COMPARISON OF SOME EMPIRICAL EQUATIONS FOR ESTIMATING THE REFERENCE EVAPOTRANSPIRATION USING METEOROLOGICAL DATA IN THI-QAR  PROVINCE

H. Al-Askri, T.S.; Naduwi, D.R.; Abdulah, M.A.

	COMPARISON OF SOME EMPIRICAL EQUATIONS FOR ESTIMATING THE REFERENCE EVAPOTRANSPIRATION USING METEOROLOGICAL DATA IN THI-QAR PROVINCE
IRAQ JOURNAL OF AGRICULTURE
Article 1, Volume 19, Issue 3, 2014, Pages 0-0
Authors
T.S. H. Al-Askri; D.R. Naduwi; M.A. Abdulah
Abstract
In this study, six empirical equations were selected to estimated the reference evapotranspiration, that belonged to three categories, three temperature-based methods and three radiation-based methods under Thi-Qar Province conditions using meteorological data for 2012 of Nasiriya meteorological station and FAO Penman-Montaith (1998) which was used as reference equation. The evaluation and comparison were made based on both the original constant values involved in each equation and the recalibrated constant values. The results showed that all equations had gave underestimations of ETo, A comparative study also shows that modifications were made by substituting recalibrated constant values for the original constant values had improved efficiency of the six equations to simulate the values of ETo were more accuracy than original equations. The statistical criteria; root mean square Error (RMSE), mean absolute error (MAE) and the coefficient of residual mass (CRM) revealed that the confidence became high in modified equations due to reducing the values of these criterions. The study of regression analysis illustrated it is necessary to make changes to the original equations when they were extrapolated to other climatic areas. The modifications achieved positive change concerning the R2 and r values for the six equations, moreover the slope of the regression equations improved in the six cases. the equation modifications obtained higher values of the index of concordance (D) ranged from 0.987 for PT equation and 0.999 for BC & HS equations, For this reason, it also obtained high values of the confidence index (C) which ranged between 0.978 for M equation to 0.998 for BC & HS equations The BC equation without adjustment indicates that the best model to estimate ETo of other equations, the formulae modification of The BC equation become more able to estimate the values of the mean monthly evapotranspiration are very close to those estimated by PM.

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