American Society of Civil Engineers

FAO-24 Reference Evapotranspiration Factors

by Richard G. Allen, M.ASCE, (Assoc. Prof., Agric. and Irrig. Engrg. Dept., Utah State Univ., Logan, UT 84322-4105) and William O. Pruitt, M.ASCE, (Former Instructor, Dept. Water, Air, and Land Sci., Univ. of California, Davis, CA 95616)

Journal of Irrigation and Drainage Engineering, Vol. 117, No. 5, September/October 1991, pp. 758-773, (doi:

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Document type: Journal Paper
Abstract: Improved equations are presented for calculating correction factors for the United Nations Food and Agriculture Organization Irrigation and Drainage Paper Number 24 (FAO-24) grass reference evapotranspiration methods. Equations duplicate tables of correction factors contained in the original FAO-24 report and simplify development of computer programming code. Correction factors are included in the original FAO-24 publication to “correct” reference evapotranspiration estimates for varying climatic conditions of windiness, humidity, and sunshine. Equations are developed in this study using regression models where independent values are comprised of various transformations of secondary parameters used in the Food and Agriculture Organization (FAO) tables.

ASCE Subject Headings:
Regression models