Drainage formula (Water Overlay)

Drainage passive: First the flow capacity is calculated

$Q_{p,t}=\mathrm{\Delta }t\cdot q_t$

$Q_b=A_d\cdot f_s\cdot \frac{(w_g-w_w)\cdot A_w}{A_d\cdot f_s+A_w}$

if w_w > w_g then: $Q_{max}=A_w\cdot (w_w-z_w)$

else:

$Q_{max}=A_d\cdot (w_d-z_d)\cdot f_s$

$Q_d=min(Q_{p,t},Q_b,Q_{max})$

where:

$w_g$ is the ground water level in meters

$w_w$ is the water level in the waterway

$z_d$ is the drainage datum height in meters.

$A_d$ is the drainage area size in square meters.

$f_s$ is the average storage percentage of the ground above the drainage.

$Q_b$ is the amount transfered for a balanced ground and surface water level

$Q_{max}$ is the amount available in the ground above the drainage.

$Q_{p_t}$ is the drainage capacity at time t

$Q_d$ is the actual drainage at time t