Submitted to: Boundary Layer Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/12/2010
Publication Date: 11/1/2010
Publication URL: http://handle.nal.usda.gov/10113/47276
Citation: Griffis, T.J., Sargent, S.S., Lee, X., Baker, J.M., Greene, J., Erickson, M., Zhang, X., Billmark, K., Schultz, N., Xiao, W., Hu, N. 2010. Determining the oxygen isotope composition of evapotranspiration with eddy covariance. Boundary Layer Meteorology. 137(2):307-326. Interpretive Summary: The oxygen isotopic composition of the water vapor transported from the surface to the atmosphere by evapotranspiration can provide important clues for studies in a variety of fields including atmospheric science, hydrology, and carbon cycle science. Unfortunately, to this point it has not been possible to directly measure the isotope ratio of evapotranspiration. We describe the development and testing of a tunable diode laser technique for continuous measurement of the isotopic components of evapotranspiration by eddy covariance (EC-TDL). The system was tested over a soybean canopy in 2008 and a corn canopy in 2009, by comparing the total flux that it measured against conventional methods for measuring total ET. The new EC-TDL system compared quite well with the other systems in measuring total evapotranspiration, and additionally provided continuous data on the isotope ratio of evapotranspiration. These data showed that the isotope ratio of evapotranspiration had a strong diurnal pattern, becoming most enriched in the afternoon, when the isotope composition of evapotranspiration was similar to that of xylem water, indicating steady-state conditions for about 4 hours. This system will provide a valuable tool for studying water and carbon cycle processes.
Technical Abstract: The oxygen isotope componsition of evapotranspiration (dF) represents an important tracer in the study of biosphere-atmosphere interactions, hydrology, paleoclimate, and carbon cycling. Here we demonstrate direct measurement of dF based on eddy covariance (EC) and tunable diode laser (EC-TDL) technique. Results are presented from laboratory experiments and field measurements in agricultural ecosystems. The field measurements were obtained during the growing seasons of 2008 and 2009. Water vapor mxing ratios (Cw) and fluxes (F) were compared using EC-TDL and traditional EC-IRGA techniques over a soybean canopy in 2008. The results indicate that Cw and F agreed to within 1% and 5% respectively. Measurements of dF above a corn canopy in 2009 revealed a dirunal pattern with an expected progressive 18O enrichment through the day ranging from about -20 per mil before sunrise to about -5 per mil in late afternoon. The isotopic composition of evapotranspiration was similar to the xylem water isotope composition (dx=-7.2 per mil) for short periods of time during 1400 to 1800 LST, indicating near steady-state conditions. Finally, the isotopic forcing values (If) reveled a diurnal pattern with mean maximum values of 0.09 m s-1 per mil at midday. The If values could be described as an exponential relation of relative humidity confirming previous model calculations and measurements over a soybean canopy in 2006. These patterns and comparisons indicate that long-term continuous isotopic water vapor flux measurements based on the EC technique are feasible and can provide new insights related to the oxygen isotope fractionation processes at the canopy scale. The broader application of this technique will provide new insights regarding water and carbon cycle processes and will increase the power of the 18O-H2O and 18O-CO2 isotope tracers for biosphere-atmosphere research.