Rational Use of The FAO Blaney‐Criddle Formula

Abstract
The FAO Blaney‐Criddle (FAO‐BC) evapotranspiration equation was calibrated and tested against a Penman combination equation with local wind function and daily lysimeter measurements of alfalfa evapotranspiration. Agreement between the calibrated FAO‐BC method and lysimeters was excellent for daily, weekly and monthly estimates when measured values for solar radiation, air temperature, relative humidity and wind speed were used. An elevation correction reduced scatter in estimates among Idaho locations. Statistics describing the daily Penman and FAO‐BC estimates deviated from those calculated for measured alfalfa reference evapotranspiration. Air temperature and relative humidity data for nonagricultural weather stations were adjusted according to fetch and aridity before being used to estimate consumptive use requirements throughout Idaho. Calibration of the FAO‐BC allowed use of alfalfa‐based crop coefficients.

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