Validation of phenological models for grapevine in the Veneto region

In this study we have compared the predictive ability of two phenological models: a traditional Thermal Time (TT) and a version of the more recently develop Unified Model (UM). Unlike TT, which quantifies the accumulation of heat units which trigger bud break and the subsequent development phases, the UM describes also the fulfilment of chilling requirements, predicting the date of dormancy break, and implements a finer description of the plant development temperature-dependency. The models were fitted and validated on phenological observations collected from 1986 and 2008 in a site of North-Eastern Italy, on the cultivars Glera, Chardonnay, Merlot and Cabernet Sauvignon. The UM fitted better to observations than TT, and yielded more accurate estimates on the validation dataset. In both models, the accuracy of estimates decreased from bud break to veraison.