Nonlinear controls on evapotranspiration in Arctic coastal wetlands

Projected increases in air temperature and precipitation due to climate change in Arctic wetlands could dramatically affect ecosystem functioning. As a consequence, it is important to define the controls on evapotranspiration, which is the major pathway of water loss from these systems. We quantified the multi-year controls on midday arctic coastal wetland evapotranspiration measured with the eddy covariance method at two vegetated drained thaw lake basins near Barrow, Alaska. Variations in near-surface soil moisture and atmospheric vapor pressure deficits were found to have nonlinear effects on midday evapotranspiration rates. Vapor pressure deficits near and above 0.3 kPa appeared to be an important hydrological threshold, allowing latent heat fluxes to persistently exceed sensible heat fluxes. Dry soils increased the bulk surface resistance (water-limited). Wet soils favored ground heat flux and therefore limited the energy available to sensible and latent heat fluxes (energy-limited). Thus, midday evapotranspiration was suppressed on both dry and wet soils through different mechanisms. We also found that wet soils (ponding excluded) combined with large atmospheric demands resulted in an increased bulk surface resistance and therefore suppressing the evapotranspiration to below its potential rate (Priestley-Taylor α < 1.26). This is likely caused by the limited ability of mosses to transfer moisture during large atmospheric demands. Ultimately, in addition to net radiation, the various controlling factors on midday evapotranspiration (near-surface soil moisture, atmospheric vapor pressure, and the limited ability of mosses that are saturated at depth to transfer water during high atmospheric vapor demands) resulted in an average evapotranspiration rate of up to 75 % of the potential evapotranspiration rate. The multiple limitations on midday evapotranspiration rates have the potential to moderate interannual variation of total evapotranspiration and dampen excessive water loss during a warmer climate. Combined with the prevailing maritime winds and the projected increase in precipitation, these dampening mechanisms will likely prevent extensive future soil drying and hence maintain the presence of coastal wetlands