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:

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

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

$h_{d}(t)=h_{c_{x,y}}+h_{v,t}(t)$

Finally, calculate the actual amount of water seeping into (or out of) the ground from the bottom.

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

Where:

• $\Delta s$ = The underground seepage which takes place, in meters.
• $\Delta t$ = Computational timestep in seconds.
• $h_{x,y}$ = Ground water head in the grid cell.
• $h_{d,t}$ = additional ground water level (or head) of the external zone causing the seepage at time t, in meters
• $h_{c_{x,y}}$ = constant ground water level (or head) of the external zone causing the seepage, in meters
• $h_{v,t}$ = additional variation of seepage head over time
• $c$ = resistance of the separating layer in days
• $ws$ = water storage fraction of the soil

Notes

• $h_{c_{x,y}}$ and $c_{x,y}$ can both be provided as spatially variable values.
• $h_{d,t}$ can be provided as a set of values, variable over time.

See also

• The formula is based on the formula of Pankow for estimating seepage
• Seepage result
• Seepage geotiffs
• Seepage fluctuation

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