Use of thermal units to estimate corn crop coefficients under semiarid climatic conditions

摘要

Two crop coefficient equations were derived as a function of fraction of thermal units from lysimeter measured corn evapotranspiration (ET sub(c-lys)) during 1997 and 1998, and reference evapotranspiration obtained from: (a) lysimeter measurements (Kc sub(mes)) or FAO Penman-Monteith (ET sub(o-PM)) estimates (Kc sub(est-PM)). For validation, corn evapotranspiration (ET sub(c-est)) was estimated in 2005 and 2006 from ET sub(o-PM) and: (a) the equation for Kc sub(mes) with (ET sub(c-est-lyslc)) or without (ET sub(c-est-lys)) locally calibrated ET sub(o-PM); (b) the equation for Kc sub(est-PM); and (c) the FAO approach (ET sub(c-est-FAO)). The ET sub(c-est_lys) estimates showed the lowest bias (0.09 mm day super(-1)); the ET sub(c-est-PM) and ET sub(c-est-FAO), the highest (0.50-0.51 mm day super(-1)). However, the root mean square error (RMSE, 1.23-1.27 mm day super(-1)) and the index of agreement (IA, around 0.94) of the ET sub(c-est-lys), ET sub(c-est-lyslc) and ET sub(c-est-PM) were similar. Therefore, ET sub(c-est-lys) is recommended although the ET sub(c-est-lyslc) was similarly accurate. The ET sub(c-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 ET sub(c-est-FAO) should be avoided as RMSE (1.35 mm day super(-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.