Comparison of several reference evapotranspiration methods for Itoshima Peninsula area

Six reference evapotranspiration (ETo) methods are compared, based on their daily performances under the given climatic condition in the western region of Fukuoka City. The Penman-Monteith equation standardized by the Food and Agriculture Organization (FAO56-PM) is used to compare with the Thornthwaite, Hargreaves, and Hamon methods. The FAO56-PM method has also been used as an index for two equations: the solar radiation (Rs) based and net radiation (Rn) based equations, which are the simplified versions of FAO56-PM. Performance analysis for the calculated values using meteorological data for 7 years (Jan 1996 – Dec 2002) was made. The standard error of estimates (SEE) was calculated on monthly basis. The estimated values by the Thornthwaite, Hargreaves, Hamon, and the Rs-based and Rn-based radiation methods were all correlated with FAO56-PM having correlation (r2) values calculated by Pearson’s correlation of 0.80, 0.69, 0.48, 0.71 and 0.45 and their standard error estimates were 0.25, 0.25, 0.25, 0.28 and 0.24 mm mon-1, respectively. Proximity in ETo estimates by the Hargreaves and the Rs-based methods showed that the solar radiation as input variable wa