Agricultural and Forest Meteorology 106 (2001) 153–168 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 a,∗ , Paul J. Hanson b , Patrick J. Mulholland b , Dennis D. Baldocchi c , Stan D. Wullschleger b a Atmospheric Turbulence and Diffusion Division, NOAA, P.O. Box 2456, Oak Ridge, TN 37831, USA b Environmental Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831, USA c Department of Environmental Science, Policy and Management, University of California at Berkeley, 151 Hilgard Hall, Berkeley, CA 94720, USA Received 17 February 2000; received in revised form 13 July 2000; accepted 13 July 2000 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. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Evapotranspiration; Eddy covariance; Catchment water balance ∗ Corresponding author. Tel.: +1-865-576-2317; fax: +1-865-576-1237. E-mail address: wilson@atdd.noaa.gov (K.B. Wilson). 1. Introduction Evapotranspiration is an important process across a wide range of disciplines, including ecology, hy- drology and meteorology. Because of this multidisci- plinary focus, a number of methodologies have been 0168-1923/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0168-1923(00)00199-4