COMPARISONOF SELECTED EVAPOTRANSPIRATION MODELS USING METEOROLOGICAL DATA TO IMPROVE THEIR PERFORMANCE FOR HYDROLOGICAL APPLICATIONS IN ETHIOPIA

Abstract

Evapotranspiration is an important process in the water cycle because it is responsible for 15% of the atmosphere's water vapor. Accurate quantification of ET is crucial in water allocation, irrigation management, evaluating the effects of changing land use on water yield, environmental assessment, and development of best management practices to protect the surface and groundwater quantity and quality. In this study, first one temperature based Temesgen-Melesse (TM) and one radiation based Abtew’s (Abt.) ET estimation method were selected and their results were compared with the standard FAO-56 Penman-Monteith method. Though both estimation methods gave reasonable results, the temperature-based model was selected for further analysis. Then the original temperature-based ET estimation method TM was modified and then calibrated using monthly averaged maximum temperature of 1-5 years, and daily (decadal-10 days) data of eight class-I meteorological stations of the study area. Then the results are calibrated and tested. An attempt was also made to estimate ‘n’ from the location latitude altitude and average of the monthly-averaged maximum temperature (T mx). The overall results have shown that the modified TM equation to be better than the original TM equation in terms of percent slope (0.8 - 12.3 against 1.3 – 15.1) and the correlation coefficient (R2) and the slope (100% good or satisfactory against 25%). The modified and calibrated equation gave the best results in terms of percent error by the slope (0.5 – 2.3), by a coefficient of efficiency (100% good or satisfactory), by R2 and slope (100% good or satisfactory) and by mean percent error (5.7 – 13.6%). Moreover, the results of this study revealed that calibration from the monthly-averaged data gave good ‘n’ values for all the stations with R2 values ranging from 0.80 to 0.92. There were no differences in the number of data points (12, 24, 36, 48 and 60 months) used. Calibration using the daily data gave satisfactory ‘n’ results for all the stations tested though the statistical parameters and performance tests did not give results that are comparable to the monthly-averaged values. xvi The results were nearly the same for 10, 20, or 30 days data points, which mean anyone of them, can be used for calibration purposes. Estimation of ‘n’ from latitude altitude and T mx gave results that are comparable to the ones with ‘n’ calibrated. The percent differences between the ‘n’ calibrated using data of 131 months and the ‘n’ values obtained from monthly data and daily data calibrations are less than 0.16% and 0.5%, respectively. The calculated ‘n’ showed a percent difference of less than 1.1%. Calculated ‘n’ can be considered as an option when PM data is not available for calibration. The study would benefit those who want to study long-term climate changes and drought patterns, which involve the use of evapotranspiration with limited data that satisfy the PM equation but have long-term data of temperature.