Date of this Version
Arellano, Maria G. (2013). Comparison of Reference (Potential) Evapotranspiration Equations and Long-term Average Frequency Distribution in Humid, Subhumid, Arid, Semiarid, and Mediterranean-Type Climates. MS thesis, University of Nebraska.
Grass-reference evapotranspiration (ETo) estimates were obtained using 12 temperature, radiation and combination-based methods in 5 locations in the United States with significantly different climatic characteristics (subhumid, semiarid, arid, humid and Mediterranean-type) in Nebraska (Clay Center and Scottsbluff), Florida (Gainesville), Arizona (Phoenix), and California (Davis). Daily, monthly and annual estimations were obtained and compared to the ETo values calculated using the Penman-Monteith method. Combination methods provided lower RMSD values in all of the locations due to the inclusion of several climate parameters. In general FAO24 Penman equation provided the lowest RMSD: 0.35, 0.42, 0.23, 0.37 and 0.37 mm for Davis, Gainesville, Phoenix Clay Center and Scottsbluff. Most temperature methods provided poor estimates in the humid location, as they do not include climate parameters that highly affect ETo under humid conditions (such as vapor pressure deficit or net radiation). Turc was the exception, providing good estimates in most of the locations. Most of the radiation methods underestimated in all the locations. Frequency distributions of grass- and alfalfa-reference evapotranspiration (ETo and ETr) were calculated and several differences were obtained between all the locations. ETo values were estimated for Davis, Phoenix, Gainesville, while ETr was selected for Clay Center and Scottsbluff. Gainesville ETo values were the lowest overall, due to its climatic characteristics: high humidity and low wind speed. The average values of ETo obtained during the peaks months were 9.3, 9.57, 4.6, 7.2 and 7.8 mm for Davis, Phoenix, Gainesville, Clay Center and Scottsbluff. The standard deviation values for those months were 1.3, 1.3, 1.18, 2.8 and 2.1 mm. The Nebraska locations presented higher variability of ETref due to their climatic characteristics.
Advisor Suat Irmak