Multi-Year Evapotranspiration and Energy Dynamics of a Reclaimed Fen in the Athabasca Oil Sands Region

Surface mining in the Athabasca Oil Sands Region (AOSR) of western Canada disrupts natural landscapes and permanently alters their hydrological functions. Wetlands cover ~55% of the region, primarily fen peatlands, and although provincial regulations require companies to restore disturbed ecosystems to a functional state, fens remain difficult to construct due to their complex hydrology and dependence on water exchange with surrounding uplands. Evapotranspiration (ET), a key component of the water balance, is particularly important in the sub-humid AOSR, where increasing reclamation activity demands accurate quantification of vertical water loss, as it influences ecohydrological feedbacks and long-term wetland sustainability. This study evaluates ET and energy dynamics in a constructed fen, built atop ~80 m deposit of composite tailings and capped with 10 m of tailings sand, using eddy covariance (EC) measurements and vegetation surveys conducted across five non-consecutive years between 2015 and 2023. Mean ET from April 1 to October 31 was 250 ± 49.9 mm, aligning with values from natural and constructed boreal peatlands. On average, ET was 15% higher in warmer and drier years. While intra-annual ET variability was mainly influenced by vapour pressure deficit and net radiation, a long-term decline in ET coincided with Typha latifolia expansion, whose canopy reduced water loss by sheltering the open water. Flux partitioning revealed that transpiration exceeded evaporation by up to 70%, highlighting the role of Typha in reducing energy input and limiting turbulent mixing over the ponded water surface. Despite declining ET, latent heat flux remained the dominant component of the energy balance, suggesting functional similarity to natural fens. With rainfall exceeding ET in 80% of the years, the study site did not experience any prolonged drought periods. These findings enhance understanding of surface-atmosphere interactions and inform wetland reclamation strategies, particularly the role of vegetation change.