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

摘要

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 (ET_0). 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 ET_0 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 ET_0 estimates.