Ortega-Farias, S.O. Facultad de Ciencias Agrarias, Escuela de Agronomia, Universidad de Talca, 747 Casilla, Talca, Chile.The Penman method using hourly and daily weather data was evaluated to estimate daily reference evapotranspiration over a well-established stand of Alta fescue under different atmospheric conditions. The performance of the Penman equation was evaluated using evapotranspiration measured by the Bowen ratio energy balance (BREB) method. Good agreement between the BREB method and Penman using hourly weather data was observed. On an hourly basis, the overall standard error of estimate (SEE) and absolute relative error (ARE) were 0.06 mms.h -1 (40.8 Ws.m -2 ) and 5.2%, respectively. On a daily basis, the SEE was 0.55 mms.d -1 and the ARE was 6.9%. The SEE and ARE for the Penman model using daily weather data were 0.74 mms.d -1 and 9.8%, respectively. These results suggest that the accuracy of the Penman equation could improve if hourly weather data were used as inputs and strongly suggest that the Penman equation with hourly weather data could be used in the Willamette Valley to predict daily evapotranspiration for irrigation purposes.