Fraction of canopy intercepted radiation relates differently with crop coefficient depending on the season and the fruit tree species

It is commonly assumed that the fraction of canopy intercepted radiation (fIR) should be well correlated with irrigation crop coefficients (Kc) throughout the season. However, in fruit trees there is some evidence that such a correlation is different between pre-harvest and postharvest periods. Over two different years, basal Kc (Kcb) data from three different weighing lysimeters (one in California growing peach trees, and the other two in Catalonia growing apple and pear trees) were analyzed using two parameters of the CropSyst growth model: full canopy Kc (Kcfc) and maximum plant hydraulic conductance (Cmax). In CropSyst, Kcb is approximated as fIR times Kcfc. The latter is usually seasonally fixed, but for this study it was made variable so that it could be adjusted every fortnight throughout the season. Variable Kcfc implies the possibility that the Kcb relationship with fIR is not constant. The objective of this study was to evaluate possible seasonal patterns in the Kcfc. The results indicated that Kcfc was variable in all species and it followed a distinctive pattern in three different time periods: (i) initial rise (spring), (ii) plateau or slight decline (mid-summer), and (iii) decline (autumn). However, the magnitude of Kcfc fluctuation was different among the three species. It fluctuated the most in the slowest growing species (pear), and the least in the fastest growing species (peach). Apple had an intermediate response. In conclusion, Kc is not a fixed function of fIR. Assumption of a fixed function will introduce errors in plant water use estimation, which could be especially large in pears and apples. This will be by 50% in pears during postharvest.