Daily reference crop evapotranspiration with reduced data sets in the humid environments of Azores islands using estimates of actual vapor pressure, solar radiation, and wind speed

Reference crop evapotranspiration (ET o ) estimations using the FAO Penman-Monteith equation (PM-ET o ) require a set of weather data including maximum and minimum air temperatures ( T max , T min ), actual vapor pressure ( e a ), solar radiation ( R s ), and wind speed ( u 2 ). However, those data are often not available, or data sets are incomplete due to missing values. A set of procedures were proposed in FAO56 (Allen et al. 1998 ) to overcome these limitations, and which accuracy for estimating daily ET o in the humid climate of Azores islands is assessed in this study. Results show that after locally and seasonally calibrating the temperature adjustment factor a d used for dew point temperature ( T dew ) computation from mean temperature, ET o estimations shown small bias and small RMSE ranging from 0.15 to 0.53 mm day −1 . When R s data are missing, their estimation from the temperature difference ( T max − T min ), using a locally and seasonal calibrated radiation adjustment coefficient ( k R s ), yielded highly accurate ET o estimates, with RMSE averaging 0.41 mm day −1 and ranging from 0.33 to 0.58 mm day −1 . If wind speed observations are missing, the use of the default u 2 = 2 m s −1 , or 3 m s −1 in case of weather measurements over clipped grass in airports, revealed appropriated even for the windy locations ( u 2 > 4 m s −1 ), with RMSE