Equation [27] which is of the type:

fp(Z1, Z2, W) - Qp = 0

has to be solved. Z2 is calculated in the case when the offtake downstream condition varies:

Z2 = ZD + ( )2/3

or Z2 = ZD + (Zo-ZD) ( )1/n

One looks for the solution by bisection algorithm.

The initial value of W is taken equal to half of the authorized maximum opening. Calculation is stopped when the difference between the offtake discharge and the targeted discharge is less than 5%.