Document: LAR-3-30-14

Seasonal and interannual variation in water vapor flux and energy balance in a northern grassland.

WEVER, L.A.*, P.J.CARLSON and L.B.FLANAGAN

University of Lethbridge, Lethbridge, Alberta, Canada ¹

Abstract:
We used the eddy covariance technique to measure exchanges of water vapor and energy in an Agropyron smithii dominated grassland near Lethbridge. Our objectives were to analyze the effects of seasonal and interannual variation in atmospheric and soil moisture deficits on evapotranspiration and energy partitioning. In contrast to 1998 when precipitation during May and June was about 67% higher than normal, 1999 was a very dry year with about 40% lower than normal precipitation during May and June. In 1998 there was a strong seasonal change in evapotranspiration and energy partitioning. During days 199-205 when volumetric soil moisture content (0-10 cm depth) was high (0.4), latent heat flux (evapotranspiration) showed peak mid-day values of 300 W m^-2, while peak sensible heat fluxes were approximately 150 W m^-2 during the same time. Shortly after this period, soil volumetric water content declined from values of 0.4 to values of about 0.15. The reduced availability of water resulted in a switch in the energy budget so that during days 234-240, the measured latent heat flux showed peak mid-day values of 100 W m^-2, while peak sensible heat fluxes were approximately 250 W m^-2. In 1999, the lower precipitation inputs and lower soil moisture resulted in reduced latent heat flux (peak seasonal values of about 100 W m^-2) and much smaller seasonal change. Sensible heat flux dominated the energy balance throughout the 1999 growing season, with peak values of 300 W m^-2 recorded during days 199-205 and during days 234-240. Higher vapor pressure deficits in 1999 also contributed to lower canopy conductance to water vapor. The amount and timing of precipitation strongly constrains evapotranspiration and energy balance in this grassland ecosystem.

Keywords: eddy covariance, climate change, water flux

This abstract is being presented at: 10:30 AM in session:
GAS EXCHANGE