ORIGINAL PAPER Calibration and validation of a remote sensing algorithm to estimate energy balance components and daily actual evapotranspiration over a drip-irrigated Merlot vineyard Carlos Poblete-Echeverrı ´a • Samuel Ortega-Farias Received: 14 August 2011 / Accepted: 12 April 2012 Ó Springer-Verlag 2012 Abstract A study was carried out to calibrate and validate a remote sensing algorithm (RSA) for estimating instantaneous surface energy balance components and daily actual evapotranspiration (ETa) over a drip-irrigated Merlot vineyard located in the Maule Region of Chile (35° 25 0 LS; 71° 32 0 LW; 125 m.a.s.l.). ETa was estimated as a function of instantaneous evaporative fraction and average daily net radiation (Rn day ) using meteorological variables in combination with reflectance data measured by a hand- held multi-spectral radiometer. The sub-models used to estimate the instantaneous net radiation (Rn ins ), soil heat flux (G ins ), and Rn day were calibrated and validated using measurements of the surface energy balance components, incoming longwave radiation ðL # ins Þ, outgoing longwave radiation ðL " ins Þ, and surface albedo (a). The validations of instantaneous sensible heat flux (H ins ), latent heat flux (LE ins ), and ETa were carried out using turbulent energy fluxes obtained from an eddy correlation (EC) system. For reducing the moderate EC imbalance (about 11 %), tur- bulent energy fluxes were recalculated using the Bowen ratio method. The validation analysis indicated that the calibrated sub-models of the RSA were able to estimate Rn ins , G ins , H ins , and LE ins with a root-mean-square error (RMSE), mean absolute error (MAE), and index of agreement (IA) ranging between 16–54, 13–44 W m -2 , and 0.72–94, respectively. Also, the RSA was able to estimate ETa with RMSE = 0.38 mm day -1 , MAE = 0.32 mm day -1 and IA = 0.96. These results demonstrate the potential use of reflectance and meteorological data to estimate ETa of a drip-irrigated Merlot vineyard. Introduction Irrigation scheduling is a critical aspect of successful grape production and wine quality (Sivilotti et al. 2005; Shellie 2006; Intrigliolo and Castel 2008). In order to obtain optimal irrigation scheduling, it is necessary to have a reliable method to quantify the water consumption of the plants, that is, daily actual evapotranspiration (ETa). For this purpose, the use of remote sensing techniques to esti- mate ETa is becoming an important tool for studying water consumption on a field and a regional scale (Allen et al. 2007; Gowda et al. 2008). Remote sensing energy balance methods provide instantaneous estimations of evapotranspiration (ET ins ), which are then used in the estimation of ETa (Chavez et al. 2008). Numerous remote sensing algorithms that use the principle of surface energy balance for estimating ETa have been developed in order to make use of remote data acquired by sensors on airborne and satellite platforms, for example: SEBI (Surface Energy Balance Index) (Menenti and Choudhury 1993), TSM (Two-source model) (Norman et al. 1995), SEBAL (Surface Energy Balance Algorithm for Land) (Bastiaanssen 1995, 2000; Bastiaanssen et al. 1998a), S-SEBI (Simplified Surface Energy Balance Index) (Roerink et al. 2000), SEBS (Surface Energy Balance System) (Su 2002), METRIC (Mapping Evapotranspiration at high Resolutions with Internal Calibration) (Tasumi 2003; Allen et al. 2007), MEBES (Surface Energy Balance to Measure Evapotranspiration) (Ramos et al. 2009) and ReSET (Remote Sensing of ET) (Elhaddad and Garcia Communicated by E. Fereres. C. Poblete-Echeverrı ´a  S. Ortega-Farias (&) Research and Extension Center for Irrigation and Agroclimatology (CITRA), Universidad de Talca, Talca, Chile e-mail: sortega@utalca.cl 123 Irrig Sci DOI 10.1007/s00271-012-0381-x