| Evaporation and energy balance characteristics were studied at the Mer Bleue bog peatland near Ottawa, Ontario, Canada. Bog peatlands exhibit a water table that is above that of the surrounding landscape, which produces novel constraints on the plants that inhabit bogs. In particular, Sphagnum mosses, by their dense growth habit, slow degradation, and capillary conductance characteristics, help maintain a consistent surface soil moisture regime over a wide range of water table depths. The presented studies were undertaken to examine evaporation and energy balance at temporal scales ranging from diurnal to interannual. Particular attention was paid to the controlling factors for evaporation dynamics and the partitioning of energy to and evaporative flux from the vascular and non-vascular vegetation. Direct measurement of evaporative flux by eddy covariance, along with measurements of radiation, air and soil temperatures, vapour pressure, soil moisture content, and water table location, began in 1998. Porometry measurements and moss and fibric peat gravimetric water contents were obtained in 1999.;Keywords. bog peatland, energy balance, evaporation, partitioning, Sphagnum moss, water table.;Evapotranspiration was a significant factor in water balance throughout the year at rates well below potential over a considerable range in water table depth. Precipitation had little direct effect on evapotranspiration. The influence of water table depth was related to the nearness of the hollow surface and the extent of the rooting zone. Net radiation, vapour pressure deficit, and surface dryness influenced the evaporation rate. As vapour pressure deficit increased, the vascular shrubs exerted stomatal control and the moss evaporation rate increased. Partitioning of latent heat flux to leaf canopy, hummock, and hollow surfaces was dominated by the leaf canopy's control on net radiation, but showed diurnal variation associated with the interplay between vapour pressure deficit and leaf and moss responses to that deficit. A three-source sparse canopy energy balance model was used diagnostically to show the variation in partitioning seen in porometry and eddy covariance measurements. In future, land surface process models could be improved by better definition of the vascular plant rooting zone, the moss capillary action zone, and leaf area variability. Improved carbon exchange modelling may result, which would be useful in assessing the climate impact of bog peatlands. |
