Infiltration model (Water Overlay)

File:Infiltration model.png
When water infiltrates from the surface, it infiltrates at a speed dictated 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 porous material present will always serve as a bottleneck for the water to flow through, even if the other layers allow for a high rate of infiltration.

Surface water infiltrates into the underground unsaturated layer. Water in the unsaturated layer is assumed to be spread equally across the entire unsaturated volume. Water then flows from the unsaturated zone into the saturated zone at the speed dictated by the underground terrain's GROUND_INFILTRATION_MD. For a given timestep, the distance the water travels is determined. The amount of water that flows from the unsaturated zone to the saturated zone is equal to the amount of water in a section of the unsaturated zone the height of which is equal to that distance. After water has been added to the saturated zone, the groundwater level (and thus the height of the saturated zone) is redetermined. The water in the unsaturated zone is redistributed uniformly across the (remaining) unsaturated zone.

Water stored in the underground saturated zone can flow horizontally from one underground cell to another, if the groundwater level relative to datum is higher than the neighboring cell's ground water level, relative to datum. The amount of water which can flow from one cell to another is dictated by the underground terrain's GROUND_INFILTRATION_MD.

Exfiltration Water stored in the underground saturated zone can also exfiltrate out of the underground and back onto the surface, if the groundwater level relative to datum exceeds the neighboring cell's surface water level relative to datum. The amount of water which can flow from the underground of one cell onto the surface of an adjacent cell is dictated by the underground terrain's GROUND_INFILTRATION_MD.