SWMM - Subcatchment - Groundwater

Groundwater - To model the flow of groundwater between an aquifer below the subsurface Catchment and a node of the drainage system, the sub-Catchment must receive a set of groundwater parameters:

• A1 - Groundwater flow coefficient
• A2 - Surface water flow coefficient
• A3 - Surface-groundwater interaction coefficient
• Aquifer - aquifer object that describes subsurface soil properties, thickness and initial conditions. Leave this field blank if you want the subsurface -Catchment does not generate any groundwater flow
• B1 - Groundwater flow exponent
• B2 - Surface water flow exponent
• Floor Aquifer Elevation - Elevation of the aquifer bed below this particular sub-catchment (EB)
• Water Table Rise - Initial water table rise at the start of the simulation for this particular sub-catchment (EB + HGW)
• Surface Elevation - Sub-Catchment ground surface elevation
• Lateral Flow Equation - The provided equation will be used in addition to the standard equation to calculate subcatchment groundwater flow (QL)
  The evaluation result of your custom equation will be added to the standard equation result.
  To override the standard equation, completely set all its coefficients to 0.
  Remember that groundwater flow units are cfs/acre for US units and cms/ha for metric units.
  
  You can use the following symbols in your expression:
  
  Hgw = height of water table above aquifer floor, feet or m
  Hsw = height of surface water above aquifer bottom, feet or m
  Hcb = height of channel bottom above aquifer bottom, feet or m
  Hgs = height of ground surface above aquifer bottom, feet or m
  Ks = saturated hydraulic Conductivity, in/h or mm/h
  K = unsaturated hydraulic Conductivity, in/h or mm/h
  Theta = moisture content of the upper unsaturated zone, fraction
  Phi = soil porosity, fraction
  Fi = surface infiltration rate, in/h or mm/h
  Fu = percolation rate of the upper soil zone, in/h or mm/h
  A = subcatchment area, ac or ha
  
  Use the STEP function to have flow only when the water table level is above a certain threshold.
  For example, the expression:
  0.001 * (Hgw - 5) * STEP (Hgw - 5)
  would generate flow only when Hgw was above 5
• Deep Flow Equation - The equation will be used to replace the standard equation for deep groundwater flow (QD)
  Flow units for deep groundwater are in/h for US units and mm/h for metric units.
  You can use the following symbols in your expression:
  
  Hgw = height of water table above aquifer floor, feet or m
  Hsw = height of surface water above aquifer bottom, feet or m
  Hcb = height of channel bottom above aquifer bottom, feet or m
  Hgs = height of ground surface above aquifer bottom, feet or m
  Ks = saturated hydraulic Conductivity, in/h or mm/h
  K = unsaturated hydraulic Conductivity, in/h or mm/h
  Theta = moisture content of the upper unsaturated zone, fraction
  Phi = soil porosity, fraction
  Fi = surface infiltration rate, in/h or mm/h
  Fu = percolation rate of the upper soil zone, in/h or mm/h
  A = (sub-Catchment area, ac or ha)
• Water table rise threshold - Minimum water table rise that must be reached before any flow occurs (EB + HCB)Leave blank to use inverted rise of receiving node
• Receiving Node - node that receives groundwater from the subcatchment
• Surface water depth - Fixed surface water depth above receiving node invert (HSW - HCB)
  Set to zero if surface water depth varies as calculated by flow routing
• Unsaturated Zone Moisture - Moisture content of the upper unsaturated zone above the water table for this particular subcatchment at the start of the simulation

A subcatchment connects to an aquifer and to a transport system node that exchanges groundwater with the sub-Catchment. It also specifies coefficients that determine the lateral groundwater flow rate between the aquifer and the node. These coefficients (A1, A2, B1, B2 and A3) appear in the following equation that calculates lateral groundwater flow as a function of groundwater and surface water levels: