# Surface infiltration formula (Water Overlay)

Surface infiltration is calculated per cell. First, the infiltration capacities are calculated:

${\displaystyle {\bar {I}}_{top}={\begin{cases}{\bar {I}}_{con},&{\text{if constructions are present on top of the surface}}\\{\bar {I}}_{terrain},&{\text{otherwise}}\\\end{cases}}}$

${\displaystyle {\bar {I}}_{surface}=min({\bar {I}}_{ground},{\bar {I}}_{top})}$

Then the actual infiltration:

${\displaystyle \Delta w=min(h,\Delta t\cdot {\bar {I}}_{surface})}$

Where:

${\displaystyle {\bar {I}}_{con}}$ = The average infiltration capacity based on constructions on top of the surface, based on GROUND_INFILTRATION_MD defined in either the building or function.
${\displaystyle {\bar {I}}_{terrain}}$ = The average infiltration capacity based on the surface terrain, defined by GROUND_INFILTRATION_MD. This value should be interpreted as the vertical conductivity (Kv) of the sub-soil.
${\displaystyle {\bar {I}}_{top}}$ = The average infiltration capacity on top of the surface.
${\displaystyle {\bar {I}}_{ground}}$ = The average infiltration capacity below the surface.
${\displaystyle {\bar {I}}_{surface}}$ = The infiltration capacity from surface to the unsaturated zone.
${\displaystyle \Delta w}$ = The surface infiltration which takes place.
${\displaystyle h}$ = The amount of water on the surface. Also known as the water column.
${\displaystyle \Delta t}$ = Computational timestep.

## Related

The following topics are related to this formula.

Formulas
Surface water level formula
Groundwater level formula
Underground infiltration formula
Models
Surface model
Underground model
Infiltration model
Tracer flow model