Reference (Potential) Evapotranspiration. I: Comparison of Temperature, Radiation, and Combination-Based Energy Balance Equations in Humid, Subhumid, Arid, Semiarid, and Mediterranean-Type Climates

Abstract

Grass-reference (potential) evapotranspiration (ETo) is often used in climate change studies to evaluate potential impacts of climate change on hydrologic balances and vegetation response. In this process, empirical temperature or radiation-based empirical equations are often used because of a lack of long-term climate data to solve the combination-based energy balance equations, but this may result in false determinations of trends and magnitudes of water use. To quantify potential differences associated with using such empirical models with respect to the FAO56 Penman-Monteith method (PM), various ETo models were evaluated in five locations that have significantly different climatic characteristics [subhumid, semiarid, arid, humid (subtropical), and Mediterranean-type] in Nebraska (Clay Center and Scottsbluff), Florida (Gainesville), Arizona (Phoenix), and California (Davis). In general, the performance of most methods not only varied with climatic conditions, but also with the time step used (i.e., daily, monthly, and long-term cumulative basis). Combination methods provided lower root mean squared difference (RMSD) in all locations due to accounting for aerodynamic and energy terms of the surface energy balance. On a daily time step, the FAO24 Penman equation provided the lowest RMSD values (0.35, 0.42, 0.23, 0.37, and 0.37 mm/d for Davis, Gainesville, Phoenix Clay Center, and Scottsbluff, respectively). In the most arid location (Phoenix) all combination methods overestimated ETo relative to the FAO56-PM, whereas the FAO24 Penman method performed best in Phoenix as well as in humid (Gainesville) and windy (Clay Center) locations. However, at wind speed values higher than 4 m/s, FAO24 Penman estimates were up to 40% higher than the FAO56-PM estimates. In general, 1948 Penman provided higher overestimations when compared to FAO56-PM for days with solar radiation values below 10 MJ/m2/d. Among temperature methods, Hargreaves method positioned first among the temperature methods in three of five locations and its RMSD of estimates in driest and most humid climates (Phoenix and Gainesville) were smallest. Most of the radiation methods underestimated in all the locations; with the exception of Gainesville, where the only method that presented this behavior was Makkink. Substantial differences were observed when comparing methods’ cumulative annual ETo to the FAO56-PM values. The performance of some of the methods differed substantially with location. For example, the FAO24 Radiation method ranked first in Davis, Clay Center, and Scottsbluff. In the extremely dry and windy location of Scottsbluff it also ranked first overall in the cumulative analysis, and second at Clay Center. This method performed poorly in Phoenix, where it ranked last on daily analysis. The results of this study can provide a reference in terms of potential errors associated with using various combination, temperature, and radiation-based empirical models in estimating ETo with respect to the FAO56-PM in various climatic conditions for different time steps.