Effect of long-term water table manipulation on peatland evapotranspiration

Continuous measurements of ecosystem scale evapotranspiration (ET) were obtained using the eddy covariance method over the 2010 and 2011 growing seasons (May-September) at three adjacent peatlands that have undergone long-term water table manipulation. The three (wet, dry and intermediate) sites represent peatlands along a hydrological gradient, with different average depths to water table (WTD) and different resulting vegetation and microform assemblages. The 2010 growing season was warmer and wetter than normal, while 2011 conditions were near normal. The difference in maximum daily ET values (95th percentiles) between sites were greater in 2010 (3.14 mm d−1-4.17 mm d−1) compared to 2011 (3.68 mm d−1-3.95 mm d−1), yielding cumulative growing season ET that followed the wet to dry gradient in both 2010 and 2011. Synoptic weather conditions (i.e. air temperature, vapour pressure deficit, and incoming solar radiation, etc.) could not explain differences in ET between sites due to their proximity to one another. Peat surface wetness was more spatially homogeneous at the wet site due to small average microtopographic variations (0.15 m) compared to the intermediate (0.30 m) and dry (0.41 m) sites. Although average Bowen ratios were less than one at all three sites, greater surface wetness and heating at the wettest site lead to differences in energy partitioning, with higher average Bowen ratios at the sites with a shallow average WTD. No significant relation between normalized ET and WTD was found at any of the sites that were consistent across both study years. In addition, the lack of a relation between ET and near-surface moisture suggests that the unsaturated hydraulic conductivity and the boundary layer resistance created by the vascular canopy combined with low surface roughness limits evaporative losses from the peat surface. This study suggests that the low ET of a dry site compared to a wet site may be due to the impact of a long-term change in WTD on leaf area and the relative distribution of plant functional groups.