Reconciling Evapotranspiration Partitioning Models with Evidence of Anomalously Low Isotopic Fractionation during Evaporation in Semi-arid Landscapes
Abstract
Partitioning land surface latent heat flux into evaporation (E) and transpiration (T) remains challenging despite a basic understanding of the underlying mechanisms. Water isotopologues are useful tracers for separating evaporation and transpiration contributions because E and T have distinct isotopic ratios. Here the isotope-based partitioning method is used at a semi-arid grassland tall-tower site in Colorado. Results suggest that under certain conditions evaporation cannot be isotopically distinguished from transpiration without modifying existing partitioning techniques. Over a 4-year period, profiles of stable oxygen and hydrogen isotope ratios of water vapor were measured from the surface to 300 m and soil water down to 1 m along with standard meteorological variables. Using these data, it was found that rainfall, equilibration, surface water vapor exchange and sub-surface vapor diffusion all contribute to the isotopic composition of evapotranspiration (ET). Applying the standard isotopic approach to find the transpiration portion of ET (i.e., T/ET), a significant discrepancy is found compared with a method to constrain T/ET based on gross primary productivity (GPP). By evaluating kinetic effects associated with soil evaporation and vapor diffusion, a significant proportion (58-84%) of evaporation following precipitation is found to be non-fractionating. This is possible when water from discrete soil layers is nearly completely evaporated as soil dries. The isotope ratio of non-fractionating evaporative flux is indistinguishable from the isotope ratio of transpiration, and may therefore explain the overestimation of T/ET from traditional "two-stream" partitioning methods. Accounting for weaker fractionation during evaporation reconciles isotope-based partitioning T/ET estimates with the GPP method.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.U13B..16K
- Keywords:
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- 0810 Post-secondary education;
- EDUCATION