Evaluation of three semi-empirical approaches to estimate the net radiation over a drip-irrigated olive orchard

Abstract

The use of actual evapotranspiration (ETa) models requires an appropriate parameterization of the available energy, where the net radiation (R-n) is the most important component. Thus, a study was carried out to calibrate and evaluate three semi-empirical approaches to estimate net radiation (R-n) over a drip-irrigated olive (Olea europaea L. 'Arbequina') orchard during 2009/2010 and 2010/2011 seasons. The orchard was planted in 2005 at high density in the Pencahue Valley, Maule Region, Chile. The evaluated models were calculated using the balance between long and short wave radiation. To achieve this objective it was assumed that T-s = T-a for Model 1, T-s = T-cv for Model 2 and T-s = T-r for Model 3 (T-s is surface temperature; Ta is air temperature; and Tcv is temperature inside of the tree canopy; T-r is radiometric temperature). For the three models, the Brutsaert's empirical coefficient (phi) was calibrated using incoming long wave radiation equation with the database of 2009/2010 season. Thus, the calibration indicated that phi was equal to 1.75. Using the database from 2010/2011 season, the validation indicated that the three models were able to predict the R-n at a 30-min interval with errors lower than 6%, root mean square error (RMSE) between 26 and 39 W m(-2) and mean absolute error (MAE) between 20 and 31 W m(-2). On daily time intervals, validation indicated that models presented errors, RMSE and MAE between 2% and 3%, 1.22-1.54 and 1.04-1.35 MJ m(-2) d(-1), respectively. The three R-n-Models would be evaluated and used in others Mediterranean conditions according to the availability of data to estimate net radiation over a drip-irrigated olive orchard planted at high density.