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United States Department of Agriculture

Agricultural Research Service

Title: The Bowen Ratio-Energy Balance Method for Estimating Latent Heat Flux of Irrigated Alfalfa Evaluated in a Semi-Arid, Advective Environment

Authors
item Todd, Richard
item Evett, Steven
item Howell, Terry

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 17, 2000
Publication Date: N/A

Interpretive Summary: The Bowen ratio-energy balance (BREB) is a micrometeorological method often used to quantify crop water use, investigate plant-water relations, and evaluate crop water use models. Its estimates of crop water use have compared favorably with those of other methods in several studies, but other studies have been less certain, especially when hot, dry air moves onto a moist field from surrounding drier areas, a common condition on the southern High Plains. We compared estimates of crop water use by the BREB method to crop water use measured by large, precision weighing lysimeters. The BREB method overestimated crop water use compared with lysimeter measurements. Overestimation was greatest when warmer, drier air moved onto the cooler, moister irrigated alfalfa field. These oasis conditions were encountered throughout the growing season, but were more common earlier in the season. Basic assumptions of the BREB method were violated under these conditions. To reestablish the validity of the BREB assumptions under oasi conditions, larger fields may be needed. These results will contribute to a better understanding of the advantages and limitations of the BREB method and will help researchers more effectively and accurately use the method.

Technical Abstract: The Bowen ratio-energy balance (BREB) is a micrometeorological method often used to estimate latent heat flux because of its simplicity, robustness, and cost. Its estimates have compared favorably with other methods in several studies, but other studies have been less certain, especially when there was sensible heat advection. The latent heat flux of irrigated alfalfa (Medicago sativa, L.) estimated by the BREB method and measured by precision weighing lysimeters was compared and contrasted over a growing season in the semi-arid, advective environment of the Southern High Plains. The BREB method overestimated latent heat flux during the daytime by 6 to 16% and underestimated at night by 40 to 73%, compared with lysimeter measurements. The smallest daytime difference of the BREB method compared with lysimeters was when Bowen ratios were between 0 and 0.3. Difference increased as Bowen ratios became more negative. Difference also increased on warmer, drier, windier days when sensible heat flux was toward the surface and an oasis effect was evident. There was evidence of the inequality of turbulent exchange coefficients under advective conditions, which violated assumptions of the BREB method.

Last Modified: 4/17/2014
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