Nonlinear controls on evapotranspiration in arctic coastal wetlands

A K Liljedahl,R Engstrom,Y Harazono,C E Tweedie,R D Hollister,D Zona,L D Hinzman,W C Oechel

doi:10.5194/bg-8-3375-2011

Abstract

Abstract. Projected increases in air temperature and precipitation due to climate change in Arctic wetlands could dramatically affect ecosystem function. As a consequence, it is important to define controls on evapotranspiration, 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 (VPD) near 0.3 kPa appeared to be an important hydrological threshold, allowing latent heat flux to persistently exceed sensible heat flux. Dry (compared to wet) soils increased bulk surface resistance (water-limited). Wet soils favored ground heat flux and therefore limited the energy available to sensible and latent heat flux (energy-limited). Thus, midday evapotranspiration was suppressed from both dry and wet soils but through different mechanisms. We also found that wet soils (ponding excluded) combined with large VPD, resulted in an increased bulk surface resistance and therefore suppressing evapotranspiration below its potential rate (Priestley-Taylor α < 1.26). This was 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 (i.e., near-surface soil moisture, atmospheric vapor pressure, and the limited ability of saturated mosses to transfer water during high VPD) resulted in an average evapotranspiration rate of up to 75% of the potential evapotranspiration rate. These multiple limitations on midday evapotranspiration rates have the potential to moderate interannual variation of total evapotranspiration and reduce excessive water loss in a warmer climate. Combined with the prevailing maritime winds and projected increases in precipitation, these curbing mechanisms will likely prevent extensive future soil drying and hence maintain the presence of coastal wetlands.

Highlights

The response of Arctic wetland hydrology to projected climate warming is uncertain
The high McNaughton and Jarvis -factor suggests that net radiation was the main control on evapotranspiration rates, but our results show that midday evapotranspiration rates are constrained during both wet and dry near-surface conditions
We estimated that current midday evapotranspiration rates represent, on average,

Summary

Introduction

The response of Arctic wetland hydrology to projected climate warming is uncertain. Evapotranspiration is the major pathway for water loss from the flat tundra landscape (Rovansek et al, 1996; Mendez et al, 1998; Bowling et al, 2003). Low-relief wetlands represent a significant portion (>400 000 km2) of the pan-Arctic landscape (Walker et al, 2005) and are unique in that they exist in an environment with a desert-like annual precipitation (∼250 mm yr−1). A negative summer net water balance is common (Mendez et al, 1998), but this is offset by an annual replenishment of water from snowmelt (Rovansek et al, 1996). Evapotranspiration is the major pathway for water loss in summer and affects the lateral exports of water

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Discussion

Conclusion