Water Table Depth and Vapour Pressure Deficit Interaction Drives Bare Peat Evaporation at Actively Extracted Peatlands in Canada

Abstract

Peat extraction alters the hydrophysical properties of the peat during the 15 to 40 plus years of extraction. Despite the importance of hydrologic conditions for driving carbon (C) emissions from this net C source, few studies in Canada have quantified the impact of peat extraction on energy partitioning and evaporation (E) rates. The removal of vegetation prior to extraction alters the controls and mechanisms of water loss, and drainage-induced subsidence is expected to enhance vertical capillary connectivity through the peat profile. We thus conducted a multi-year study using eddy covariance to understand the energy balance and daytime E rates from actively extracted sites in Quebec and Alberta, Canada. Despite being a partially drained system, available energy was largely partitioned into latent heat. The relative importance of surface and atmospheric controls of E varied with hydrologic conditions; with greater water table depth (WTD), the relative importance of vapour pressure deficit decreased, and the relative importance of WTD increased. Our results highlight a need for continuous surface moisture measurements for accurate E prediction. During active extraction, site managers harrow (till) the top few cm of peat to create a drier, hydrologically isolated layer that is then extracted and processed. A weighing bucket lysimeter experiment found that while harrowing initially elevated E rates, by ~4 h post harrowing, the newly dry layer acted as a barrier to further water loss from the peat profile. These sites provide a unique opportunity to further our understanding of water availability and transport of water to the evaporating surface from bare peat, and will inform future modelling efforts to partition evapotranspiration from peatlands. An understanding of the impact of site management on E rates informs site water balance calculations and can aid in optimizing harvesting practices and effective restoration strategies post-extraction.