where \(\text{fr}_{\text{LAImx}}\) is the fraction of the plant‘s maximum leaf area index (LAI) corresponding to a given fraction of potential heat units for the plant, \(\text{fr}_{\text{PHU}}\) is the fraction of potential heat units accumulated for the plant on a given day in the growing season, and \(l_{1}\) and \(l_{2}\) are shape coefficients.
The shape coefficients are calculated by solving Equation 4 and 5 using two known points (\(\text{fr}_{LAI,1}\), \(\ \text{fr}_{PHU,1}\)) and (\(\text{fr}_{LAI,2}\), \(\ \text{fr}_{PHU,2}\)):