Using High-Frequency Water Vapor Isotopic Measurements as a Novel Method to Partition Daily Evapotranspiration in an Oak Woodland

Christopher Adkison,Rajit Patankar,Caitlyn Cooper-Norris,Georgianne W Moore

doi:10.3390/w12112967

Christopher Adkison, Rajit Patankar + Show 2 more

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https://doi.org/10.3390/w12112967

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Abstract

Partitioning evapotranspiration (ET) into its constituent fluxes (transpiration (T) and evaporation (E)) is important for understanding water use efficiency in forests and other ecosystems. Recent advancements in cavity ringdown spectrometers (CRDS) have made collecting high-resolution water isotope data possible in remote locations, but this technology has rarely been utilized for partitioning ET in forests and other natural systems. To understand how the CRDS can be integrated with more traditional techniques, we combined stable isotope, eddy covariance, and sap flux techniques to partition ET in an oak woodland using continuous water vapor CRDS measurements and monthly soil and twig samples processed using isotope ratio mass spectrometry (IRMS). Furthermore, we wanted to compare the efficacy of δ2H versus δ18O within the stable isotope method for partitioning ET. We determined that average daytime vapor pressure deficit and soil moisture could successfully predict the relative isotopic compositions of soil (δe) and xylem (δt) water, respectively. Contrary to past studies, δ2H and δ18O performed similarly, indicating CRDS can increase the utility of δ18O in stable isotope studies. However, we found a 41–49% overestimation of the contribution of T to ET (fT) when utilizing the stable isotope technique compared to traditional techniques (reduced to 4–12% when corrected for bias), suggesting there may be a systematic bias to the Craig-Gordon Model in natural systems.

Highlights

Evapotranspiration (ET) is an important process in the terrestrial hydrological cycle that accounts for evaporation (E) from the soil, open water, and canopy-intercepted water and transpired (T) water from vegetation
This study demonstrated utility using a combination of stable isotopes, sap flux, covariance techniques tothe partition
This study demonstrated a novel methodology for partitioning ET using a combination of the high-resolution stable isotope, sap flux, and eddy covariance techniques over multiple seasons in a natural oak woodland site

Summary

Introduction

Evapotranspiration (ET) is an important process in the terrestrial hydrological cycle that accounts for evaporation (E) from the soil, open water, and canopy-intercepted water and transpired (T) water from vegetation. ET constitutes a large percentage of the water cycle in most environments, and up to 95% in arid environments, deeming it an important water flux at a variety of spatial scales [1,2]. An improved understanding of ET partitioning and the contribution of T to ET (fT ) is necessary for refined water resource management and quantification of vegetative responses to climate change and alterations to the carbon cycle. Complex dynamics exist between carbon fluxes and precipitation at multiple scales, and the water-use efficiency of plants varies with ecosystem aridity and vegetation cover [3,4]. Partitioning of ET is necessary because T is seen as a desirable aspect of the water cycle that allows vegetation to grow, while water that is evaporated is generally seen as being “lost” from the system.

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