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 (a) identify 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 wildfire. 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 by fire 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 the 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

“…The results of the LMM showed that the most significant control on DOC concentration at Poplar Fen was the WT at the beginning of each hydrologic year (i.e., HY-WT). Elmes et al (2018) discussed ; 2012, 2013, 2016), concrete ground ice was detected the following spring and water had been stored over the winter as ground ice. Conversely, when fall WT were lower (2014,2015,2017), concrete ground ice was not detected the following spring and the entire peat column was free to recharge the underlying aquifer.…”

“…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, 2018), 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, 2018). However, certain fen areas were left virtually unburned where DOB equalled 0 cm.…”

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

“…The generally weak relationships (Kendall tau < 0.3) for each microtopographic form may also be due to the site location. 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. Further study is needed to assess this relationship under dry and wet spring conditions.…”

Wildfire overrides hydrological controls on boreal peatland methane emissions

“…This could lead to a reduction in CH 4 emissions or even uptake of CH 4 via oxidation (Strack et al, 2004;Turetsky et al, 2008;Moore et al, 2011). Conversely, high water tables can occur post-fire (Kettridge et al, 2015), although they are often associated with low surface moisture contents due to hydrophobicity of the peat (Doerr et al, 2000). Low soil moisture rates can also occur under increased ash deposition after fire, with increased closure of soil pores by ash causing reduced capacity to hold water and increased runoff (Heydari et al, 2017).…”