Annual and seasonal variability in evapotranspiration and water table at a shrub-covered bog in southern Ontario, Canada

Evapotranspiration (ET) was measured via the eddy covariance technique at a shrub bog peatland in southeastern Ontario for 5 years. For most of the study period the temperature was above normal. Precipitation was variable, but, in 2 years, late summer dry periods resulted in an extended period of deep drawdown of the water table (WT). Growing-season (May-September) daily ET varied considerably; maximum ET rates were 4 to 5 rum day(-1). Winter ET rates were an order of magnitude smaller than in summer, yet the total winter ET loss was important, accounting for 23 to 30% of the annual ET water loss. Annual precipitation exceeded annual ET by 1.55 to 1.94 times. During the growing season, daily ET was closely related to daily potential evaporation (PET); however, the slope of this relationship was statistically different in some years. In contrast, ET and WT were only weakly related in most years. When ET was sorted into 5 cm WT classes there was no difference in mean ET across most WT classes; only the two deepest WT classes had significantly smaller mean ET. The ratio ET/PET followed the same pattern. We present a conceptual model of ET that relates WT, soil hydraulic properties and moss and vascular plant processes. Copyright (c) 2005 John Wiley & Sons, Ltd.