Interannual climatic sensitivity of surface energy flux densities and evapotranspiration in a disturbed and rewetted ombrotrophic bog

Abstract

This study quantified surface energy balance and evapotranspiration in a Sphagnum-dominated ombrotrophic bog located near their southern limit in western North America (49.13${}^{\circ }$ N, 122.98${}^{\circ }$ W) from summer 2014 through 2022 to assess the bog's sensitivity to future climatic conditions, particularly to increasing severity and duration of drought conditions. Precipitation exceeded evapotranspiration in winter, but net surface water exchange was negative for between four and six months in summer. Shifts in surface water exchange towards net gain occurred consistently between early September and late October; the timing of the shift towards net loss was less consistent in spring, ranging from mid-February to mid-May. Daily evapotranspiration was primarily driven by net radiation and vapour pressure deficit. Surface moisture availability, as represented by water table depth, was a secondary control. Evapotranspiration tended to decline with increasing water table depth, with a slight flattening of the relation below a depth of about 0.058 m. The initial, more rapid, rate of decline is hypothesized to reflect a decrease of surface ponding and the decreased effect at greater depths to be associated with continued supply of water to the peat surface by capillary transport. Albedo increased from about 0.10 to 0.14 over each growing season, but the negative feedback on available energy for evapotranspiration at the peat surface was minimal. Net radiation did not vary substantially among years, and maximum seasonal water table drawdown appeared to be most strongly associated with growing season vapour pressure deficit, and was not correlated with the duration of seasonal net water loss to the atmosphere. In a climate change context, this study suggests that the ecohydrological response of ombrotrophic bogs will be most sensitive to changes in summertime vapour pressure deficit, which is projected to increase in the future.