Hydrological Function of a Mountain Fen at Low Elevation Under Dry Conditions

Abstract

Mountain fens are limited in their spatial extent but are vital ecosystems for biodiversity, habitat, and carbon and water cycling. Studies of fen hydrological function in northern regions indicate the timing and magnitude of runoff is variable, with atmospheric and environmental conditions playing key roles in runoff production. How the complex ecohydrological processes of mountain fens that govern water storage and release as well as peat accumulation will respond to a warmer and less snowy future climate is unclear. To provide insight, we studied the hydrological processes and function of Sibbald fen, located at the low end of the known elevation range in the Canadian Rocky Mountains, over a dry period. We added an evapotranspiration function to the Spence hydrological function method to better account for storage loss. When frozen in spring and early summer, the fen primarily transmits water. When thawed, the fen's hydrological function switches from water transmission to water release, leading to a summertime water table decline of nearly 1 m. Rainfall events larger than 5 mm can transiently switch fen hydrological function to storage, followed by contribution, depending on antecedent conditions. The evapotranspiration function was dominant only for a brief period in late June and early July when rainfall was low and the ground was still partially frozen, even though evapotranspiration accounted for the largest loss of storage from the system. This research highlights the mechanisms by which mountain peatlands supply baseflow during drought conditions, and the importance of frozen ground and rainfall in regulating their hydrological function. The study has important implications for the sustainability of low elevation mountain fens under climate change.

A review of groundwater in high mountain environments

“…Several studies have examined the hydrologic function of wetlands and meadows in high mountain catchments (Chignell, Laituri, Young, & Evangelista, 2019; Chimner et al, 2019; Cooper et al, 2010, 2019; Lowry, Loheide, Moore, & Lundquist, 2011; Millar, Cooper, & Ronayne, 2018; Mosquera, Lazo, Célleri, Wilcox, & Crespo, 2015; Mosquera et al, 2016; Polk et al, 2017; Streich & Westbrook, 2019). Due to their excess of water and decreasing permeability with drying, wetlands can self‐regulate to keep the water table near the land surface (Rezanezhad et al, 2016).…”

Beyond fill and spill: Hydrological connectivity in a sub‐arctic bog‐fen‐tributary complex in the Hudson Bay Lowlands, Canada

“…Bog frost tables were thinner and formed later than those in the fen (Figure 4; Ulanowski, 2014) and, combined with a lack of upgradient inflow, resulted in a faster decline in water tables (Figure 2b). Though not measured here, studies in other northern peatlands have shown a correlation between higher localized snow depth and thinner frost tables as well as delayed formation of ground ice (Moore, 1987; Streich & Westbrook, 2020). Frost tables were thinnest at this site in bog hollows (Figure 4), suggesting this may be an area of preferential snow accumulation.…”

“…These water table depths are comparable to those defined in other northern peatland‐dominated catchments. In a permafrost peatland system, bog‐to‐bog connectivity was associated with water table depths ~10 cm bgs (Connon et al, 2015), while in boreal fens active groundwater contribution to downgradient tributaries occurred at water tables between 5 and 10 cm bgs (Goodbrand et al, 2019), except for in extreme environments such as a montane fen underlain by well drained sediment, which contributed groundwater flow to water tables up to 90 cm bgs (Streich & Westbrook, 2020). It is likely that connectivity thresholds are landscape specific and defined by the local topography, peatland morphology and climate; the modelling of these systems would greatly aid in assessing the sensitivity of connectivity thresholds on system functioning in different environments.…”

“…These water table depths are comparable to those defined in other northern peatland-dominated catchments. In a permafrost peatland system, bog-to-bog connectivity was associated with water table depths ~10 cm bgs (Connon et al, 2015), while in boreal fens active groundwater contribution to downgradient tributaries occurred at water tables between 5 and 10 cm bgs (Goodbrand et al, 2019), except for in extreme environments such as a montane fen underlain by well drained sediment, which contributed groundwater flow to water tables up to 90 cm bgs (Streich & Westbrook, 2020).…”

“…However connectivity was low for the remainder of the year in these peat plateaus due to the isolating effect of permafrost. In a boreal montane fen system (Streich & Westbrook, 2020), disproportionally high runoff was generated during rain events where frost tables were present in the fen; the presence of frost also reduced the depletion of fen storage which preserved water levels into the summer. In the peatland complex studied here, the effects of spring temperature on snowmelt and ground thaw carried over to patterns in summer connectivity (Figures 7 and 8).…”