Comparison of permafrost thaw-related changes to hydrological response and land cover in subarctic peatland-dominated landscapes

The hydrological implications of discontinuous permafrost thaw in peatland-dominated basins are not well understood. While there is evidence suggesting that permafrost-thaw-driven land cover change increases annual runoff and the runoff ratio in the Taiga Plains of northwestern Canada, few studies have evaluated the impact on small to medium-sized basins (< 105 km2) outside this ecoregion. Here, we assess runoff, runoff ratio, and precipitation trends for 34 peatland-dominated basins, of which 28 are in the discontinuous and sporadic permafrost zones and 6 in adjacent permafrost-free environments. We calculated annual and monthly trends between 1970 and 2016 using the Mann-Kendall test and found that annual runoff, runoff ratio, and precipitation increased significantly in 25%, 16%, and 13% of basins respectively, at a 5% significance level, and decreased significantly in 3%, 19%, and 9% of basins, respectively. Increased annual runoff ratios occurred exclusively in basins overlying permafrost, while increases and decreases in annual runoff and precipitation were found in both permafrost and permafrost-free basins. Increases in annual runoff and runoff ratio occurred independently of precipitation changes in only the Taiga Plains and the Western Siberian Plain. Runoff during winter increased significantly in all ecoregions and occurred independently of the areal extent of permafrost, although the magnitude of these increases was small compared with those of April and May.\nThe Hudson Plains, Canada is one of the largest, undisturbed peatland regions (370,000 km2) in the world. Air temperature in the Hudson Plains is increasing rapidly leading to unprecedented permafrost thaw. The region’s remoteness has hindered our knowledge of how permafrost thaw alters peatland land cover and hydrological response. However, such relationships have been intensively studied in the Taiga Plains of northwestern Canada and have demonstrated that the thaw and subsidence of permafrost peatlands in sporadic (30% – 80% areal) permafrost zone and quantify land cover changes over 40 years using multiple remote sensing datasets (lidar, air photographs, and high-resolution satellite imagery). We then evaluate these landscapes at a fundamental hydrological unit, the peatland complex, identify five peatland complexes, and conceptualize their potential hydrologic response using circuitry analogs. We partitioned peatland complexes into their component peatland forms (e.g., permafrost peatland, bog, fen) and represented each using an electrical component (e.g., generator, switch, conductor). Using this approach, we suggest a 60 km latitudinal segment (54.5 ° N – 54.9 ° N) where peatland complexes are most vulnerable to permafrost-thaw-induced land cover and hydrological change.\nWe compared trends in temperature, hydrology, and land cover between 1980 and 2019 at two peatland-dominated basins in the zone of discontinuous permafrost, Angling River, where no changes to runoff ratio (total annual runoff/ total annual precipitation) had previously been detected and Jean-Marie River, where increases to hydrological response had previously been detected. We used the Mann-Kendall test to assess runoff, precipitation, and runoff ratio trends. To assess changes to land cover, we estimated Theil-Sen slopes on a collection of Landsat scenes for which Tasseled Cap indices of brightness, greenness, and wetness were calculated. We found that between 1980 and 2019, the mean annual air temperature in the Jean-Marie River basin remained warmer and is presently closer to 0 °C than in the Angling River basin. While no statistically significant annual runoff, runoff ratio, or precipitation trends were found in the Angling River basin, some evidence suggests a recent increase in runoff ratio. Overall, between 1980 and 2019 we found statistically significant increases in annual runoff and runoff ratio in Jean-Marie River, however, elevated annual runoff and runoff ratios exclusively occurred from 1996 through 2012. Since 2012, annual runoff and runoff ratios have not been substantially different than those before 1996. Although we found evidence of permafrost thaw in the Angling basin, slopes of Tasseled Cap indices indicated that vegetation growth and tundra greening were the main land cover changes. While in Jean-Marie, slopes of Tasseled Cap indices demonstrated a mixture of wetting, drying, and vegetation growth, processes associated with permafrost-thaw-related disturbances.

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