Evaluation of Class A pan coefficients for estimating reference evapotranspiration in humid location

S. Irmak, D. Z. Haman, J. W. Jones

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Evaporation pans [Class A pan, U.S. Weather Bureau (USWB)] are used extensively throughout the world to measure free-water evaporation and to estimate reference evapotranspiration (ETo). However, reliable estimation of ETo using pan evaporation (Epan) depends on the accurate determination of pan coefficients (Kpan). Two equations developed by Frevert et al. in 1983 and Snyder in 1992 to estimate daily Kpan values were evaluated using a 23-year climate dataset in a humid location (Gainesville, Florida). The ETo data, calculated using daily Kpan values from these equations, were compared to the Food and Agricultural Organization (FAO)-Penman-Monteith (FAO56-PM) method. The two equations resulted in significantly different daily Kpan values that produced different daily, monthly, and annual total ETo estimates. The ETo values calculated using Frevert et al.'s 1983 Kpan coefficients were in very good agreement with the FAO56-PM method with daily, monthly, and annual mean percent errors (PE) of 5.8, 5.5, and 5.7%, respectively. The daily and annual mean-root-mean-square error (RMSE) of the estimates using this method were as low as 0.33 and 7.3 mm, respectively. Snyder's 1992 equation overestimated FAO56-PM ETo with daily, monthly, and annual mean PEs of 16.3, 13.8, and 13.2%, respectively. The daily and annual mean RMSEs for this method were higher (0.6 and 18 mm) than those obtained with Frevert et al.'s 1983 coefficients. The overestimations with Snyder's 1992 method were highest in the peak ETo month of May and in summer months. The performances of the Kpan equations were also evaluated using randomly selected individual years (1979, 1988, 1990, and 1994) of climate data that had different climate characteristics than the 23-year average dataset. Frevert et al.'s 1983 coefficients resulted in good ET0 estimates with lower annual mean PEs of 7.0, 0.1, 15.7, and 1.3% for 1979, 1988, 1990, and 1994, respectively, compared to Snyder's 1992 equation, which resulted in considerably higher PEs of 17.6, 9.1, 26.2, and 14.3% in 1979, 1988, 1990, and 1994, respectively. It was concluded that using Frevert et al.'s 1983 equation to calculate daily Kpan provided more accurate ETo estimates, relative to the FAO56-PM method, from Epan data compared to Snyder's 1992 equation under the humid-region climatic conditions in this study. The method is very useful in computer calculations of ETo since it does not require "table lookup" for Kpan values.

Original languageEnglish (US)
Pages (from-to)153-159
Number of pages7
JournalJournal of Irrigation and Drainage Engineering
Issue number3
StatePublished - May 2002

All Science Journal Classification (ASJC) codes

  • Civil and Structural Engineering
  • Water Science and Technology
  • Agricultural and Biological Sciences (miscellaneous)


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