Daily reference evapotranspiration estimates by the “Copais” approach

In a previous paper [Alexandris, S., Kerkides, P., 2003. New empirical formula for hourly estimations of reference evapotranspiration. Agric. Water Manage. 60, 181–198.] the Copais empirical function for estimating hourly reference evapotranspiration ETo using data of three attributes (solar radiation, temperature and relative humidity) was presented. In this work, a similar empirical equation for estimating daily ETo is proposed. Bilinear surface regression analysis is used to determine the equation coefficients. Results are compared with corresponding estimates obtained by the widely used physically sound methods of ASCE Penman–Monteith, CIMIS–Penman, FAO-56 Penman–Monteith and the empirical hourly Copais and daily Hargreaves–Samani models. Comparison shows that the new empirical function operates quite consistently and satisfactorily, providing reliable daily ETo estimates, slightly (on average by 3.5%) underestimating the summed hourly estimates by the combination equations, applied under three climatic regimes. Behaviour of the other empirical model (Hargreaves–Samani) is climate depending. The proposed empirical model may become a useful tool for routine daily reference evapotranspiration estimations.