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Calibration of the Angström-Prescott solar radiation model for accurate estimation of reference evapotranspiration in the absence of observed solar radiation

Mousavi, Roya, Sabziparvar, Ali Akbar, Marofi, Safar, Ebrahimi Pak, Niaz Ali, Heydari, Majid
Theoretical and applied climatology 2015 v.119 no.1-2 pp. 43-54
climate, evapotranspiration, lysimeters, models, solar radiation, Iran
The Angström-Prescott (AP) model is widely suggested for estimating global solar radiation (Rₛ), especially for reference crop evapotranspiration (ET₀) calculations. Many previous studies indicated that the coefficients of the AP model depend on the climatic and geographical characteristics of study sites. In the present work, calibrated AP models were developed in some climates in Iran. Different radiation scenarios from estimated and measured Rₛwere applied to calibrate the AP coefficients. Radiation scenarios included measured Rₛ(M) and estimated Rₛfrom the models of Daneshyar (D) and Hargreaves (H). Daily reference evapotranspiration was calculated using the coefficients obtained from different Rₛscenarios. The estimated daily ET₀values were compared with field lysimeter data for validation. Results showed that scenarios D and H present the minimum ET₀deviation from lysimeter data. Findings suggested that at the majority of the study sites, calibration of the AP model improved the accuracy of estimated ET₀. Results also indicated the applicability of reliable Rₛestimation methods to calibrate the AP model. Apart from applied Rₛscenarios, the daily ET₀is underestimated by the FAO56 Penman-Monteith model.