# 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:

${\bar {I}}_{top}={\begin{cases}{\bar {I}}_{con},&{\text{if constructions are present on top of the surface}}\\{\bar {I}}_{terrain},&{\text{otherwise}}\\\end{cases}}$ ${\bar {I}}_{surface}=min({\bar {I}}_{ground},{\bar {I}}_{top})$ Then the actual infiltration:

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

${\bar {I}}_{con}$ = The average infiltration capacity based on constructions on top of the surface, based on GROUND_INFILTRATION_MD defined in either the building or function.
${\bar {I}}_{terrain}$ = 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.
${\bar {I}}_{top}$ = The average infiltration capacity on top of the surface.
${\bar {I}}_{ground}$ = The average infiltration capacity below the surface.
${\bar {I}}_{surface}$ = The infiltration capacity from surface to the unsaturated zone.
$\Delta w_{t}$ = The surface infiltration which takes place at time $t$ .
$h_{t}$ = The amount of water on the surface at time $t$ . Also known as the water column.
$\Delta t$ = Computational timestep.
$f_{i,t}$ = The interpolated infiltration factor at time $t$ . Configured using INFILTRATION_FACTOR_S.

## 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