AN036: Water Stable Isotope Technique to Determine Evapotranspiration Partitioning

AN036: Water Stable Isotope Technique to Determine Evapotranspiration Partitioning

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. In these guidelines, we demonstrate how laser-based absorption spectroscopy, and in particular, Cavity Ring-Down Spectroscopy (CRDS), can be applied to many steps of ET analyses.

Abstract: 

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. Transpiration occurs through stomatal pores which are the same pores through which the plants uptake CO2 for building carbon compounds and ultimately gaining biomass.  Stomata are tightly controlled by physiological signals to optimize carbon gain per water lost. Water-use efficiency of a plant species or crop type is related to both plant genetics, as well as acclimation to irrigation regime.

The characterization of those plant processes was historically performed through cumbersome and inaccurate water flux measurements. However, 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 (FET and FT).

In these guidelines, we demonstrate how laser-based absorption spectroscopy, and in particular, Cavity Ring-Down Spectroscopy (CRDS), can be applied to many steps of ET analyses, including (i) characterization of partial pressure and the isotopic composition 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 combination with the Craig-Gordon model, to determine the evaporation flux signature, (iii) direct measurement of isotopic signature of transpiration in leaf chambers to differentiate the isotope signature of this source.