Evaluation of the Bowen ratio/energy balance method for determining forest evapotranspiration

Abstract

The Bowen ratiolenergy balance method using periodic reversal of the psychrometers to remove systematic errors is evaluated. Temperature and vapour‐pressure differences can be measured with accuracies of ±0.005 °C and ±1 Pa, respectively. For a 3‐m vertical separation of the psychrometers and the Bowen ratio (ß), 0 < ß < 4, the probable relative error in the forest evapotranspiration (E) is < ±15% if the temperature and vapour‐pressure gradients are large, and ranges from ±10 to ± 60% if the gradients are small. The error in E is from two to five times these values for ß < 0. Measurements of E made with the Bowen ratiolenergy balance method are compared with those made concurrently with an eddy correlation/energy balance method, a stomatal diffusion resistance method and a soil water balance method. Agreement is generally within ±20% and frequently within ±10%, well within the errors associated with the methods.