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United States Department of Agriculture

Agricultural Research Service

Title: Discussion of "adjusting Temperature Parameters to Reflect Well-Watered Conditions"

Author
item Howell, Terry

Submitted to: Journal of Irrigation and Drainage Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 2, 1999
Publication Date: N/A

Interpretive Summary: A paper was published that recommended that air temperature and dew point temperature data be adjusted to correspond to "near" saturated conditions (high air humidity) to remove site "aridity" bias effects from increasing computed crop water use. Crop water use was characterized using standard equations. The authors of the published paper derived relationships and data adjusting procedures for maximum (Tmax), minimum (Tmin), and dew poin temperature (Tdew) based on monthly averaged data compared with the monthly ration of "effective precipitation" to the computed crop water use. The procedures advocated in the published paper were tested against data at Bushland, TX, in a semi-arid, continental climatic regime. Climate data from 1 July 1995 through December 1998 were used because they contained two relatively "dry" or drought periods and a long "wetter" period. The derived relationships between Tmin, Tmax, and Tdew at Bushland, TX, did not texactly correspond to the authors' notions in the published paper. But they were not too dissimilar from the published arid site data. The data adjustments did improve the correlation between measured grass water use and computed grass water use, but they also increased the bias and decreased the sensitivity. This was opposite to the published work for a Mediterranean climate site in California. Any data adjustments should be viewed cautiously, and a greater emphasis placed on weather station siting, instrument maintenance, and data processing for irrigated agriculture.

Technical Abstract: This is a discussion of a published paper that recommends that air temperature and dew point temperature data be adjusted to correspond to "near" saturated conditions to remove site "aridity" bias from estimated reference evapotranspiration (ETo). The authors of the published paper derived relationships and data adjusting algorithms for maximum (Tmax), minimum (Tmin), and dew point (Tdew) based on monthly averaged data compared with the monthly ratio of "effective precipitation" to ETo (called PRATIO). The authors' concepts were tested against data at Bushland, TX, in a semi-arid, continental climatic regime. Climate data from 1 July 1995 through December 1998 were used because they contained two relatively "dry" or drought periods and a long "wetter" period. The relationships between Tmin, Tmax, and Tdew to PRATIO at Bushland, TX, did not exactly correspond to the authors' notions in the published paper, although not too dissimilar rfrom the published arid site data. The data adjustments did improve the correlation between measured grass evapotranspiration and computed ETo, but they also increased the bias (Y-axis intercept) and decreased the sensitivity (slope). This was opposite to the published work for a Mediterranean climate site in California. Any data adjustments should be viewed cautiously, and a greater emphasis placed on weather station siting, instrument maintenance, and data processing for irrigated agriculture.

Last Modified: 12/21/2014
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