Cavity Ring-Down Spectroscopy As a Tool for Evapotranspiration Partitioning Characterization Using Water Isotopes.

As fresh water is becoming a scarcity, there has been a global effort to preserve and efficiently use it. In agriculture, evapotranspiration (ET), or the flux of water from a vegetated surface via both evaporation (E) and transpiration (T) by plants, is an important component of the water budget. Water loss via transpiration can be considered “good” water use, while water loss via evaporation can be considered “wasted” water use. The characterization of those plant processes was historically performed through cumbersome and inaccurate water flux measurements. With the recent advancement of laser-based water vapor isotope analyzers, various calculation models have been developed to correlate real-time, spatial, and temporal isotopic measurements with evaporation and transpiration fluxes.

Here we demonstrate how laser-based absorption spectroscopy, and in particular, Cavity Ring-Down Spectroscopy (CRDS), can be optimized in many types of ET analyses, including (i) the characterization of the partial pressure and isotopic content of the vertical water vapor profiles to determine the bulk ET signal through a Keeling Mixing model, (ii) the use of soil water isotopic composition, in combined with the Craig-Gordon model, to determine the evaporation flux signature, (iii) the direct measures of isotopic signature of transpiration in leaf chambers to differentiate the isotope signature of this source. In addition, we present results of field experiments supporting the method of vertical water vapor profiling.