Ground bottom flow formula (Water Overlay)

Underground seepage (from (or to) outside the project area) is calculated per cell.

The seepage is calculated using the following formula:

$\Delta s=\frac{h_d(t)-h_{x,y}}{c}\cdot \frac{\Delta t}{24\cdot 60\cdot 60}$

The external seepage head is variable over time by the introduction of h_v,t

$h_d(t)=h_c+h_{v,t}(t)$

Finally, calculate the actual amount of water infiltrating.

$\Delta w=\frac{\Delta s}{ws}$

Where:

• Δs = The underground seepage which takes place, in meters.
• Δt = Computational timestep in seconds.
• h_d = ground water level (or head) of the external zone causing the seepage, in meters
• h_{v> = additional variation in head of hd over time}
• c = resistance of the separating layer in days
• ws = soil water storage fraction

• H_unsat = The height of the unsaturated zone.
• S = Ratio of water to height in the unsaturated zone.
• C_inf = The height in the unsaturated zone which can be subject to infiltration to the saturated zone.

• W_unset = The amount of water in the unsaturated zone. The height of the water column if the equivalent amount of water was placed on the surface.
• WL_underground = The groundwater level, relative to datum.
• H_surface = The terrain height in the cell, relative to datum.
• I_und = The GROUND_INFILTRATION_MD of the underground terrain type.

Related

The following topics are related to this formula.

Formulas

Groundwater level formula

Surface infiltration formula

Models

Underground model

Infiltration model

Substance flow model