SWMM Subcatchments Basics: Creating and Configuring Drainage Areas

Subcatchments are the fundamental building blocks of SWMM hydrology. They represent land areas that receive rainfall and generate runoff. Understanding subcatchments is essential for accurate stormwater modeling.

This guide follows the object definitions and input fields in the EPA SWMM 5.2 User’s Manual. Treat example parameter ranges as starting points only; derive project values from survey, land-cover, soil, calibration, and governing-agency data.

What Is a Subcatchment?

A subcatchment in SWMM is a land surface area that:

• Receives rainfall from an assigned rain gage
• Generates runoff based on surface characteristics
• Discharges to a node, another subcatchment, or an outlet
• May include infiltration into pervious portions

Think of subcatchments as the watershed areas that contribute flow to your drainage system.

Creating Subcatchments

Using the Subcatchment Tool

1. Click the Subcatchment tool in the Map Toolbar
2. Click on the map to start drawing the subcatchment polygon
3. Continue clicking to add vertices
4. Double-click to close the polygon

Drawing Tips

• Draw subcatchments roughly to scale on the map
• Vertices define the visual boundary (used for area calculation if Auto-Length is enabled)
• The shape does not affect hydraulic calculations - only the properties matter
• Keep boundaries logical (follow property lines, roads, drainage divides)

Alternative: Adding Without Drawing

You can add a subcatchment without drawing:

1. Right-click Subcatchments in the Browser Panel
2. Select Add
3. Enter properties manually (including area)

This is useful when you have calculated areas from GIS or CAD software.

Key Subcatchment Properties

Understanding each property is crucial for accurate modeling.

Area

The total surface area of the subcatchment.

Width

The characteristic width is the width of overland flow used by SWMM’s nonlinear-reservoir runoff calculation. It is a calibration-sensitive conceptual parameter, not necessarily a measured plan width.

An often-used initial estimate is:

Example Calculation:

• Subcatchment area: 5 acres (217,800 sq ft)
• Representative maximum overland flow length: 400 feet
• Initial width estimate: 217,800 / 400 = 545 feet

Slope

The average slope of the land surface, expressed as a percent.

Slope affects:

• Overland flow velocity
• Time of concentration
• Peak runoff timing

Percent Imperviousness

The percentage of the subcatchment covered by impervious surfaces (roofs, pavement, etc.).

Manning’s n Values

Manning’s roughness coefficients control overland flow velocity.

N-Imperv (impervious surfaces):

N-Perv (pervious surfaces):

Depression Storage

Depression storage represents the small depressions, puddles, and surface irregularities that must fill before runoff begins.

Dstore-Imperv (impervious depression storage):

Dstore-Perv (pervious depression storage):

Percent Zero-Imperv

The percentage of the subcatchment’s impervious area assigned zero impervious depression storage. This fraction can begin generating runoff without first filling the Dstore-Imperv depth. It does not state how much impervious area is directly connected and is not interchangeable with DCIA.

Represent connectivity separately with SWMM’s subarea routing settings: choose whether impervious runoff goes directly to the subcatchment outlet or is routed onto the pervious subarea, and set the percentage routed consistently with the mapped drainage connections. Support %Zero-Imperv, impervious depression storage, and subarea routing independently from surface and drainage data or calibration.

Infiltration Parameters

For pervious portions of subcatchments, SWMM needs infiltration parameters based on your selected infiltration model.

Horton Infiltration

The most common infiltration model uses three parameters:

Typical Values by Soil Type:

Green-Ampt Infiltration

An alternative model based on soil physics:

SCS Curve Number

Uses the familiar NRCS curve number method:

Calculate your curve number with our tool

Connecting Subcatchments

Each subcatchment must have an outlet where runoff discharges.

Outlet Options

1. Node (Junction, Outfall, Storage): Most common - runoff enters the pipe network
2. Another Subcatchment: Creates cascading runoff between areas

Setting the Outlet

1. Select the subcatchment
2. In the Property Editor, find Outlet
3. Enter the ID of the destination node or subcatchment

Cascading Subcatchments

You can model sheet flow from one area to another:

• Subcatchment A (parking lot) outlets to Subcatchment B (grassed swale)
• Subcatchment B outlets to Junction J1

This models the time delay and volume reduction as runoff flows through intermediate areas.

Rain Gage Assignment

Every subcatchment participating in rainfall/runoff simulation needs an assigned rain gage.

Single Rain Gage

For small sites, one rain gage serves all subcatchments:

1. Create one rain gage (RG1)
2. Assign RG1 to all subcatchments

Multiple Rain Gages

For large sites or when spatial rainfall variation matters:

1. Create multiple rain gages with different data
2. Assign each subcatchment to the nearest or most appropriate gage

Best Practices for Subcatchment Delineation

When to Create Multiple Subcatchments

Create separate subcatchments when:

• Land use differs: Separate commercial from residential
• Slopes vary significantly: Different hillsides
• Discharge points differ: Each inlet should have its contributing area
• You need detailed results: More subcatchments = more spatial detail

When to Combine Areas

Combine areas into one subcatchment when:

• They share characteristics: Similar slope, imperviousness, soil
• They drain to the same point: Single outlet
• Simplification is acceptable: Screening-level analysis

There is no universal acreage range for a SWMM subcatchment. Resolution should preserve the spatial differences, drainage paths, and output locations material to the analysis without adding detail unsupported by the input data.

Common Mistakes to Avoid

Incorrect Width

Problem: Treating a measured plan width or Area / flow length as an automatically correct value Solution: Use a documented initial estimate, then review sensitivity and calibration evidence

Missing Rain Gage

Problem: Subcatchment has no rain gage assigned Solution: Assign a rain gage in properties before running

Unrealistic Infiltration

Problem: Infiltration rates do not match soil conditions Solution: Support the selected method’s inputs with applicable site data, traceable published values, and calibration where observations exist. Do not translate soil texture directly into empirical Horton parameters.

Disconnected Outlet

Problem: Outlet node does not exist Solution: Verify outlet ID matches an existing node or subcatchment

Verifying Subcatchment Setup

Before running your model, check these items:

1. All subcatchments have assigned rain gages
2. All subcatchments have valid outlets
3. Total area matches your site (sum all subcatchment areas)
4. Imperviousness values are reasonable for land use
5. Width values produce reasonable flow paths

Next Steps

Now that you understand subcatchments:

1. Learn about pipes: Junctions and Conduits
2. Run simulations: Running Simulations
3. Calculate runoff manually: Rational Method Calculator

Summary

Subcatchments are the hydrologic engine of SWMM models. Key takeaways:

• Area defines how much rainfall is captured
• Width controls runoff timing and peak flow
• Imperviousness, depression storage, subarea routing, and the event affect runoff volume
• Slope and roughness affect flow velocity
• Infiltration parameters govern how much water enters the soil

Accurate subcatchment definition is essential for reliable SWMM results. Take time to properly characterize your drainage areas before moving on to the pipe network.