# Surface infiltration formula (Water Overlay)

Surface infiltration is the amount of water that infiltrates from the surface to the unsaturated zone below the surface.

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

${\displaystyle {\bar {I}}_{top}={\begin{cases}{\bar {I}}_{building},&{\text{if building are present on top of the surface}}\\{\bar {I}}_{terrain},&{\text{otherwise}}\\\end{cases}}}$
${\displaystyle {\bar {I}}_{surface}=min({\bar {I}}_{ground},{\bar {I}}_{top})}$

Then the actual infiltration:

${\displaystyle \Delta w_{t}=min(h_{t},{\bar {I}}_{surface}\cdot \Delta t\cdot f_{i,t})}$

Where:

${\displaystyle {\bar {I}}_{building}}$ = The average infiltration capacity based on buildings 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.
${\displaystyle \Delta w_{t}}$ = The surface infiltration which takes place at time ${\displaystyle t}$.
${\displaystyle h_{t}}$ = The amount of water on the surface at time ${\displaystyle t}$. Also known as the water column.
${\displaystyle \Delta t}$ = Computational timestep.
${\displaystyle f_{i,t}}$ = The interpolated infiltration factor at time ${\displaystyle t}$. Configured using INFILTRATION_FACTOR_S.

## Related

The following topics are related to this formula.

Formulas
Surface water level formula (Water Overlay)
Groundwater level formula (Water Overlay)
Ground infiltration formula (Water Overlay)
Models
Surface model (Water Overlay)
Ground model (Water Overlay)
Infiltration model (Water Overlay)
Tracer flow model