A comparison of methods for determining forest evapotranspiration and its components: Sap-flow, soil water budget, eddy covariance and catchment water balance

Kell B. Wilson, Paul J. Hanson, Patrick J. Mulholland, Dennis D. Baldocchi, Stan D. Wullschleger

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639 Scopus citations

Abstract

A multi-year, multi-technique study was conducted to measure evapotranspiration and its components within an uneven-aged mixed deciduous forest in the Southeastern United States. Four different measurement techniques were used, including soil water budget (1 year), sap flow (2 years), eddy covariance (5 years), and catchment water budget (31 years). Annual estimates of evapotranspiration were similar for the eddy covariance and catchment water balance techniques, averaging 571 ± 16 mm (eddy covariance) and 582 ± 28 mm (catchment water balance) per year over a 5-year period. There were qualitative similarities between sap flow and eddy covariance estimates on a daily basis, and sap flow estimates of transpiration were about 50% of annual evapotranspiration estimated from eddy covariance and catchment studies. Soil evaporation was estimated using a second eddy covariance system below the canopy, and these measurements suggest that soil evaporation explains only a small portion of the difference between sap flow estimates of transpiration and eddy covariance and catchment water budget estimates of evapotranspiration. Convergence of the catchment water balance and eddy covariance methods and moderately good energy balance closure suggests that the sap flow estimates could be low, unless evaporation of canopy-intercepted water was especially large. The large species diversity and presence of ring-porous trees at our site may explain the difficulty in extrapolating sap flow measurements to the spatial scales representative of the eddy covariance and catchment water balance methods. Soil water budget estimates were positively correlated with eddy covariance and sap flow measurements, but the data were highly variable and in error under conditions of severe surface dryness and after rainfall events.

Original languageEnglish
Pages (from-to)153-168
Number of pages16
JournalAgricultural and Forest Meteorology
Volume106
Issue number2
DOIs
StatePublished - Jan 27 2001

Funding

This work was funded by a grant from NASA/GEWEX and the US Department of Energy (Terrestrial Carbon Program) and is a contribution to the Ameriflux and FLUXNET projects. Funding for ORNL researchers (PJH, PJM, SDW) was provided by the NSF/DOE/NASA/USDA/EPA/NOAA Interagency Program on Terrestrial Ecology and Global Change (TECO) through NASA’s Earth Science Enterprise Program under Interagency Agreement No. 2013-K057-A1, and the Program for Ecosystem Research, Environmental Sciences Division, Office of Health and Environmental Research, U.S. Department of Energy under contract DE-AC05-00OR22725 with University of Tennessee-Battelle LLC. Research was conducted on the Oak Ridge National Environmental Research Park. E. Falge and D. Schindler provided data on species inventory. M. Brewer, M. Hall and D. Auble provided field and laboratory assistance. J. Herewhe and R. Dobosy provided internal reviews.

FundersFunder number
NASA/GEWEX
NASA’s Earth Science Enterprise Program2013-K057-A1
NSF/DOE/NASA
Office of Health and Environmental Research
US Department of Energy
USDA/EPA/NOAA
University of Tennessee-Battelle LLC
U.S. Department of EnergyDE-AC05-00OR22725

    Keywords

    • Catchment water balance
    • Eddy covariance
    • Evapotranspiration

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