Determination of Forest Evapotranspiration Using Bowen Ratio and Eddy Correlation Measurements

Abstract

Rates of evapotranspiration from a 14 m high Douglas fir forest on the southwest coast of British Columbia were obtained using the energy balance/Bowen ratio method and an energy balance/eddy correlation method. In the former method, the Bowen ratio was measured using reversing diode psychrometers. In the latter, the sensible heat flux was obtained by eddy correlation analysis of data obtained from a fast response thermistor and Gill anemometers mounted horizontally and at 30° from the vertical. The generally low wind speed above the forest resulted in occasional stalling of the anemometers and made obtaining adequate eddy correlation data difficult. Spectral analysis of the eddy correlation data indicates that a significant fraction of the sensible heat flux was at low frequencies. The regression relationship between evapotranspiration rate obtained using the energy balance/eddy correlation method (Ee) and that obtained using the energy balance/Bowen ratio method (E?) was found to be Ee = 0.96E? ? 0.02 [mm h?1], R2 = 0.93, sy·z = 0.07 mm h?1 The experiment suggests that an eddy correlation system using mechanical anemometers is not suitable for extended water balance studies of forests where low wind speeds predominate.