Subcatchment Data Fields (SWMM)

Subcatchment data can be edited on either the Subcatchment Grid Window of the Subcatchments Grid or the Subcatchment Property Sheet.

Most data can be edited on the grid, but some fields cannot be displayed in the grid format.

To view and edit all subcatchment data, use the Property Sheet.

The following describes all the data which can be used to define a subcatchment. For details of results fields, see the Subcatchment Results Data Fields (SWMM) topic.

Common Data Fields

Fields that are common to the majority of objects can be found in the Common Fields topic.

Subcatchment Data

Database Table Name: sw_subcatchment

Show Columns

Database Names

Size, Type and Units

Defaults and Error Limits

Field Name

Help Text

Database Field

Data Type

Size

Units

Precision

Default

Error Lower Limit

Error Upper Limit

Warning Lower Limit

Warning Upper Limit

Subcatchment ID

A unique name for the subcatchment.

subcatchment_id

Text

254

 

0

 

 

 

 

 

Rain gage ID The name or identifier of the Rain Gage associated with this subcatchment. raingauge_id Text 254              
Drains to

The type of object to which this subcatchment drains. The choices are:

Option

Description
sw_node Indicates that this subcatchment drains to a node. The Outlet field's dropdown list is automatically populated with the names of all nodes in the current network when this option is selected.
sw_subcatchment Indicates that this subcatchment drains to another subcatchment. The Outlet field's dropdown list is automatically populated with the names of all subcatchments in the current network when this option is selected.
sw_drains_to Text 20     sw_node        

Outlet

The name of the node or subcatchment to which this subcatchment drains. The name can be either be typed in or selected from a dropdown list. The list is automatically populated when an option is selected in the Drains to field.

outlet_id

Text

254

 

0

 

 

 

 

 

Area

Area of the subcatchment.

The default area is calculated from the subcatchment's geometry.

area

Double

 

CA

3

 

0

 

 

 

x

The X co-ordinate at the centre of the subcatchment boundary region.

If the system default flag (#D) is set, the X co-ordinate at the centroid of the subcatchment will be determined by ICM.

x

Double

 

XY

1

 

 

 

 

 

y

The Y co-ordinate for the centre of the subcatchment boundary region.

If the system default flag (#D) is set, the X co-ordinate at the centroid of the subcatchment will be determined by ICM.

y

Double

 

XY

1

 

 

 

 

 

Width

The width of the overland flow path for sheet flow runoff.

width Double   L     0      

Slope

The average gradient at which the subcatchment drains to the node.

catchment_slope

Double

 

S

3

0.005

 

 

 

 

Imperviousness The percentage of the subcatchment's land area (excluding the area used for LID controls) which is impervious. percent_impervious Double     3 0.01 0 100    
Impervious roughness

The value of Manning's n for overland flow over the impervious portion of the subcatchment.

roughness_impervious Double     8 0.01 0   0  
Use area-averaged rain

Applicable when using spatial rainfall (from Events or TSDB).

Check this option to calculate rainfall for the subcatchment as the area-average of the rainfall of all rainfall polygons or radar cells that overlap the subcatchment. Any areal reduction/calibration factors set for the rainfall polygons are incorporated in the calculation of the area-averaged rainfall.

A rainfall profile is created for each subcatchment which is then passed to the simulation engine.

If this option is not checked, the rainfall polygon or radar cell that contains the centroid of the subcatchment will be used to provide rainfall data for the entire subcatchment. The rainfall rate for every radar cell is sent to the simulation engine as a separate rainfall profile, and the engine checks the centroids to determine which profile to use.

See Spatially Varying Rainfall in SWMM Networks and Using Spatial Rainfall in SWMM Simulations for further details.

Note

The simulation engine can process up to 2,147,483,647 TSDB generated rainfall profiles in a single simulation. Therefore if radar images with more cells than this are imported, spatial rain zones must be used to crop them to less than 2,147,483,647 cells for any given simulation (and similarly for subcatchment-averaged rainfall, where per-subcatchment profiles also need to be taken into account).

area_average_rain Boolean     0          
Pervious roughness The value for Manning's n for overland flow over the pervious portion of the subcatchment. roughness_pervious Double     8 0.1 0      
Impervious storage depth The depth of depression storage on the impervious portion of the subcatchment. storage_impervious Double   SO 3 0.00127 0      
Pervious storage depth The depth of depression storage on the pervious portion of the subcatchment. storage_pervious Double   SO 3 0.00127 0      
Percent no storage The percentage of the impervious area with no depression storage. percent_no_storage Double     3 25 0 100    
Routing

The type of runoff routing. The choices are:

Option

Description
Impervious Runoff from a pervious area flows to an impervious area.
Pervious Runoff from an impervious area flows to a pervious area.
Outlet (Default) Runoff from both areas flows directly to an outlet.
route_to Text 10     Outlet        
Percent routed The percentage of runoff routed between sub-areas. percent_routed Double       100 0 100    
Infiltration model

Specifies how the infiltration of rainfall into the upper soil zone in this subcatchment is to be modelled.

The choices are:

Option

Description
Default Uses the infiltration model that is set in this network's model parameter options.
Horton Uses the Horton infiltration model.
Modified_Horton (Modified Horton) Uses the Modified Horton infiltration model.
Green_Ampt (Green Ampt) Uses the Green-Ampt infiltration model.
Modified_Green_Ampt (Modified Green Ampt) Uses the Modified Green-Ampt infiltration model.
Curve_number (Curve number) Uses the Curve number infiltration model.

The infiltration models are described in the SWMM5 Reference Manual, Volume 1 - Hydrology.

infiltration Text 32     Default        
Soils

A series of records that assign different soils to percentage areas of this subcatchment. These are entered on a grid that is displayed by clicking on the button.

 

Show Columns

Database Names

Size, Type and Units

Defaults and Error Limits

Field Name

Help Text

Database Field

Data Type

Size

Units

Precision

Default

Error Lower Limit

Error Upper Limit

Warning Lower Limit

Warning Upper Limit

Soil

The name of a soil.

The name of a new soil can be typed in or an existing one can be selected from a list of defined types of soils.

soil Text 254              
Area The percentage of this subcatchment that this soil applies to. area Double     3 0 0 100    

 

Note that only the infiltration parameters defined for each soil, which are applicable for the selected Infiltration model, will be applied during a simulation.

If no soils are assigned to any areas in this grid, then the applicable infiltration parameters ClosedInitial infiltration, Limiting infiltration, Decay factor, Maximum infiltration volume, Average capillary suction, Saturated hydraulic conductivity, Initial moisture deficit, Curve number and Drying time, specified in the Non-soils infiltration section of the subcatchment's properties, will be used.

Similarly, if the soils, which are specified in the grid, apply to less than 100% of the subcatchment area, and if any applicable infiltration parameters are specified in the Non-soils infiltration section of the subcatchment's properties, then these will be applied to the remaining area.

Soil Structure                
Initial infiltration

Used by the Horton Infiltration model.

Initial infiltration () from the equation to calculate rate:

initial_infiltration Double   I 3 76.2 0      
Limiting infiltration

Used in the Horton Infiltration model.

Limiting infiltration () from the equation to calculate rate:

limiting_infiltration Double   I 3 12.7 0      
Decay factor

Used in the Horton Infiltration model.

The infiltration rate decay constant for the Horton curve (1/hours). Typical values range between 2 and 7.

decay_factor Double   HR 3 4        
Maximum infiltration volume

Used in the Horton Infiltration model.

Maximum infiltration volume possible.

max_infiltration_volume Double   RD 3 0 0      
Average capillary suction The average value of soil capillary suction. Used by the Green Ampt model. average_capillary_suction Double   RD 1 76.2 0      
Saturated hydraulic conductivity Soil saturated hydraulic conductivity. Used by the Green Ampt model. saturated_hydraulic_conductivity Double   R 1 76.2 0

 

 

 

Initial moisture deficit

The fractional difference between soil porosity and actual moisture content.

Used by the Green Ampt model.

initial_moisture_deficit Double     2   0 1    
Curve number

The SCS number of the curve associated with this subcatchment.

Used by the Curve Number model. A list of curve number values that can be used can found here.

curve_number Double     2 3 0 100    
Drying time

Used in the Horton Infiltration and Curve Number models.

Time in days for a fully saturated soil to dry completely. Typical values range from 2 to 14 days

drying_time Double   TD 3 7        
Aquifer ID

The identifier for the aquifer (if any) associated with this subcatchment.

Any aquifer already defined can be selected from the dropdown list. Alternatively, a new one can be added or an existing one modified using the button located to the left of the field. This displays the non-visual object menu, from which you can select the appropriate option.

aquifer_id Text 254   0          
Aquifer node ID The name of node that receives groundwater from the aquifer. aquifer_node_id Text 254   0      

 

 

Elevation

The elevation of the subcatchment's ground surface above the aquifer.

elevation Double   Z 3      

 

 

Groundwater coefficient The groundwater coefficient. The A1 value in the standard lateral groundwater flow equation. groundwater_coefficient Double     3 0    

 

 

Groundwater exponent The groundwater exponent. The B1 value in the standard lateral groundwater flow equation. groundwater_exponent Double     3 0    

 

 

Surface coefficient Surface water flow coefficient. The A2 value in the standard lateral groundwater flow equation. surface_coefficient Double     3          
Surface exponent Surface water flow exponent. The B2 value in the standard lateral groundwater flow equation. surface_exponent Double     3          
Surface groundwater coefficient The surface groundwater coefficient. The A3 value in the standard lateral groundwater flow equation. surface_groundwater_coefficient Double     3 0        
Surface depth

The depth of surface water above the Aquifer node ID’s invert level.

Set to zero if surface water depth will vary as computed by flow routing.

surface_depth Double Y   3 0        
Groundwater threshold

The minimum level that the groundwater must reach before any flow occurs.

If no value is specified, the invert level of the Aquifer node ID will be used.

groundwater_threshold     Z 3      

 

 

Aquifer elevation The elevation at the bottom of the aquifer. aquifer_elevation Double   Z 3          
Aquifer initial ground water elevation

The level of the groundwater at the start of the simulation for the aquifer associated with this subcatchment.

If no value is specified, the value specified for the aquifer's Initial groundwater parameter will be used.

aquifer_initial_groundwater Double   Z 3      

 

 

Aquifer initial ground water moisture content

The moisture content of the unsaturated zone of the aquifer for this subcatchment at the start of the simulation (volumetric fraction).

If no value is specified, the value specified for the aquifer's Initial moisture content parameter will be used.

aquifer_initial_moisture_content Double     3   0 1

 

 

Lateral groundwater flow equation

The button displays the Groundwater Flow Equation Editor which can be used to create or edit a custom lateral groundwater flow equation.

If a custom equation is specified, it will be used in addition to the standard lateral groundwater flow equation to compute groundwater outflow from the subcatchment. However, if the Groundwater coefficient, the Surface coefficient and the Surface groundwater coefficient fields are set to 0, then standard equation will be replaced by the custom equation.

lateral_gwf_equation Memo                
Deep groundwater flow equation

The button displays the Groundwater Flow Equation Editor which can be used to create or edit a custom deep groundwater flow equation. If a custom equation is specified, this will be used instead of the standard deep groundwater flow equation to calculate deep groundwater flow.

deep_gwf_equation Memo     3      

 

 

Snow pack ID

The name of the snow pack associated with the subcatchment.

Any snow pack already defined can be selected from the dropdown list. Alternatively, a new one can be added or an existing one modified using the button located to the left of the field. This displays the non-visual object menu, from which you can select the appropriate option.

snow_pack_id

Text

254

 

0

 

0

 

   

LIDS controls

A series of records defining the LID controls structure parameters for the subcatchment. The records are entered on the Subcatchment LID Controls grid which is displayed by clicking on the button. See the Subcatchment LID Controls Grid topic for further information.

Each LID structure has one or more LID control object associated with it. The LID control objects are defined on the LID control tab of the Subcatchment Grid. A new LID control object can also be added using the button located to the left of the field. This displays the Insert New LIDS control Object option that allows you to add the new object.

sud_controls

Structure

 

 

 

 

 

 

 

 

Land uses

A series of records defining the land uses which apply to this subcatchment. These are entered on a grid that is displayed by clicking on the button.

Database Table Name: sw_sub_coverage

Show Columns

Database Names

Size, Type and Units

Defaults and Error Limits

Field Name

Help Text

Database Field

Data Type

Size

Units

Precision

Default

Error Lower Limit

Error Upper Limit

Warning Lower Limit

Warning Upper Limit

Land use

The name of a land use.

The name of a new land use can be typed in or an existing one can be selected from a list of defined land uses.

land_use Text 254              
Area The percentage of this subcatchment that this land use applies to. area Double     3 0 0.001 100    
coverage Structure                
Initial buildup

A series of records defining the initial build up of pollutants which apply to this subcatchment. These are entered on a grid that is displayed by clicking on the button.

Database Table Name: sw_sub_loading

Show Columns

Database Names

Size, Type and Units

Defaults and Error Limits

Field Name

Help Text

Database Field

Data Type

Size

Units

Precision

Default

Error Lower Limit

Error Upper Limit

Warning Lower Limit

Warning Upper Limit

Pollutant The name of the pollutant. The name of a new pollutant can be typed in or an existing one can be selected from a list of defined pollutants. pollutant Text 254   0          
Initial build up The build up of this pollutant in the subcatchment at the start of a simulation. build_up Double   SM 3   0.001      
loadings Structure                
Curb length

The total length of curbs in the subcatchment.

Enabled when the pollutant build-up Unit is set to Curb length.

curb_length Double   L     0

 

 

 

Pervious surface roughness pattern The name of the time pattern that is to be used to adjust the pervious surface roughness for this subcatchment on a monthly basis. n-perv_pattern Text 254              
Depression storage pattern The name of the time pattern that is to be used to adjust the depression storage or this subcatchment on a monthly basis. dstore_pattern Text 254              
Infiltration capacity pattern The name of the time pattern that is to be used to adjust the infiltration capacity for this subcatchment on a monthly basis. infil_pattern Text 254              

Subcatchments Grid

Property Sheets