Ground model (Water Overlay): Difference between revisions

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[[File:underground_model.png]]
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The water model includes an underground model which dictates the movement of water in the soil. The underground model includes the flow of water from the surface into the underground via infiltration, the flow of water from one underground location to another, and the exfiltration of water from the soil back onto the surface layer.
The Water Module also contains a simplified multi-layer ground model, for [[Infiltration model (Water Overlay)|infiltration]], [[Evaporation model (Water Overlay)|evapotranspiration]], [[#Bottom flow|bottom flow]] and [[#Horizontal flow and aquifers|horizontal flow]] in the ground layer. Additionally, the model also applies a simplified form of exfiltration of water from the soil back onto the surface layer.


The underground model can be explicitly activated or deactivated by setting the [[Ground water model attribute (Water Overlay)|GROUND_WATER]] attribute of the water overlay to the appropriate value. If the underground model is deactivated, no water can move from or to the underground in any form, including [[Underground evaporation formula (Water Overlay)|underground evaporation]].
==Groundwater modes==
For the [[Water Module]] there exist three [[ground water (Water Overlay)|modes for handling groundwater]]:
* None
* Infiltration only (selected by default for the [[Flooding_(Overlay)|flooding]] and [[Rainfall_(Overlay)|rainfall overlay]])
* Complete (selected by default for the [[Groundwater_(Overlay)|groundwater overlay]])


The underground is bounded vertically by the [[Terrain height|surface of the terrain]] at the top, and an assumed 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 model attribute (Water Overlay)|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 follows the profile of the surface.
==Multi Layered model==
The assumption is made that ground is bound vertically by the [[terrain height (Water Overlay)|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 ground, is equal to [[Ground bottom distance m model attribute (Water Overlay)|ground bottom distance]]. In other words, the impenetrable ground 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 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 saturated with water. This water is assumed to work as a continuous volume of water able to flow horizontally. The unsaturated zone is the region of the underground above the saturated zone. The edge between the unsaturated and saturated zone can be considered the groundwater level.
The fraction of water the ground can store per volume is defined with the [[Terrain water storage percentage (Water Overlay)|WATER_STORAGE_PERCENTAGE]] attribute of the ground layer terrain.


The groundwater level, and thus the height of the saturated zone, is determined both by the amount of water in the saturated zone, and the underground terrain's [[Water storage percentage (Terrain) (Water Overlay)|WATER_STORAGE_PERCENTAGE]]. The lower the water storage percentage of the soil, the greater the volume of soil that is saturated by the same amount of water, and thus the higher the the groundwater level will become.
The ground layer is modeled using two layers: the '''unsaturated zone''' and the '''saturated zone'''. The saturated zone is the region of the ground where the soil is fully saturated with water. The unsaturated zone is the region of the ground 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 underground water level is initialized with the values of the [[Ground water (Overlay)|ground water GeoTIFF]] connected to the water model. If no ground water data is connected, the ground water level relative to [[Datum|datum]] is equal to the surface water level relative to datum, as defined by the [[Water level (Water Overlay)|WATER_LEVEL]] attribute of the [[Water level area (Water Overlay)|water level area]] in that location.
The edge between the unsaturated and saturated zone is defined as the ''groundwater level'', also known as the watertable.


When water infiltrates from the surface, it infiltrates at a speed dictated by the surface terrain's [[Ground infiltration md (Terrain) (Water Overlay)|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.
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 model attribute (Water Overlay)|ground bottom distance]] and the ground terrain's [[Terrain water storage percentage (Water Overlay)|WATER_STORAGE_PERCENTAGE]].


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.
===Initialization===
The  ground water level is initialized with the values of the [[Groundwater Prequel (Water Overlay)|ground water prequel]] 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 (Water Overlay)|WATER_LEVEL]] attribute of the [[Water area (Water Overlay)|water area]] in that location. If no water area exists there either, the groundwater level is set to one meter below the [[elevation model (Water Overlay)|surface]].


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.
===Vertical flow===
Vertical ground water flow is modeled as [[Infiltration model (Water Overlay)|infiltration]] and [[Evaporation model (Water Overlay)|evapotranspiration]].


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.
===Horizontal flow and aquifers===


{{Template:WaterOverlay_nav}}
Ground flow is different from surface flow, since it has to account for the slowdown and porousness of the medium. In general, [[Ground flow formula (Water Overlay)|horizontal ground flow]] is calculated using formulas described in Harbaugh 2005<ref name="Harbaugh"/><ref name="Modflow"/>. Depending on the [[Hydraulic conductivity with thickness (Water Overlay)|configuration of the Water Overlay ]], the flow can also be calculated using a [[Terrain hydraulic conductivity with thickness md (Water Overlay)|KD]] values instead of [[Terrain hydraulic conductivity md (Water Overlay)|K]]. However, when an [[Aquifer (Water Overlay)|aquifer]] is present, the [[Ground flow formula (Water Overlay)#Aquifer formula|horizontal aquifer flow]] variant is applied.
[[File:Undergroundflow2.png|left]]
{{clear}}
 
===Bottom flow===
Water can also flow into the ground layer from deeper confined layers due to pressure from higher regions outside the project area. The speed at which the [[Ground bottom flow formula (Water Overlay)|ground bottom flow]] occurs is dependent on the local ground water levels, the [[Bottom pressure prequel (Water Overlay)|head (pressure) of the higher regions]] and the [[Bottom resistance prequel (Water Overlay)|resistance of the layer]] separating the saturated zone of the freatic ground layer and deeper ground layer.
 
Note that ground bottom flow can also be negative, allowing water to flow out of the project area to the deeper confined layers to regions elsewhere.
 
===Drainage===
[[Drainage (Water Overlay)|Drainages]] are hydraulic structures that provide connections between the ground and nearby waterways, draining groundwater to levels which are optimal for agriculture. Drainage can be passive, where water drains based on water head differences, or active, where the water is pumped from the drain to the nearby waterway. Additionally, the datum height below ground and the overflow height in the waterway can be configured.
 
 
{{article end
|notes=
* The [[sewer model (Water Overlay)|sewer]] is explicitly ''not'' part of the ground model, and is not affected by the [[Ground water (Water Overlay)|GROUND_WATER]] attribute nor by the ground model directly.
|related=
The following topics are related to this models.
; Formulas
: [[Groundwater level formula (Water Overlay)]]
: [[Ground flow formula (Water Overlay)]]
: [[Ground evaporation formula (Water Overlay)]]
: [[Surface infiltration formula (Water Overlay)]]
: [[Ground bottom flow formula (Water Overlay)]]
: [[Ground infiltration formula (Water Overlay)]]
; Models
: [[Surface model (Water Overlay)]]
: [[Evaporation model (Water Overlay)]]
: [[Infiltration model (Water Overlay)]]
: [[Tracer flow model (Water Overlay)]]
|references=
<references>
<ref name="Modflow">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)</ref>
<ref name="Harbaugh">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.</ref>
</references>
}}
 
{{WaterOverlay formula nav}}

Latest revision as of 15:03, 5 March 2024

  • Water module layers.jpeg
  • Water module flows.jpeg

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

Groundwater modes

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

Multi Layered model

The assumption is made that ground 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 ground, is equal to ground bottom distance. In other words, the impenetrable ground 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 ground can store per volume is defined with the WATER_STORAGE_PERCENTAGE attribute of the ground layer terrain.

The ground layer is modeled using two layers: the unsaturated zone and the saturated zone. The saturated zone is the region of the ground where the soil is fully saturated with water. The unsaturated zone is the region of the ground 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 and the ground terrain's WATER_STORAGE_PERCENTAGE.

Initialization

The ground water level is initialized with the values of the ground water prequel 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 area in that location. If no water area exists there either, the groundwater level is set to one meter below the surface.

Vertical flow

Vertical ground water flow is modeled as infiltration and evapotranspiration.

Horizontal flow and aquifers

Ground flow is different from surface flow, since it has to account for the slowdown and porousness of the medium. In general, horizontal ground flow is calculated using formulas described in Harbaugh 2005[1][2]. Depending on the configuration of the Water Overlay , the flow can also be calculated using a KD values instead of K. However, when an aquifer is present, the horizontal aquifer flow variant is applied.

Undergroundflow2.png

Bottom flow

Water can also flow into the ground layer from deeper confined layers due to pressure from higher regions outside the project area. The speed at which the ground bottom flow occurs is dependent on the local ground water levels, the head (pressure) of the higher regions and the resistance of the layer separating the saturated zone of the freatic ground layer and deeper ground layer.

Note that ground bottom flow can also be negative, allowing water to flow out of the project area to the deeper confined layers to regions elsewhere.

Drainage

Drainages are hydraulic structures that provide connections between the ground and nearby waterways, draining groundwater to levels which are optimal for agriculture. Drainage can be passive, where water drains based on water head differences, or active, where the water is pumped from the drain to the nearby waterway. Additionally, the datum height below ground and the overflow height in the waterway can be configured.


Notes

  • The sewer is explicitly not part of the ground model, and is not affected by the GROUND_WATER attribute nor by the ground model directly.

Related

The following topics are related to this models.

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

References

  1. ↑ 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.
  2. ↑ 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)
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