C.6 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 an aquifer and to a node of the drainage system that exchanges groundwater with the aquifer. It also specifies coefficients that determine the rate of groundwater flow between the aquifer and the node. These coefficients (A1, A2, B1, B2 and A3) appear in the following equation that computes groundwater flow as a function of groundwater and surface water levels:

Q_gw = A1 (H_gw - H)^B1 - A2(H_sw - H*)^B2 + A3H_gwH_sw

Where:

Q_gw = groundwater flow (cfs per acre or cms per hectare)

H_gw = height of saturated zone above bottom of aquifer (ft or m)

H_sw = height of surface water at receiving node above aquifer bottom (ft or m)

H* = threshold groundwater height (ft or m).

The properties listed in the editor are as follows:

·        Aquifer Name: Name of aquifer object that supplies groundwater. Leave this field blank if you want the subcatchment not to generate any groundwater flow.

·        Receiving Node: Name of node that receives groundwater from the aquifer.

·        Surface Elevation: Elevation of ground surface for the subcatchment that lies above the aquifer in feet or meters.

·        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.

·        Fixed Surface Water Depth: Fixed depth of surface water at the receiving node (feet or meters) (set to zero if surface water depth will vary as computed by flow routing). This value is used to compute HSW.

·        Threshold Groundwater Elevation: Groundwater elevation that must be reached before any flow occurs (feet or meters). Leave blank to use the receiving node's invert elevation.

The values of the flow coefficients must be in units that are consistent with the groundwater flow units of cfs/acre for US units or cms/ha for metric units.

If 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 groundwater flux 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 negative fluxes 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.