The Bowen ratio-energy balance method for estimating latent heat flux of irrigated alfalfa evaluated in a semi-arid, advective environment

The Bowen ratio-energy balance (BREB) is a micrometeorological method often used to estimate latent heat flux because of its simplicity, robustness, and cost. Estimates of latent heat flux have compared favorably with other methods in several studies, but other studies have been less certain, especially when there was sensible heat advection. We compared the latent heat flux of irrigated alfalfa (Medicago sativa, L.) estimated by the BREB method with that measured by lysimeters over a growing season in the semi-arid, advective environment of the southern High Plains. Difference statistics from the comparison and indicators of sensible heat advection were used to analyze the performance of the BREB method relative to lysimeters. Latent heat flux was calculated from mass change measured by two precision weighing lysimeters and from two BREB systems that used interchanging temperature and humidity sensors. Net radiation (Rn), soil heat flux (G), and other meteorological variables were also measured. Difference statistics included the root mean square difference (RMSD) and relative RMSD (normalized by mean lysimeter latent heat flux). Differences between lysimeters averaged 5–15% during the day, and 25–45% at night. Estimates of latent heat flux by the two BREB systems agreed closely (relative RMSD=8%) when they were at the same location with sensors at the same height. Differences increased when the location was the same but sensors were at different heights, or when the sensor height was the same but location in the field different, and probably was related to limited fetch and the influence of different source areas beyond the field. Relative RMSD between lysimeter and BREB latent heat fluxes averaged by cutting was 25–29% during the first two cuttings and decreased to 16–19% during the last three cuttings. Relative RMSD between the methods varied from 17 to 28% during morning hours with no pattern based on cutting. Afternoon relative RMSD was 25% during the first two cuttings and decreased to 15% during subsequent cuttings. Greatest differences between the two methods were measured when the Bowen ratios were less than 0, on days that were hot, dry and windy, or when the latent heat flux exceeded the available energy (Rn−G). These conditions were likely to be encountered throughout the growing season, but were more common earlier in the season.