HOURLY GRASS EVAPOTRANSPIRATION IN MODIFIED MARITIME ENVIRONMENT

The Penman method using hourly and daily weather data was evaluated to estimate daily reference evapotranspiration over a well-established s tand of Alta fescue under different atmospheric conditions. The perfor mance of the Penman equation was evaluated using evapotranspiration me asured by the Bowen ratio energy balance (BREB) method. Good agreement between the BREB method and Penman using hourly weather data was obse rved. On an hourly basis, the overall standard error of estimate (SEE) and absolute relative error (ARE) were 0.06 mm . h(-1) (40.8 W . m(-2 )) and 5.2%, respectively. On a daily basis, the SEE was 0.55 mm . d(- 1) and the ARE was 6.9%. The SEE and ARE for the Penman model using da ily weather data were 0.74 mm . d(-1) and 9.8%, respectively. These re sults suggest that the accuracy of the Penman equation could improve i f hourly weather data were used as inputs and strongly suggest that th e Penman equation with hourly weather data could be used in the Willam ette Valley to predict daily evapotranspiration for irrigation purpose s.