The Groundwater global database of subcatchment groundwater data. This group of parameters is referenced from individual subcatchments. 


Groundwater flows only (no water quality) are routed. Infiltration may be routed through an unsaturated zone lumped storage, followed by routing through a saturated zone lumped storage. Outflow may occur from the saturated zone to conduits or may be lost from the simulation to deep groundwater. Evapo-transpiration from both the upper and lower zone may also be simulated.

The groundwater table is dynamic. If it rises to the surface, the upper zone disappears and infiltration is stopped. If it drops below the bottom elevation of the conduit, groundwater outflow will cease.

Note: In most cases any water routed through the subsurface zones will be clean and thus act to dilute concentrations in downstream elements.  However, it is possible to add a constant concentration to groundwater flows based on Runoff Pollutant→Landuse specific data.  In this case pollutant load can be generated based on the Landuse breakdown and be routed when Interface Files are used to pass flows and concentrations. 

Upper Zone (H2 - GRELEV)

The initial depth of the water table below the ground surface, in ft. or m.

Lower Zone (H2 - STG)

The initial depth of the water above the aquifer base, in ft. or m.

Use Conduit Depth (H2 - TW = 0)

This option uses the depth of water in the drainage conduit at the end of the previous time step to simulate the effect of conduit tailwater levels on groundwater outflow.

Constant Channel Depth (BC) (H2 - TW > 0)

If the 'Constant BC' option is set enter the average depth of water in the conduit for the duration of the run. This depth is used to simulate the effect of tailwater levels on groundwater outflow. This option assumes a constant depth of flow in the drainage conduit to simulate the effect of conduit tailwater levels on groundwater outflow.

Elevation of Channel Base (H2 - BC)

Depth of the channel bottom relative above the aquifer base.

Evapo-transpiration

Groundwater evaporation/transpiration parameters. Evapo-transpiration from the upper zone represents soil moisture lost via cover vegetation and by direct evaporation from the pervious area of the subcatchment. Evapo-transpiration from the lower zone is typically small compared to other terms.

Potential Evaporation available for subsurface water loss is the difference between total evaporation input to the model and evaporation used by the surface routing.

Wilting Point (H3 - WP)

The soil moisture content at which plants can no longer obtain enough moisture to meet transpiration requirements; they wilt and die unless water is added to the soil. The moisture content at a tension of 15 atmospheres is accepted as a good estimate of wilting capacity. Wilting point must be less than the field capacity, since it occurs at higher tensions. The following table indicates typical values (Linsley et al, 1982).

Soil TypeWilting Point
(fraction moisture content) 
Sand 0.03
Sandy Loam 0.07
Loam0.14
Silt Loam0.17
Clay Loam0.19
Clay0.26
Peat 0.30

Field Capacity (H3 - FC)

The amount of water a well-drained soil holds after free water has drained off, or the maximum amount it can hold against gravity, expressed as a moisture content fraction. This occurs at soil moisture tensions of anywhere from 0.1 to 0.7 atmospheres; often the moisture content at a tension of 0.33 atmosphere is used.

Field capacity must be greater than the wilting point (since it occurs at lower tensions), and less than 0.9 times the porosity. Typical values of field capacity are given in the following table (Linsley et al, 1982).

Soil Type

Field Capacity
(fraction moisture content)

Sand 0.08
Sandy Loam0.17
Loam 0.26
Silt Loam0.28
Clay Loam 0.31
Clay 0.36
Peat 0.56

Fraction of Evapo-transpiration Assigned to Zone Fraction (H4 - CET)

Fraction of maximum Evapo-transpiration rate assigned to the upper zone; a fraction in the range 0.0 - 1.0. The upper zone evapo-transpiration is computed by multiplying this parameter with the available evaporation. Available evaporation is the difference between total evaporation and evaporation used by surface routing.

Max Depth of Significant Lower Zone Transpiration (H4 - DET)

Maximum depth over which significant lower zone transpiration occurs, ft [m]. Lower zone evapo-transpiration occurs after upper zone evapo-transpiration by removing the remaining fraction linearly as a function of depth to the water table. If the water table drops below this depth no lower zone evapo-transpiration occurs.

Infiltration/Percolation

Groundwater infiltration percolation parameters. Percolation represents the flow of water from the unsaturated zone to the saturated zone, and is the only inflow for the saturated zone. The percolation equation is given by:

PERC = HKTH * (1 + PCO*(TH-FD)/(DWT1/2))

HKTH = HKSAT * EXP((TH-PR)*HCO)

Where

PERC = percolation rate (positive downward) (zero if TH < FD)

HKTH = hydraulic conductivity

* HKSAT = saturated hydraulic conductivity

* HCO = calibration parameter

* TH = moisture content

* PR = porosity

* FD = field capacity

* DWT1 = depth of water table below surface

Parameters with an asterisk (*) are entered as part of the groundwater parameter set.

Infiltration

Saturated Hydraulic Conductivity (H3 - HKSAT). Saturated hydraulic conductivity, in./hr [cm/hr].

Porosity (H3 - POR). Porosity expressed as a fraction in the range 0.0 - 1.0. Porosity should be greater than the initial upper zone moisture in order to give a positive initial available groundwater volume. Porosity is critical for percolation computations because of its role in determining moisture storage.

Curve Fitting Parameter (H4 - HCO). Hydraulic conductivity vs. moisture content curve-fitting parameter, dimensionless. This parameter can be estimated from an exponential fit of hydraulic conductivity to soil moisture, assuming such data is available. This parameter is a sensitive calibration parameter for movement of unsaturated water into the saturated zone.

Initial Upper Zone Moisture (H3 - TH1). Initial upper zone moisture expressed as a fraction in the range 0.0 - 1.0. Initial upper zone moisture should be less than the porosity in order to give a positive initial available groundwater volume.

Percolation

Coeff for Unquantified Losses (H4 - DP). Coefficient for unquantified losses, in./hr [cm/hr]. Deep percolation represents a lumped sink term for unquantified losses from the saturated zone. The two primary losses are assumed to be percolation through the confining layer, and lateral outflow to somewhere other than the receiving water. The model provides for a first order decay, typical of water table recession curves.

Tension / Soil Moisture Slope (H4 - PCO). Average slope of tension versus soil moisture curve, ft/fraction [m/fraction]. This parameter can also be used for calibration, since it is likely that a better estimate of this parameter can be obtained than for the curve fitting (HCO) parameter.

Groundwater Outflow

Global groundwater outflow calculation parameters. Groundwater discharge represents lateral flow from the saturated zone to the receiving water. To this end, a general equation is provided to formulate the groundwater flow. The variables used in the equation are defined in the parent dialog. Note that if the water table becomes less than the channel invert, then flow is set to zero.

Because of the general nature of the equation, a variety of functional forms can be approximated. A linear reservoir can be selected by setting the Groundwater Flow Exponent (B1) to 1 and the Channel Water Influence Coefficient (A2) and the Groundwater/Channel Water Coefficient (A3) to 0.

One very important rule to remember, regardless of the functional form chosen, is that the groundwater flow should never be allowed to be negative. Although negative flow may occur in reality (ie. bank recharge), SWMM cannot subtract flow from the channel, since flow routing is not coupled to groundwater flow routing.

A simple way to ensure positive groundwater flow is to set Groundwater Flow Coefficient (A1) >= Channel Water Influence Coefficient (A2) and Groundwater Flow Exponent (B1) >= Channel Water Influence Exponent (B2).

Groundwater Flow Coefficient (H3 - A1)

Coefficient to the term dealing with the depth of the water table above the conduit invert.

Groundwater Flow Exponent (H3 - B1)

Exponent to the term dealing with the depth of the water table above the conduit invert.

Channel Water Influence Coefficient (H3 - A2)

Coefficient to the term dealing with the depth of water in the conduit.

Channel Water Influence Exponent (H3 - B2)

Exponent to the term dealing with the depth of water in the conduit.

Groundwater / Channelwater Coefficient (H3 - A3)

Coefficient for the cross product of water table depth above conduit invert and water depth in the conduit.