Evapotranspiration Dynamics During the Ecosystem Development of a Constructed Upland‐Fen Watershed

ABSTRACT In recognition of the need for sustainable land use practices, environmental regulations require landscapes affected by resource extraction in the Athabasca Oilsands Region to be reclaimed to their pre‐disturbance functionality. Thus, pilot‐scale watersheds have been constructed to examine the viability of reclamation endeavours in the region. As water availability is a primary driver of ecosystem function, an understanding of water usage and movement during the different phases of ecosystem development is required to ensure successful reclamation. This study captured the evolving evapotranspiration (ET) regime during the initial 7 years post‐construction of a fen‐upland watershed. Both landscapes exhibited significant biophysical evolution, from bare ground to fully vegetated ecosystems. During bare ground conditions, ET was driven by atmospheric and edaphic controls. In the fen, ponded conditions resulted in consistently high rates of surface evaporation throughout the growing season (3.8 mm/day). In contrast to the drier upland, which exhibited limited evaporation rates (1.2 mm/day), punctuated by small increases in response to precipitation events. Once a plant community was well established, edaphic controls decreased, and ET fluxes were largely driven by plant‐mediated responses to atmospheric conditions. In the fen, widespread plant coverage and the establishment of a thick litter layer suppressed surface evaporative losses but increased transpiration, ultimately resulting in lower ET rates (2.5 mm/day). In the upland, the growth and development of treed species resulted in a marked increase in ET rates (2.6 mm/day). These rates are comparable to those of natural landscapes in the region, suggesting successful establishment of ecohydrological function.