Abstract
The isotopic compositions of water fluxes provide valuable insights into the hydrological cycle and are widely used to quantify biosphere–atmosphere exchange processes. However, the combination of water isotope approaches with water flux components remains challenging. The Iso-SPAC (coupled heat, water with isotopic tracer in soil–plant–atmosphere-continuum) model is a useful framework for simulating the dynamics of water flux and its components, and for coupling with isotopic fractionation and mixing processes. Here, we traced the isotopic fractionation processes with separate soil evaporation (Ev) and transpiration (Tr), as well as their mixing in evapotranspiration (E) for simulating diurnal variations of isotope compositions in E flux (δE). Three sub modules, namely isotopic steady state (ISS), non-steady-state (NSS), and NSS Péclet, were tested to determine the true value for the isotope compositions of plant transpiration (δTr) and δE. In situ measurements of isotopic water vapor with the Keeling-plot approach for δE and robust eddy covariance data for E agreed with the model output (R2 = 0.52 and 0.98, RMSD = 2.72‰, and 39 W m−2), illustrating the robustness of the Iso-SPAC model. The results illustrate that NSS is a better approximation for estimating diurnal variations in δTr and δE, specifically during the alternating periods of day and night. Leaf stomata conductance regulated by solar radiation controlled the diurnal variations in transpiration fraction (Tr/E). The study emphasized that transpiration and evaporation, respectively, acted to increase and decrease the δ18O of water vapor that was affected by the diurnal trade-off between them.
Highlights
Isotopic compositions of water fluxes provide valuable insights into the hydrological cycle and are widely used to quantify biosphere–atmosphere exchange processes [1,2,3]
The Iso-SPAC model was applied to simulate the diurnal variations in energy fluxes and isotopic compositions in E
Energy flux estimated by the Iso-SPAC model agreed very well with observed values
Summary
Isotopic compositions of water fluxes provide valuable insights into the hydrological cycle and are widely used to quantify biosphere–atmosphere exchange processes [1,2,3]. Evapotranspiration flux in terrestrial ecosystems is a combination of two or three different pathways (e.g., plant transpiration, soil evaporation, and canopy interception) of water vaporization [4,5]. Using the isotopic composition of soil evaporation (Ev), transpiration (Tr), and evapotranspiration (E) provided an independent approach to partitioning E in various ecosystems [6,7,8,9]. Combined measurements of the isotopic compositions of water vapor are often used to diagnose the local impacts on E, as well as its components (Ev and Tr) on atmospheric moisture [10,11]. Studies on water flux and its component coupling with isotope fractionation and mixing processes remain few and challenging [12], in particular on a diurnal timescale [13].
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