New empirical formula for hourly estimations of reference evapotranspiration

A new empirical equation for estimating hourly reference evapotranspiration ET₀ is proposed. This equation requires data for three pertinent meteorological attributes, solar radiation R_s, air temperature T, and relative humidity RH. A routine surface polynomial regression analysis in three stages, is employed in order to estimate the factors c_i entering the empirical model ET₀=f(R_s, RH, T; c_i). For the calibration of the proposed model, data sets collected from Copais (Greece) were used. Verification of the validity of the model was obtained using independent data from Copais as well as data from CIMIS (Davis, Sacramento, CA). A comparative evaluation of the model was performed against some of the most widely used and strongly recommended models for estimating hourly ET₀. Among them are the recently proposed Penman–Monteith (FAO56-Penman–Monteith), CIMIS version of Penman (CIMIS-Penman), and the American Society of Civil Engineers version of Penman–Monteith (ASCE-PM). Statistics and scatterplots, using ASCE-PM as the standard model indicate that the new empirical equation (Copais) operates quite satisfactorily for both regions, therefore may provide the premises to become a tool, for routine hourly reference evapotranspiration calculations.