An Improved Estimation of the Angstrom–Prescott Radiation Coefficients for the FAO56 Penman–Monteith Evapotranspiration Method

Ali Sabziparvar (), Roya Mousavi, Safar Marofi, Niaz Ebrahimipak and Majid Heidari

Water Resources Management: An International Journal, Published for the European Water Resources Association (EWRA), 2013, vol. 27, issue 8, 2839-2854

Abstract: The FAO56 Penman–Monteith (FAO56-PM) method is known as the standard method for estimating reference evapotranspiration (ET 0 ) in a variety of climate types. Global solar radiation (R s ) is one of the essential inputs of this model, which is usually estimated from the Angstrom–Prescott (AP) method. The major drawback of the FAO56 pre-defined AP coefficients application is that the AP coefficients might need local calibration, to estimate ET 0 accurately. The aim of this study is to compare the effect of the FAO56 pre-defined AP coefficients (i.e. a and b) and the locally calibrated ones, on estimating daily ET 0 in 15 sites over Iran. Using long-term (1980–2007) experimental global solar radiation data (R s ), new locally calibrated (a) and (b) coefficients are suggested and new ET 0 values are determined accordingly. It was found that the range of the calibrated AP coefficients (a, b) are climate dependent and locally different from those of recommended by the FAO56-PM method. Estimated ET 0 at daily scale, improved up to 72.7 % when the calibrated AP coefficients were applied instead of FAO56 pre-defined AP coefficients. Based on the results, applying the FAO56 pre-defined AP coefficients (i.e. a = 0.25 and b = 0.50) in northern subtropical-humid and southern hot climates caused larger ET 0 errors. By contrast, the least ET 0 errors were found in cool arid and cool semi-arid inland climates, locating about 1,330 above sea level. The correlations between the calibrated AP coefficients and geographical factors are also discussed in this research. Copyright Springer Science+Business Media Dordrecht 2013

Keywords: Angstrom–Prescott coefficients; FAO56 Penman–Monteith; Evapotranspiration model; Calibration (search for similar items in EconPapers)
Date: 2013
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DOI: 10.1007/s11269-013-0318-z

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