Multi-scale water balance analysis of a thawing boreal peatland complex near the southern permafrost limit in northwestern Canada

Permafrost thaw profoundly changes landscapes in the Arctic-boreal region, affecting ecosystem composition, structure, function and services and their hydrological controls. The water balance provides insights into water movement and distribution within a specific area and thus helps understand how different components of the hydrological cycle interact with each other. However, the water balances of small- (<10¹ km²) and meso-scale basins (10¹–10³ km²) in thawing landscapes remain poorly understood. Here, we conducted an observational study in three small-scale basins (0.1–0.3 km²) of a thawing boreal peatland complex. The three small-scale basins were situated in the headwater portion of Scotty Creek, a meso-scale low-relief basin (drainage area estimated to range between 130–202 km²) near the southern permafrost limit in the Taiga Plains ecozone in northwestern Canada. By measuring water losses (discharge and evapotranspiration [ET]), inputs (rainfall [R] and snow water equivalent [SWE]) and storage change (ΔS), and by calculating runoff (Q), we (1) aimed to quantify the growing season water balances (May–September, 2014–2016) of the three small-scale headwater sub-basins. After (2) comparing monthly sub-basin and corresponding basin water losses through ET and Q, we aimed to (3) assess the long-term (1996–2022) annual basin water balances using publicly available observations of discharge (and thus calculated Q), R and SWE in combination with simulated ET. (1) Growing season water balance residuals (RES) for the sub-basins ranged from −81 to +122 mm. The monthly growing season water balance for the sub-basin for which all water balance components throughout the 3 year study period were recorded exhibited large positive RES for May (+117 to +176 mm), since it included late-winter SWE routinely estimated in late March right before snowmelt. In contrast, lower monthly and negative RES were obtained for June–September (−41 to 0 mm). For two sub-basins, we provide two different drainage area estimates, highlighting the challenges associated with automated terrain analysis using digital elevation models (DEMs) in low-relief landscapes. Drainage areas were similar for one sub-basin, but they exhibited a fivefold difference for the other. This discrepancy was attributed to the high degree of landscape heterogeneity and resulting hydrological connectivity, with implications for Q calculations and RES. (2) Spring freshet contributed 41 % to 100 % (sub-basins) and 50 % to 79 % (basin) of the April–September Q. Spring freshet peaks were comparable, except for the driest year (2014), when the basin Q was more than 10 times lower than in the sub-basins. At both scales, ET was the dominant source of water loss, more than twice Q. (3) Over the long term (1996–2022), the increase in the basin runoff ratio (the ratio of runoff to precipitation) from 1996 to 2012 (0.1 to 0.5) has been attributed to the increased connectivity of wetlands to the drainage network due to permafrost thaw. However, the smaller mean and more variable runoff ratio from 2013 to 2022 may be due to wetland drying and/or changes in precipitation patterns. Overall, we demonstrate how the hydrological responses of rapidly thawing boreal peatland complexes – at both the sub-basin and basin scales – are shaped by complex factors that extend beyond year-to-year changes in precipitation and ET. Long-term hydrological monitoring is crucial to identify and understand potential threshold effects (e.g. changes in land cover and hydrological connectivity) and ecohydrological feedbacks affecting the local (e.g. subsistence activities), regional (e.g. water storage) and global ecosystem services (e.g. carbon storage) provided by thawing boreal peatland complexes.