EVALUATION AND ENHANCING THE ACCURACY OF SOME METHODS FOR COMPUTING REFERENCE EVAPOTRANSPIRATION IN EAST DELTA

Ten different methods for computing the reference evapotranspiration ET0 have been selected for evaluation in East Delta of Egypt. Some of these methods were recommended by FAO or ASCE and the others are widely used. The evaluation was carried out by comparing daily computed ET0 values with the measured lysimeter values. After evaluation of the ten methods, four methods were selected to improve their accuracy. Improving the accuracy of each method was carried out through two attempts. The first attempt uses regression coefficients to remove any overestimation or underestimation from the calculated ET0 values. In the second approach the sensitivity of the ET0 estimation methods to changes in the climatic parameter were analyzed. This is done to find the most effective governing parameter and the required adjustment factors for each parameter. Data of Inshas agro-meteorological station, located in Sharkia Governorate, were used. The study shows that using the regression coefficient for enhancement is not recommended. Significant improvements of accuracy can be obtained by applying the resulting adjustment factors, as obtained from the sensitivity analysis. The FAO Penman-Monteith method with the measured value of the solar radiation multiplied with 1.5 is recommended to improve the accuracy by 20% in East Delta.