Hydrology

Refactoring the SWMM 5 Help File – Groundwater in SWMM5

Groundwater in SWMM5

Aquifers

Aquifers are sub-surface groundwater zones used to model the vertical movement of water infiltrating from the subcatchments that lie above them. They also permit the infiltration of groundwater into the drainage system, or exfiltration of surface water from the drainage system, depending on the hydraulic gradient that exists. Aquifers are only required in models that need to explicitly account for the exchange of groundwater with the drainage system or to establish baseflow and recession curves in natural channels and non-urban systems.

The parameters of an aquifer object can be shared by several subcatchments but there is no exchange of groundwater between subcatchments. A drainage system node can exchange groundwater with more than one subcatchment.

Aquifers are represented using two zones – an un-saturated zone and a saturated zone. Their behavior is characterized using such parameters as soil porosity, hydraulic conductivity, evapotranspiration depth, bottom elevation, and loss rate to deep groundwater. In addition, the initial water table elevation and initial moisture content of the unsaturated zone must be supplied.

Aquifers are connected to subcatchments and to drainage system nodes as defined in a subcatchment’s Groundwater Flow property. This property also contains parameters that govern the rate of groundwater flow between the aquifer’s saturated zone and the drainage system node.

See Also

Aquifer Editor

Groundwater Flow Editor

Aquifer Editor

The Aquifer Editor is invoked whenever a new Aquifer object is created or an exisitng Aquifer object is selected for editing. It contains the following data fields:

Name

User-assigned aquifer name.

Porosity

Volume of voids / total soil volume (volumetric fraction).

Wilting Point

Soil moisture content at which plants cannot survive (volumetric fraction).

Field Capacity

Soil moisture content after all free water has drained off (volumetric fraction).

Conductivity

Soil’s saturated hydraulic conductivity (in/hr or mm/hr).

Conductivity Slope

Average slope of log(conductivity) versus soil moisture deficit (i.e., porosity minus moisture content) curve (unitless).

Tension Slope

Average slope of soil tension versus soil moisture content curve (inches or mm).

Upper Evaporation Fraction

Fraction of total evaporation available for evapotranspiration in the upper unsaturated zone.

Lower Evaporation Depth

Maximum depth below the surface at which evapotranspiration from the lower saturated zone can still occur (ft or m).

Lower Groundwater Loss Rate

Rate of percolation to deep groundwater when the water table reaches the ground surface (in/hr or mm/hr).

Bottom Elevation

Elevation of the bottom of the aquifer (ft or m).

Water Table Elevation

Elevation of the water table in the aquifer at the start of the simulation (ft or m).

Unsaturated Zone Moisture

Moisture content of the unsaturated upper zone of the aquifer at the start of the simulation (volumetric fraction) (cannot exceed soil porosity).

Upper Evaporation Pattern

The name of a monthly time pattern used to adjust the Upper Evaporation Fraction for different months of the year. Leave blank if not applicable.

Groundwater Flow Editor

The Groundwater Flow Editor dialog is invoked when the Groundwater property of a Subcatchment is being edited. It is used to link a subcatchment to both a parent aquifer and to a node of the conveyance system that exchanges groundwater with the subcatchment. It also specifies coefficients that determine the rate of lateral groundwater flow between the aquifer and the node. These coefficients (A1, A2, B1, B2, and A3) appear in the following equation that computes lateral groundwater flow as a function of groundwater and surface water levels:

QL = A1(HGW – HCB)B1 – A2(HSW – HCB)B2 + A3(HGW HSW)

where QL = lateral groundwater flow (cfs per acre or cms per hectare), HGW = height of saturated zone above bottom of aquifer (ft or m), HSW = height of surface water at receiving node above aquifer bottom (ft or m), and HCB = height of channel bottom above aquifer bottom (ft or m). Note that QL can also be expressed in inches/hr for US units.

The rate of seepage to deep groundwater, QD, in in/hr (or mm/hr) is given by the following equation:

QD = LGLR * HGW / HGS

where LGLR is the lower groundwater loss rate parameter assigned to the subcatchment’s aquifer (in/hr or mm/hr) and HGS is the distance from the ground surface to the aquifer bottom (ft or m).

In addition to the standard lateral flow equation, the dialog allows one to define a custom equation whose results will be added onto those of the standard equation. One can also define a custom equation for deep groundwater flow that will replace the standard one. Finally, the dialog offers the option to override certain parameters that were specified for the aquifer to which the subcatchment belongs. The properties listed in the editor are as follows:

Aquifer Name

Name of the aquifer object that describes the subsurface soil properties, thickness, and initial conditions. Leave this field blank if you want the subcatchment not to generate any groundwater flow.

Receiving Node

Name of the node that receives groundwater from the subcatchment.

Surface Elevation

Elevation of the subcatchment’s ground surface (ft or m).

Groundwater Flow Coefficient

Value of A1 in the groundwater flow formula.

Groundwater Flow Exponent

Value of B1 in the groundwater flow formula.

Surface Water Flow Coefficient

Value of A2 in the groundwater flow formula.

Surface Water Flow Exponent

Value of B2 in the groundwater flow formula.

Surface-GW Interaction Coefficient

Value of A3 in the groundwater flow formula.

Surface Water Depth (HSW – HCB)

Fixed depth of surface water above the receiving node’s invert (ft or m). Set to zero if surface water depth will vary as computed by flow routing.

Threshold Water Table Elevation (EB + HCB)

Minimum water table elevation that must be reached before any flow occurs (feet or meters). Leave blank to use the receiving node’s invert elevation.

Aquifer Bottom Elevation (EB)

Elevation of the bottom of the aquifer below this particular subcatchment (ft or m). Leave blank to use the value from the parent aquifer.

Initial Water Table Elevation (EB + HGW)

Initial water table elevation at the start of the simulation for this particular subcatchment (ft or m). Leave blank to use the value from the parent aquifer.

Unsaturated Zone Moisture

Moisture content of the unsaturated upper zone above the water table for this particular subcatchment at the start of the simulation (volumetric fraction). Leave blank to use the value from the parent aquifer.

Custom Lateral Flow Equation

Click the ellipsis button (or press Enter) to launch the Custom Groundwater Flow Equation editor for lateral groundwater flow (QL). The equation supplied by this editor will be used in addition to the standard equation to compute groundwater outflow from the subcatchment.

Custom Deep Flow Equation

Click the ellipsis button (or press Enter) to launch the Custom Groundwater Flow Equation editor for deep groundwater flow (QD). The equation supplied by this editor will be used to replace the standard equation for deep groundwater flow.

The coefficients supplied to the lateral groundwater flow equations must be in units that are consistent with the groundwater flow units, which can either be cfs/acre (equivalent to inches/hr) for US units or cms/ha for SI units.

Note that elevations are used to specify the ground surface, water table height, and aquifer bottom in the dialog’s data entry fields but that the groundwater flow equation uses depths above the aquifer bottom.
If lateral groundwater flow is simply proportional to the difference in groundwater and surface water heads, then set the Groundwater and Surface Water Flow Exponents (B1 and B2) to 1.0, set the Groundwater Flow Coefficient (A1) to the proportionality factor, set the Surface Water Flow Coefficient (A2) to the same value as A1, and set the Interaction Coefficient (A3) to zero.
Note that when conditions warrant, the lateral groundwater flow can be negative, simulating flow into the aquifer from the channel, in the manner of bank storage. An exception occurs when A3 ≠ 0, since the surface water – groundwater interaction term is usually derived from groundwater flow models that assume unidirectional flow. Otherwise, to ensure that a negative flow will not occur, one can make A1 greater than or equal to A2, B1 greater than or equal to B2, and A3 equal to zero.
To completely replace the standard lateral groundwater flow equation with the custom equation, set all of the standard equation coefficients to 0.

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