Comparison of Some Calibration Methods of Hargreaves Equation for Evapotranspiration Estimation(Case Study: Shiraz and Zarghan Synoptic Stations in Fars Province)
Subject Areas : Farm water management with the aim of improving irrigation management indicators
1 - Associated Professor of Irrigation and Drainage, Marvdasht Branch, Islamic Azad University, Marvdasht, Iran.
Keywords: Evapotranspiration, Monthly effective temperature, Hargreaves equation, Penman-Monteith equation,
Abstract :
Background and Aim: The Hargreaves equation is one of the simplest methods for estimation of reference crop potential evapotranspiration which has two constant coefficients of 17.8 and 0.0023. The Hargreaves equation has been calibrated in many studies around world based on lysimeter data or the Penman-Monteith method. The main objective of this study was the calibration and validation of Hargreaves equation with three conditions based on the Penman-Monteith equation in Shiraz and Zarghan synoptic stations in Fars province. Method: In this study, the Hargreaves equation has been calibrated and evaluated with three different conditions based on the Penman-Monteith method in Shiraz and Zarghan synoptic stations in Fars province. In the first condition (H1), the monthly effective temperature was used instead of monthly mean temperature, and also the constant coefficients of 17.8 and 0.0023 were calibrated. The monthly effective temperature is the function of minimum and maximum monthly temperature and a constant coefficient that must be determined in calibration stage. In the second condition (H2), the constant coefficients of 17.8 and 0.0023 were calibrated, and in the third condition (H3) only the constant coefficient of 0.0023 was calibrated. For this study, 25 years (1988-2012) of monthly weather data of Shiraz and Zarghan synoptic stations in Fars province were used to calibrate the three mentioned conditions, and the coefficients of each condition for each month and each station were determined separately by using the Solver menu of Excel software. Then, 10 years (2013-2022) of monthly data of mentioned stations were used to evaluate the obtained results. Also, Root Mean Square Error (RMSE), Normalized Root Mean Square Error (NRMSE), Mean Bias Error (MBE) and model Efficiency Factor (EF) were employed to compare the results in calibration and evaluation stages.Results: The results in calibration and evaluation stages for both stations showed that the H3 condition for estimating evapotranspiration was better than the other conditions, and only determining a coefficient instead of 0.0023is needed. The value of this coefficient was changed between 0.0022 to 0.0027 in both stations.Conclusion: The results indicated that similar to many previous studies in the world, it is better to calibrate the coefficients of the Hargreaves equation for estimating reference crop potential evapotranspiration for study region. On the other hand, the results showed that using monthly effective temperature for estimating reference crop potential evapotranspiration in Hargreaves equation didn’t lead to the improvement of evapotranspiration estimation.
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