\frac{q_O}{q_{\rm max}} = 1 - a \, \frac{p_{wf}}{p_r} - (1-a) \Bigg(\frac{p_{wf}}{p_r} \Bigg)^2  \quad , \quad p_b > p_r > p_{wf}, \quad \{ 0<a<1 \}