Time and Place Calibration of the Hargreaves Equation for Estimating Monthly Reference Evapotranspiration under Different Climatic Conditions

Houshang Ghamarnia,Hossein Mirzaei,Vahid Rezvani,Erfan Khodaei

doi:10.5539/jas.v4n3p111

Abstract

The standardized FAO-56 Penman–Monteith (PM) equation requires a great number of input parameters that are difficult to record. The Hargreaves method is another approach. It has high accuracy, while it requires fewer input parameters. In the present study, the Hargreaves-Samani method was calibrated by the PM equation under different climate conditions, in the western Iran. Different correction coefficients were used for each station under study instead of the constant coefficient 0.0023 in the Hargreaves equation based on monthly and annual scales. The corrected coefficients obtained for different regions can be used to estimate ETo for the PM method under the same climates where reliable data are available. The results also showed that as a larger study area with more weather stations is used and a larger amount of data is obtained, the coefficient of 0.0023 in the Hargreaves equation does not need further corrections for local calibration purposes.

Highlights

The estimation of evapotranspiration is the first and the most important step towards designing, planning and managing different irrigation networks, water distribution systems, water application, calculation of lake surface evaporation losses, implementation of hydrological and agro-hydrological models, environmental studies, water balance, and water management practices (Landras et al, 2008)
A comparison shows that the lowest monthly root mean square error (RMSE) index belonged to the Aleshtar Station, with dry sub-humid climate and with a value of 0.095 in December
The results showed that the RMSE values in warm months were higher than those in cold months of the year, causing differences between evapotranspiration values calculated by the Hargreaves and Penman-Monteith methods

Summary

Introduction

The estimation of evapotranspiration is the first and the most important step towards designing, planning and managing different irrigation networks, water distribution systems, water application, calculation of lake surface evaporation losses, implementation of hydrological and agro-hydrological models, environmental studies, water balance, and water management practices (Landras et al, 2008). In semiarid climates where water resources are www.ccsenet.org/jas limited and seriously overexploited, precise estimation of crop water requirements is needed for effective management and planning of available water resources (Jabloun and Sahli, 2008). The different methods applied to estimate evapotranspiration are categorized into two main groups, namely direct and indirect, or computational methods (Alizadeh and Kamali, 2007). By using different climate factors in an indirect method, the potential evapotranspiration is calculated, and by considering different crop coefficients at different crop growth stages, the desired plant evapotranspiration can be calculated (Allen et al, 1998; Savana and Frenken, 2002; Alizadeh and Kamali, 2007)

Objectives

Methods

Results

Conclusion