ECALTOOL: fuzzy logic based computer program to calibrate the Hargreaves equation for accurate estimation of evapotranspiration

Abstract

A reference evapotranspiration rate (ET0) is vital information for irrigation scheduling as well as plant growth modeling.  Estimation of evapotranspiration using environmental variables is a convenient approach compare to direct measurement.  Several mathematical models have been proposed for the estimation of ET0.  Among all these, the Hargreaves equation has received maximum attention of the agricultural fraternity as it needs minimum weather data.  However, requirement of minimal weather data in the Hargreaves equation makes it a simple and pragmatic model for estimation of ET0.  The minimalist approach makes the equation incapable to estimate ET0 accurately under extreme weather conditions.  The calibration or adjustment of the Hargreaves equation parameter CH and EH for different climate conditions is well accepted approach to accomplish error free estimation from the equation.  The establish calibration methods are empirical and time consuming.  Further, the results obtained by these methods are valid only for the restricted area and season only.  This paper presents Evapotranspiration CALibration TOOL (ECALTOOL), which is a fuzzy based universal computer program developed on the platform of NI LabVIEW to calibrate CH and EH.  The key features of the tool are its ability to provide calibration of ET0 for 1100 locations of 190 countries, and its user friendliness.  The performance of the tool is compared and validated against the benchmark Penman-Monteith equation as well as the experimentally calibrated values for various locations with diverse climate conditions.  Mean Absolute Error (MAE) and Mean Absolute Percentage Error (MAPE) is in the range of 0.4856 – 1.1562 and 1.21%– 2.06% respectively.  The calibrated values of CH and EH are proved to be accurate in comparison with experimentally carried out calibration.  The developed tool eliminates the need of location specific experimental calibration process for the Hargreaves equation.
Keywords: evapotranspiration, fuzzy logic, Hargreaves equation, LabVIEW