The Groundwater Flow tab is used to edit the Groundwater property of a Subcatchment.
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:
| Field | Description |
| 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.