Evaluation of an Hourly Empirical Method Against ASCE PM (2005), for Hyper Arid to Subhumid Climatic Conditions of the State of California

Accurate estimations of ETo is critical for hydrologic studies, efficient crop irrigation, water resources management and sustainable development. The evaluation of an empirical method for estimating hourly ETo, utilizing the incoming solar radiation and a relation between a function of relative humidity and vapor pressure deficit as an aerodynamic term was calibrated under semi arid conditions, and assessed against the ASCE PM (2005) method for hyper arid to sub humid climatic regimes in the State of California. For hyper arid climatic conditions, the empirical method underestimated and had R2 values from 0.88 to 0.95 and RMSE values from 0.062 to 0.115 mm h-1. Hyper arid climatic conditions, correspond to lower R2 and different relations between the VPD and the inverse of the natural logarithm of relative humidity (1/lnRH). For arid semi arid and sub humid climatic conditions the empirical method performed satisfactorily. The RMSE was calculated for various intervals of the range of the observed wind speed values and it was satisfactory for >99% of all data. The RMSE was also calculated for various intervals of the observed values of VPD and was satisfactory for >97% of all data except for hyper arid stations (59% of u2 and 60% of VPD data).