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

A multi-year, multi-technique study was conducted to measure evapotranspira tion and its components within an uneven-aged mixed deciduous forest in the Southeastern United States. Four different measurement techniques were use d, including soil water budget (1 year), sap flow (2 years), eddy covarianc e (5 years), and catchment water budget (31 years). Annual estimates of eva potranspiration were similar for the eddy covariance and catchment water ba lance techniques, averaging 571 +/- 16 mm (eddy covariance) and 582 +/- 28 mm (catchment water balance) per year over a 5-year period. There were qual itative similarities between sap flow and eddy covariance estimates on a da ily basis, and sap flow estimates of transpiration were about 50% of annual evapotranspiration estimated from eddy covariance and catchment studies. S oil evaporation was estimated using a second eddy covariance system below t he canopy, and these measurements suggest that soil evaporation explains on ly a small portion of the difference between sap flow estimates of transpir ation and eddy covariance and catchment water budget estimates of evapotran spiration. Convergence of the catchment water balance and eddy covariance m ethods and moderately good energy balance closure suggests that the sap flo w estimates could be low, unless evaporation of canopy-intercepted water wa s especially large. The large species diversity and presence of ring-porous trees at our site may explain the difficulty in extrapolating sap flow mea surements to the spatial scales representative of the eddy covariance and c atchment water balance methods. Soil water budget estimates were positively correlated with eddy covariance and sap flow measurements, but the data we re highly variable and in error under conditions of severe surface dryness and after rainfall events. (C) 2001 Elsevier Science B.V. All rights reserv ed.