\frac{q_O}{q_{O,\rm max}} = 1 - \sum_{k=1}^N a_k \, \left( \frac{p_{wf}}{p_r} \right)^k  \quad , \quad \{ p_b > p_r > p_{wf} \}, \quad \sum_{k=1}^N a_k =1, \quad \{ 0<a_k<1 \}_{k=1..N}