Local Calibration of the Hargreaves and Priestley-Taylor Equations for Estimating Reference Evapotranspiration in Arid and Cold Climates of Iran Based on the Penman-Monteith Model

The Food and Agricultural Organization of the United Nations (FAO)-56 version of Penman-Monteith (PMF-56) model has been established as a standard for calculating reference evapotranspiration (ETo). An important constraint of application of the PMF-56 model is the requirement of solar radiation, wind speed, air temperature, and humidity data, which may not be available for a given location, especially in developing countries. The Hargreaves (HG) and Priestley-Taylor (P-T) equations are simple equations that require few weather data inputs, although regional calibration of the equations is needed for acceptable performance before applying them for ETo estimation. In this study, the HG and P-T equations were calibrated on the basis of the PMF-56 method in arid and cold climates of Iran using data from 12 stations during 1994-2005. After calibration of the HG equation, the average value of the adjusted HG coefficient for arid climate was 0.0031, which is about 34% higher than the original value (0.0023). Similarly, the average value of the new HG coefficient for cold climate was 0.0028, which is about 22% higher than the original value. The results showed that the original P-T coefficient of 1.26 was very low for the climatic regions, and the new P-T coefficients of 1.82 and 2.14 have the best fit as compared with the PMF-56 method in cold and arid climates, respectively. Overall, calibration of the HG and P-T equations resulted in improvements of the equations by reducing the errors of the ETo estimates. DOI: 10.1061/(ASCE)HE.1943-5584.0000366. (C) 2011 American Society of Civil Engineers.