# Underground model (Water Overlay)

The Water Module also contains a simplified 2-layer underground model, for infiltration, evaporation and horizontal flow in the underground. Additionally, the model also applies a simplified form of exfiltration of water from the soil back onto the surface layer.

## Contents

## Groundwater modes

For the Water Module there exist three modes for handling groundwater:

- None
- Infiltration only (selected by default for the flooding and rainfall overlay)
- Complete (selected by default for the groundwater overlay)

## Two Layered model

The assumption is made that underground is bound vertically by the surface of the terrain at the top and by an impenetrable layer at the bottom. The distance between the surface and the impenetrable layer, and thus the effective height of the underground, is equal to GROUND_BOTTOM_DISTANCE_M. In other words, the impenetrable underground layer is assumed to be a set distance below the surface. The distance is uniform across the entire project area, and thus the impenetrable follows the profile of the surface.

The fraction of water the underground can store per volume is defined with the WATER_STORAGE_PERCENTAGE attribute of the underground layer terrain.

The underground is composed of 2 layers: the **unsaturated zone** and the **saturated zone**. The saturated zone is the region of the underground where the soil is fully saturated with water. The unsaturated zone is the region of the underground located directly above the saturated zone. This region can also contain water, but the amount contained is always less than the maximum storage fraction. Water in the unsaturated zone is assumed to be uniformly distributed across the entire height of the zone.

The edge between the unsaturated and saturated zone is defined as the *groundwater level*, also known as the watertable.

The groundwater table is the height of the top of the saturated zone, relative to datum. The amount of water in the saturated zone is determined with the datum height of the groundwater table, the datum height of the surface, the GROUND_BOTTOM_DISTANCE_M and the underground terrain's WATER_STORAGE_PERCENTAGE.

### Initialization

The underground water level is initialized with the values of the ground water GeoTIFF connected to the water model. If no ground water data is connected, the ground water level relative to datum is equal to the surface water level relative to datum, as defined by the WATER_LEVEL attribute of the water level area in that location. If no water level area exists there either, the underground is assumed to be fully saturated, with the groundwater level equal to the surface height relative to datum.

### Vertical flow

Vertical underground water flow is modeled as infiltration.

### Horizontal flow and aquifers

Underground flow is different from surface flow, since it has to account for the slowdown and porousness of the medium. In general, horizontal underground flow is calculated using formulas described in Harbaugh 2005^{[1]}^{[2]}. However, when an aquifer is present, the horizontal aquifer flow variant is applied.

## Notes

- The sewer is explicitly
*not*part of the underground model, and is not affected by the GROUND_WATER attribute nor by the underground model directly.

## Related formulas

- Groundwater level formula
- Underground flow formula
- Underground evaporation formula
- Surface infiltration formula
- Underground infiltration formula

## Related models

## References

- ↑ Harbaugh, A.W., 2005, MODFLOW-2005, the U.S. Geological Survey modular ground-water model-the Ground-Water Flow Process: U.S. Geological Survey Techniques and Methods 6-A16, variously paginated.
- ↑ Langevin, C.D., Hughes, J.D., Banta, E.R., Niswonger, R.G., Panday, Sorab, and Provost, A.M. (2017) ∙ Documentation for the MODFLOW 6 Groundwater Flow Model: U.S. Geological Survey Techniques and Methods, book 6, chap. A55 ∙ p 31 ∙ found at: https://doi.org/10.3133/tm6A55 (last visited 2019-02-04)