The definitive editor version is available at: http://link.springer.com/journal/271 http://www.springerlink.com/content/h7270424k3j76459/Two crop coefficient equations were derived as a function of fraction of thermal units from lysimeter measured corn evapotranspiration (ETc-lys) during 1997 and 1998, and reference evapotranspiration obtained from: a) lysimeter measurements (Kcmes) or FAO Penman-Monteith (ETo-PM) estimates (Kcest-PM). For validation, corn evapotranspiration (ETc-est) was estimated in 2005 and 2006 from ETo-PM and: a) the equation for Kcmes with (ETc-est-lyslc) or without (ETc-est-lys) locally calibrated ETo-PM; b) the equation for Kcest-PM; and c) the FAO approach (ETc-est-FAO). The ETc-est_lys estimates showed the lowest bias (0.09 mm day-1); the ETc-est-PM and ETc-est-FAO, the highest (0.50-0.51 mm day-1). However, the root mean square error (RMSE, 1.23-1.27 mm day-1) and the index of agreement (IA, around 0.94) of the ETc-est-lys, ETc-est-lyslc and ETc-est-PM were similar. Therefore, ETc-est-lys is recommended although the ETc-est-lyslc was almost as accurate. The ETc-est-PM is less recommended due to poorer bias and systematic mean square error, and a general underestimation except for low corn ET values. For real time irrigation scheduling, the ETc-est-FAO should be avoided as RMSE (1.35 mm day-1), IA (0.93) and bias were slightly worse, corn ET was overestimated but for high values, and the length of the four phenological stages must be known in advance.This research has been funded through projects HID96-1295-C04-04 and AGL-2004-06675-CO3-02 (Spanish Ministry of Education), and PIP090/2005 (Regional Government of Aragón).Peer reviewed