Increasing runoff trends throughout circumpolar regions

Abstract:

Permafrost (ground that remains frozen for two or more years) in the discontinuous permafrost zone (30–80% areal) often occurs in peatland ecosystems. Observations over the past 30–40 years have documented permafrost thaw resulting in the subsidence of raised peat plateaus and replacement by permafrost-free peatlands. Across the discontinuous permafrost zone of northwestern Canada permafrost-thaw induced landcover change has led to increased annual runoff and runoff ratios (total runoff/total precipitation) in peatland-dominated basins. For this study we explored whether those trends are present in 36 circumpolar peatland-dominated basins. For each basin, we analyzed Mann-Kendall trends and calculated Theil-Sens slopes of annual (and monthly) runoff, runoff ratio, and precipitation. Linkages between permafrost thaw and runoff regime change were explored over two overlapping analysis periods, 1970–2016 and 1990–2016. Annual runoff significantly (α ≤ 0.05) increased in 7 basins between 1970 and 2016, and in 9 basins between 1990 and 2016. We found that annual runoff significantly decreased in only 1 basin between 1970–2016 and 3 basins between 1990–2016. Increased annual runoff ratios occurred in 6 (1970–2016) and 10 (1990–2016) basins. Conversely, runoff ratios decreased in 4 (1970–2016) and 2 (1990–2016) basins. The number of basins with significantly increased runoff in at least 1 month was 15 over each analysis period. We found the greatest number of increased runoff trends in April, May, and June. Similarly, runoff ratios over those same months increased in 11 (1970 –2016) and 14 (1990–2016) basins. Like previous studies in permafrost regions, increased snow period (November–March) runoff was the most widespread significant trend, however the volume change, both year-to-year and total, did not account for annual runoff increases. We also observed few instances of significant changes to annual and monthly precipitation. Precipitation changes did not account for increases in annual runoff. Overall, the greatest number of basins with increased runoff and runoff ratio trends occurred in northwestern Canada.