Estimation of crop reference evapotranspiration fro two locatioons (Coastal and Andean Region) of Ecuador

In two locations of Ecuador with similar latitude but different altitude (Portoviejo, Coastal zone and Riobamba, Andean zone) the Holdrigde (Ho) and Hargreaves-Samani (H-S) models and three modifications of these (daily, monthly and annual with constants CHO and KE and KT, respectively) were fitted in order to estimate crop reference evapotranspiration (ETo) for a range of moving averages in time, for a three day period. Once CHO, KE and KT were calibrated those ETo estimates were compared with ETo estimated by the Pan Evaporation Method (ETo Pan). Among CHO (Ho) and KE (H-S) coefficients, the best performance was achieved relating CHO and KE to vapor pressure deficit, since it corrected the vertical thermal gradient and increased the precision in ETo prediction, while relating the KT (H-S) coefficient to air temperature allowed the correction of the solar radiation. When comparing the three modifications of both models with ETo Pan, the daily CHO model and the monthly H-S model generated slightly better ETo estimates in Portoviejo and Riobamba, respectively. Finally, in terms of quality of ETo estimates, any of the three modifications of both models were better than those with Penman-Monteith in both locations.