Daily Grass and Alfalfa-Reference Evapotranspiration Estimates and Alfalfa-to-Grass Evapotranspiration Ratios in Florida

Efficient use of natural water resources in agriculture is becoming an important issue in Florida because of the rapid depletion of freshwater resources due to the increasing trend of industrial development and population. Reliable and consistent estimates of evapotranspiration (ET) are a key element of managing water resources efficiently. Since the 1940s numerous grass- and alfalfa-reference evapotranspiration $({\mathrm{ET}}_o$ and ${\mathrm{ET}}_r\text{,}$ respectively) equations have been developed and used by researchers and decision makers, resulting in confusion as to which equation to select as the most accurate reference ET estimates. Twenty-one ${\mathrm{ET}}_o$ and ${\mathrm{ET}}_r$ methods were evaluated based on their daily performance in a humid climate. The Food and Agriculture Organization Penman-Monteith (FAO56-PM) equation was used as the basis for comparison for the other methods. Measured and carefully screened daily climate data during a 23-year period (1978–2000) were used for method performance analyses, in which the methods were ranked based on the standard error of estimate (SEE) on a daily basis. In addition, the performance of the four alfalfa-based ET $\left({\mathrm{ET}}_r\right)$ equations and the ratio of alfalfa ET to grass ET $(K_r$ values) were evaluated, which have not been studied before in Florida’s humid climatic conditions. The peak month ${\mathrm{ET}}_o$ estimates by each method were also evaluated. All methods produced significantly different ${\mathrm{ET}}_o$ estimates than the FAO56-PM method. The 1948 Penman method estimates were closest to the FAO56-PM method on a daily basis throughout the year, with the daily SEE averaging 0.11 mm⋅d⁻¹; thus this method was ranked the second best overall. Although 1963 Penman (with the original wind function) slightly overestimated ET, especially at high ${\mathrm{ET}}_o$ rates, it provided remarkably good estimates as well and ranked as the third best method, with a daily average SEE value of 0.14 mm⋅d⁻¹. Both methods produced peak month ${\mathrm{ET}}_o$ estimates closest to the FAO56-PM method among all methods evaluated, with daily peak month SEEs averaging 0.07 and 0.09 mm⋅d⁻¹, respectively. Significant variations were observed in terms of the performance of the various forms of Penman’s equations. For example, the original Penman-Monteith method produced the poorest ${\mathrm{ET}}_o$ estimates among the combination equations, with a daily SEE for all months and peak month averaging 0.50 and 0.35 mm⋅d⁻¹, respectively and ranked 11th. An average value of 1.18 was used to convert ${\mathrm{ET}}_r$ estimates to ${\mathrm{ET}}_o$ values for alfalfa-reference methods. The $K_r$ value of 1.18 resulted in reasonable estimates of ${\mathrm{ET}}_o$ throughout the year by the Kimberley forms of the Penman equations. Another ${\mathrm{ET}}_r$-based equation, Jensen-Haise, gave consistently poor estimates. The Stephens-Stewart radiation method was the highest-ranked (10th) noncombination method overall. The temperature-based McCloud method (ranked 19th) produced the poorest ${\mathrm{ET}}_o$ estimates among all methods with a daily SEE for all months and for the peak month averaging 1.93 and 1.22 mm⋅d⁻¹, respectively. In general, the results obtained from the temperature methods suggest that all of the temperature methods, with the possible exception of the Turc method, can only be applicable for these climatic conditions after they are calibrated or modified locally or regionally. The FAO and Christiansen pan evaporation methods (ranked 17th and 18th, respectively) produced poor ${\mathrm{ET}}_o$ estimates and had the largest amount of point scatter in daily ${\mathrm{ET}}_o$ estimates relative to the FAO56-PM ${\mathrm{ET}}_o\text{.}$ Both methods resulted in the highest daily SEE of 1.18 and 1.19 mm⋅d⁻¹ for all months, after the McCloud method (1.93 mm⋅d⁻¹), and with the highest SEE of 1.30 and 1.24 mm⋅d⁻¹ for the peak month of all methods evaluated. The FAO56-PM method uses solar radiation, wind speed, relative humidity, and minimum and maximum air temperature to estimate ${\mathrm{ET}}_o\text{.}$ It has been recommended that the FAO56-PM be used for estimating ${\mathrm{ET}}_o$ when all the necessary input parameters are available. However, all these input variables may not be available, or some of them may not be reliable for a given location if the FAO56-PM equation is used, and one may need to choose other temperature, radiation, or pan evaporation methods based on the availability of data for estimating ${\mathrm{ET}}_o\text{.}$ The results of this study can be used as a reference tool to provide practical information on which method to select based on the availability of data for reliable and consistent estimates of daily ${\mathrm{ET}}_o$ relative to the FAO56-PM method in a humid climate.