Comparison of six evapotranspiration models for a surface irrigated maize agro-ecosystem in Northern Italy

The approaches for the estimation of evapotranspiration (ET) can be classified in “direct” methods, based on the original Penman–Monteith (PM) equation, in which the canopy resistance rc is modelled, and “indirect” methods, based on the preliminary calculation of ET for a well-watered reference grass (ETo) with a constant rc, which is then multiplied by a crop coefficient Kc to obtain ET. Even if the latter approaches are more widely adopted for their practical simplicity, many authors show that the former often provide better ET estimates in absence of calibration of crop parameters. In this study the performances of different direct and indirect methods were evaluated in the case of a surface irrigated maize grown in the Padana Plain (Northern Italy). The “one-layer” original PM equation with three different models for rc (Monteith, Jarvis, Katerji–Perrier), the “two-layers” PM model proposed by Shuttleworth and Wallace, the “single” and “double crop coefficient” models illustrated in the Paper FAO-56 were compared to latent heat fluxes measured in 2006 by eddy-covariance techniques. Results confirm that direct methods are more performing. The FAO-56 models with generalized crop coefficients overestimate ET, especially during the middle growth stage.