…”

Water table dynamics in a constructed wetland, Fort McMurray, Alberta

“…Upland–wetland connections in the WBP are complicated by antecedent moisture and climatic conditions (Devito et al, ; Wells et al, ). For example, peatlands can supply adjacent uplands with water during dry periods, whereas uplands often recharge peatlands during wet periods (Ferone & Devito, ), with some natural fen peatlands in the AOSR being supplied primarily by local groundwater flow systems that originate in local topographic uplands (Elmes, Thompson, Sherwood, & Price, ) These hydrologic connections between peatlands and their surroundings (i.e., adjacent uplands or ponds) are important to maintain wetness and sustain wetland processes in undisturbed landscapes within the subhumid WBP region, especially during dry periods (Devito et al, ; Ferone & Devito, ; Petrone, Silins, & Devito, ; Wells et al, ; Wells & Price, ; Wood et al, ). Similarly, these connections are assumed to be important for the maintenance and permanence of constructed peatland systems in the postmining setting, and understanding the nature of these water exchanges is crucial for interpreting and evaluating landscape designs.…”