Infiltration model (Water Overlay)
The Water Module supports two types of infiltration; surface to unsaturated zone and unsaturated to saturated zone. Furthermore, exfiltration can also occur due to horizontal groundwater flow.
Surface water can infiltrate into the underground unsaturated layer. When it does, it infiltrates at a speed defined by the surface terrain's GROUND_INFILTRATION_MD attribute, the underground terrain's GROUND_INFILTRATION_MD attribute, or (if present) by the construction's GROUND_INFILTRATION_MD, whichever value is lowest. The least conductive layer will therefore be the bottleneck, even if the other layers allow better infiltration.
Water infiltrating into the unsaturated layer is assumed to be spread equally across the entire unsaturated column within the grid cell.
Water can flow further down into the saturated zone at a speed defined by the underground terrain's GROUND_INFILTRATION_MD. For a given timestep and the infiltration speed, the distance the water can travel downwards is determined. This distance can be projected on this unsaturated zone as a subsection. The amount of water that flows from the unsaturated zone to the saturated zone is then equal to the amount of water in this subsection. After water has been added to the saturated zone, the groundwater level (and thus the height of the saturated zone) is redetermined. The water remaining in the unsaturated zone is redistributed uniformly across the (remaining) unsaturated zone.
Water stored in the underground saturated zone can also exfiltrate out of the underground and back onto the surface, if the groundwater table exceeds to surface elevation. This situation can occur due to horizontal underground flow.