where yield is the grape yield (kg/vine), nShoot is the shoot number per vine, fBud is the bud fertility (bunch number per shoot),nBerry is the berry number per bunch and wBerry the mean berry weight (g). The abbreviations sim and obs refer to simulated and observed values, respectively. In this model, the yield components that are formed during season 1 are predicted by the relationships presented in Fig. 2.
As shoot number per vine mostly resulted from pruning practices, observed values for each year and treatment were used. Mean berry weight is the only component formed only during season 2;therefore the mean value over all years and treatments was used for each cultivar (Shiraz: wBerry = 1.62 g, Aranel: wBerry = 1.21 g) and the model did not account for variations in berry weight. Therefore, this model only accounted for yield variations due to variations of environmental factors during season 1. Comparison of yields simulated with Eq. (1) with observed yields are presented in Fig. 4. For both cultivars, more than 60% of the yield in season 2 was explained by the grapevine water and nitrogen status at flowering in season 1