Calibration of the Penman wind function using the Bowen ratio energy balance method

Abstract

The Bowen ratio energy balance (BREB) method was used to calibrate the wind function constants ( W f = a w + b w u 2) in the aerodynamic term of the Penman equation for computation of the hourly and 24 h alfalfa-reference evapotranspiration ( E tr) at an irrigated semi-arid valley in northern Utah during the 1991–1992 growing season. Statistical analysis of the 28 non-rainy days (when the conditions of alfalfa-reference evapotranspiration were met) showed that the hourly constants in the wind function were a w = 2.677 and b w = 0.127 (with u 2 expressed in km h −1). The hourly relationship between the alfalfa-reference evapotranspiration estimated by the BREB method ( E tr(BREB)) and that computed by the Penman method ( E tr(Penman)) is E tr(Penman) = 0.976E tr(BREB), with the correlation coefficient ( R) equal to 0.974. The daily (24 h) constants were a w = 1.00 and b w = 0.014 (with u 2 expressed in km day −1). The 24 h relationship is E tr(Penman) = 1.009 E tr(BREB), with R = 0.988. The wind function constants found here can be applied for computation of hourly and 24 h alfalfa-reference evapotranspiration and irrigation scheduling in this semi-arid valley.