Determining the Oxygen Isotope Composition of Evapotranspiration Using Eddy Covariance

摘要

The oxygen isotope composition of evapotranspiration (t F ) represents an important tracer in the study of biosphere-atmosphere interactions, hydrology, paleoclimate, and carbon cycling. Here, we demonstrate direct measurement of t F based on the eddy-covariance and tunable diode laser spectroscopy (EC-TDL) techniques. 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 vapour mixing ratios (s w ) and fluxes (F) were compared using EC-TDL and traditional eddy-covariance and infrared gas analyser techniques over a soybean canopy in 2008. The results indicate that s w and F agreed to within 1 and 6%, respectively. Measurements of t F above a corn canopy in 2009 revealed a diurnal pattern with an expected progressive p#iO enrichment through the day ranging from about 20 before sunrise to about 5 in late afternoon. The isotopic composition of evapotranspiration was similar to the xylem water isotope composition (t x = 7.2) for short periods of time during 1400-1800 LST, indicating near steady-state conditions. Finally, the isotopic forcing values (I F ) revealed a diurnal pattern with mean maximum values of 0.09msp# at midday. The I F 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 vapour flux measurements based on the eddy-covariance technique are feasible and can provide new insights related to the oxygen isotope fractionation processes at the canopy scale.