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Research Project: Understanding Water-Driven Ecohydrologic and Erosion Processes in the Semiarid Southwest to Improve Watershed Management

Location: Southwest Watershed Research Center

Title: Partitioning evapotranspiration using long-term carbon dioxide and water vapor fluxes: New approach to ET partitioning

item Scott, Russell - Russ
item Biederman, Joel

Submitted to: Geophysical Research Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/22/2017
Publication Date: 7/12/2017
Citation: Scott, R.L., Biederman, J.A. 2017. Partitioning evapotranspiration using long-term carbon dioxide and water vapor fluxes: New approach to ET partitioning. Geophysical Research Letters. 44:6833-6840.

Interpretive Summary: Most of water from precipitation that falls on land returns back to the atmosphere. This process called evapotranspiration (ET) follows two pathways: one is through plants called transpiration (T), and the other is through direct evaporation (E) of water lying on the land surface. Determining these two pathways is important to understanding the amount of water available to support plant productivity rather than simply lost to the atmosphere. In this study we propose a new method to determine T and E independently from much more widely available evapotranspiration data. We find that our estimates agree with limited, direct measurements and consistent with expectations of seasonal changes in the relative strengths of T and E. Our method should allow for new estimates of E and T that will lead to improved understanding of how water is recycled by ecosystems.

Technical Abstract: The separate components of evapotranspiration (ET) provide critical information about the pathways and time scales over which water is returned to the atmosphere, but ecosystem-scale measurements of transpiration (T) and evaporation (E) remain elusive. We propose a novel determination of average E and T based on multiyear eddy covariance estimates of gross ecosystem photosynthesis (GEP). The method can be used at water-limited sites over flux integration periods during which a linear regression line between GEP (abscissa) and ET (ordinate) yields a positive x-axis intercept, an estimate of E. At four semiarid sites driven by summer rainfall, we find increasing monthly T/ET as the growing season progresses, consistent with previous, direct measurements. The seasonal growth of T/ET is related to increasing leaf area index and declining frequency of rainy days and the wet surface conditions which promote E, suggesting both surface and climatic controls on ET partitioning.