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, a nd cost. Estimates of latent heat flux have compared favorably with other m ethods in several studies, but other studies have been less certain, especi ally when there was sensible heat advection. We compared the latent heat Au x of irrigated alfalfa (Medicago sativa, L,) estimated by the BREB method w ith that measured by lysimeters over a growing season in the semi-arid, adv ective environment of the southern High Plains. Difference statistics from the comparison and indicators of sensible heat advection were: used to anal yze the performance of the BREB method relative to lysimeters. Latent heat flux was calculated from mass change measured by two precision weighing lys imeters and from two BREB systems that used interchanging temperature and h umidity sensors. Net radiation (R-n), soil heat Aux (G), and other meteorol ogical variables were also measured. Difference statistics included the roo t mean square difference (RMSD) and relative RMSD (normalized by mean lysim eter latent heat flux). Differences between lysimeters averaged 5-15% durin g the day, and 25-45% at night. Estimates of latent heat Aux by the two BRE B systems agreed closely (relative RMSD=8%) when they were at the same loca tion with sensors at the same height. Differences increased when the locati on was the same but sensors were at different heights, or when the sensor h eight was the same but location in the held different, and probably was rel ated to limited fetch acid the influence of different source areas beyond t he field. Relative RMSD between lysimeter and BREB latent heat fluxes avera ged 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 vari ed from 17 to 28% during morning hours with no pattern based on cutting. Af ternoon relative RMSD was 25% during the first two cuttings and decreased t o 15% during subsequent cuttings. Greatest differences between the two meth ods 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 en ergy (R-n-G). These conditions were likely to be encountered throughout the growing season, but were more common earlier in the season. (C) 2000 Elsev ier Science B.V. All rights reserved.