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The Western Boreal Plain of North Central Alberta comprises a mosaic of wetlands and aspen ( Populus tremuloides) dominated uplands where precipitation (P) is normally exceeded by evapotranspiration (ET). As such these systems are highly susceptible to the climatic variability that may upset the balance between P and ET. Above canopy evapotranspiration (ETC) and understory evapotranspiration (ETB) were examined using the eddy covariance technique situated at 25.5 m (7.5 m above tree crown) and 4.0 m above the ground surface, respectively. During the peak period of the growing seasons (green periods), ETC averaged 3.08 mm d−1 and 3.45 mm d−1 in 2005 and 2006, respectively, while ETB averaged 1.56 mm d−1 and 1.95 mm d−1. Early in the growing season, ETB was equal to or greater than ETC once understory development had occurred. However, upon tree crown growth, ETB was lessened due to a reduction in available energy. ETB ranged from 42 to 56% of ETC over the remainder of the snow-free seasons. Vapour pressure deficit (VPD) and soil moisture (θ) displayed strong controls on both ETC and ETB. ETC responded to precipitation events as the developed tree crown intercepted and held available water which contributed to peak ETC following precipitation events >10 mm. While both ETC and ETB were shown to respond to VPD, soil moisture in the rooting zone is shown to be the strongest control regardless of atmospheric demand. Further, soil moisture and tension data suggest that rooting zone soil moisture is controlled by the redistribution of soil water by the aspen root system. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

In the Canadian Western Boreal Plain (WBP), wetlands (ponds and peatlands) comprise up to 50% of the landscape and represent unique habitat where summer precipitation is often outpaced by evapotranspiration and hillslope groundwater position does not follow topography. In this sub-humid location, groundwater fluxes and stores in riparian peatlands influence pond water levels and root zone moisture sources for forested uplands. To accurately describe the transport and retention of water in peat, it is important to consider peat subsidence. This paper quantifies the amount and effect of seasonal subsidence in a riparian peatland in the Utikuma Lake region in north-central Alberta, Canada. Results demonstrate that the deep and poorly decomposed peat deposits are resistant to compression, and that thick (and persistent) ground frost hinders pore collapse (shrinkage) above the water table until late summer when the ground has thawed. Even then, subsidence is still limited to the top 50 cm and is not closely related to changes in peatland water table or pond water level. Thus the water balance of these ponds and riparian areas appears to be less sensitive to peat volume changes than it is to the persistence of a substantial frost layer well into the snow-free period. [ABSTRACT FROM AUTHOR]

The relative contributions to total actual evapotranspiration (AET) from pond and riparian areas in a pond-wetland complex in the Western Boreal Plain (WBP) of northern Alberta are measured using the Bowen ratio energy balance technique. Measurements show that a pond typical of the WBP evaporates at a rate more than twice that of the adjacent riparian peatland. Relating the actual to potential evapotranspiration over both surfaces yields Priestley-Taylor α, coefficients of 0.60 and I-II for the peatland and pond respectively. Further results demonstrate that the sheltering and turbulent influences of the adjacent forested areas must be considered in the processes governing the permanence of WBP ponds. That is, forestry practices may inadvertently enhance the evaporative losses from the ponds, over and above the controls exerted by the regional climate. [ABSTRACT FROM AUTHOR]