Measurement and partitioning of evapotranspiration for application to vadose zone studies

Authors: Anderson, Raymond - Ray; ZHANG, XUDONG - Shenyang Agricultural University; Skaggs, Todd

Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/21/2017
Publication Date: 1/18/2018
Citation: Anderson, R.G., Zhang, X., Skaggs, T.H. 2018. Measurement and partitioning of evapotranspiration for application to vadose zone studies. Vadose Zone Journal. 16(13):1-9. doi:10.2136/vzj2017.08.0155.
DOI: https://doi.org/10.2136/vzj2017.08.0155

Interpretive Summary: Partitioning evapotranspiration into its components, evaporation and transpiration, is critical for multiple hydrological uses, including evaluating the success of irrigation strategies to reduce non-productive evaporation and evaluating the performance of numerous hydrologic models. Unfortunately, partitioning evapotranspiration is challenging because most measurement techniques cannot separate evaporation and transpiration, and many previously developed methods have serious limitations. In this study, we assess recent developments and advancements in partitioning evapotranspiration. We focus particularly on flux variance partitioning, a micrometeorological method that enables use of widely acquired eddy covariance data. We present some analyses on the impact of water use efficiency parameterization for the performance of flux variance partitioning. We conclude that flux variance partitioning and remote-sensing analysis hold the best potential for partitioning evapotranspiration across large scales due to the large number of datasets that can be retrospectively analyzed. This research is of interest to irrigators, ecologists, and modelers who need independent estimates of evaporation and transpiration.

Technical Abstract: Partitioning evapotranspiration (ET) into its constituent components, soil evaporation (E) and plant transpiration (T), is important for vadose zone studies because E and T are often parameterized separately. However, partitioning ET is challenging, and many longstanding approaches have significant spatial and temporal limitations and other challenges. In the past 15 years, new micrometeorological and remote sensing approaches have been developed that allow long-term partitioning of ET as well as partitioning at spatial scales ranging from the pedon to the globe. In this update paper, we review some recent methodological developments for partitioning ET. These include micrometeorological approaches involving the flux variance partitioning of high frequency eddy covariance observations and proxies for photosynthesis and transpiration such as measurements of isotopic fractionation and carbonyl sulfide uptake Due to its potential for retrospective analysis, we focus particularly on flux variance partitioning and present some analyses of the impact of water use efficiency parameterization on ET partitioning. Finally, we discuss advances in partitioning the energy balance between canopy and soil using remote sensing that can enable partitioning with satellite, aerial, and near surface sensors. We conclude that the flux variance partitioning with eddy covariance and two-source energy balance with remote sensing platforms may have the widest utility for partitioning ET because large public repositories of eddy covariance and satellite data exist which could be reprocessed on a wide scale to partition ET across numerous sites.