THE TRANSITION OF THE BLANEY-CRIDDLE FORMULA TO THE PENMAN-MONTEITH EQUATION IN THE WESTERN UNITED STATES

by: Theodore W. Sammis, Junming Wang, David R. Miller

Volume 2011, No. 1, 21 Feb 2011

This paper reviews the history of calculating “consumptive water use,” later termed evapotranspiration (Et), for plants in the western U.S. The Blaney-Criddle formula for monthly and seasonal consumptive water use was first developed for New Mexico in 1942 for limited crops. The formula was based on the input of monthly mean air temperature and an empirical monthly/seasonal coefficient. Subsequent changes improved the Blaney-Criddle formula by adding more weather and crop variables. The availability of data from automated weather stations, after about 1980, that measure more weather input variables has allowed the empirical Blaney-Criddle formula to be replaced by the mechanistic standardized Penman-Monteith equation with an appropriate crop coefficient to calculate Et. The Penman-Monteith equation calculates Et under non-stressed conditions and represents the maximum Et and associated yield of the crop.

Water rights in the western U.S. have historically, and continue to be, adjudicated using variations of the Blaney-Criddle formula. The Blaney-Criddle formula, derived in farmers’ fields under water stress conditions, calculates an Et that is most closely related to average county yields during the years the measurements were taken. But the empirical relationship and the originally derived coefficients are outdated and invalid for today’s agriculture production systems and should be replaced with the Penman-Monteith equation when adjudicating water rights.

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