Usefulness of class A Pan coefficient models for computation of reference evapotranspiration for a semi-arid region

The reliability of estimates of reference evapotranspiration (ETo) using pan evaporation (E-pan) depends on the accurate determination of pan coefficients (K-pan). Six ETo models were evaluated for their usefulness using 33-year climatological dataset of a semi-arid region of the Gujarat state of India. The equations compared include Cuenca (1989), Allen and Pruitt (1991), Snyder (1992), Modified Snyder (Grismer et al., 2002), Orang (1998), and Pereira et al. (1995). The ET data, calculated using daily K-pan values from these equations, were compared to the Food and Agricultural Organization (FAO)-Penman-Monteith (FA056-PM) method as a reference. Based on the visual comparison as well as from the statistical criteria, ETo values computed using Modified Snyder and Orang model have very close agreement with the FA056-PM method for daily, monthly, and annual estimates as compared to other approaches. The sequential performances of the explored models was found as: Modified Snyder (Eqn. 5) > Orang (Eqn. 6) > Cuenca Eqn. (2) > Allen & Pruitt (Eqn. 3) > Snyder (Eqn. 4) > Pereira et al. (Eqn. 7) model. Therefore, the Modified Snyder model (Grismer et al., 2002) could be recommended as the best model for ETo computations under these prevailing climatic conditions for a semi arid region.