Quick Start Guide: Setting Up Your First GeoSWMM Model

Introduction

This Quick Start Guide introduces hydrologic and hydraulic (H&H) modeling concepts to beginners.

Comprehensive H&H modeling begins with understanding how rainfall and snowmelt generate surface runoff, how this water travels through a watershed, and how these processes can be represented digitally.

GeoSWMM combines powerful ArcGIS Pro–based tools with the physics-based SWMM simulation engine, allowing users to model how water moves across the landscape and through drainage systems. GeoSWMM’s ArcGIS Pro–based tools help process digital data to gain locational insights, efficiently and accurately prepare model inputs, and visualize data in maps, tables, and charts.

Representation of Conveyance Networks in Hydrologic and Hydraulic Models

Hydrologic models represent real-world watershed processes using a structured network of simplified conceptual units. In most modern modeling frameworks, a watershed is first divided into smaller drainage units called subcatchments, typically delineated based on topography and flow direction so that each subcatchment drains to a unique outlet point. Within each subcatchment, rainfall is partitioned into infiltration, evaporation, depression storage, and surface runoff using physically based or empirical methods.

Excess rainfall, more precisely referred to in hydrology as rainfall excess, becomes runoff that is translated to a drainage point. This point is commonly referred to as an outlet or pour point (a term widely used in the GIS community) and is represented in the model by a node or junction. From these nodes, runoff is conveyed downstream through pipes, channels, or stream reaches. In some models, such as SWMM, these pipes or channel segments are referred to as conduits or links.

Flow through the links is routed using established mathematical routing approaches, including unit hydrographs, nonlinear reservoir methods, Muskingum routing, or, more universally, solutions of the Saint-Venant equations.

This node–link representation forms the backbone of most modern hydrologic and hydraulic modeling systems, including SWMM, HEC-HMS, and HEC-RAS. A notable exception is HSPF, in which subcatchments discharge directly into reaches, and flow routing occurs from upstream reaches to downstream reaches without explicit representation of nodes or junctions. Despite these differences in conveyance network representation, all hydrologic models follow the same fundamental sequence observed in natural systems: divide the watershed into smaller subwatersheds, generate runoff from the subwatersheds, and route flow downstream based on terrain-controlled connectivity, whether represented as link-to-link or node-to-link-to-node relationships.

From the perspective of FEMA and practicing engineers, a hydraulic model differs from a hydrologic model in that it does not generate runoff but instead routes flow through the conveyance system. Hydraulic models focus on how water moves once flow enters channels, pipes, or other conveyance elements and are generally capable of representing complex conditions such as the effects of hydraulic structures, pumps, flow controls, and other man-made components.

The following sections build on these concepts and describe how they are implemented in practice to help you understand the workflow for setting up GeoSWMM or other hydrologic and hydraulic models.

The Model Components in GeoSWMM

As water flows downhill under the influence of gravity, individual water drops combine to form larger flows. Any point on land or along a flow path therefore has an associated upstream area that collects runoff from rainfall and snowmelt and drains through this common point, called the outfall. In hydrology, this drainage area is referred to as a watershed or catchment. An outfall is the terminal point of a watershed and, therefore, the two must always be considered together.

Flow through an outfall can be estimated using hydrologic formulas. However, when a watershed is large, accurate flow estimation requires consideration of flow routing through streams, lakes, and man-made conveyance components such as stormwater pipes, channels, and hydraulic structures. This routing is represented by defining a stream or conveyance network that carries water from upstream locations to the outfall. The upstream locations where runoff enters the network are represented by outlets or junctions in GeoSWMM.

To estimate flows entering the conveyance network and passing through individual conduits or stream segments (called links in GeoSWMM), the watershed is divided into smaller subwatersheds, also referred to as subcatchments. This subdivision allows the model to capture localized variations in land use, soil type, slope, and flow paths and to discretize the watershed into smaller computational units. Each subcatchment defines the drainage area contributing runoff to a specific outlet, and together the subcatchments represent the entire watershed.

Build the Initial Model in GeoSWMM

Locate the Outfall

Identify the point on the map where you would like to estimate flow or generate a hydrograph through H&H modeling. This point represents the outfall location.

An example of an outfall location may be the site of a proposed culvert at Golden Hills Canyon Road near Salt Lake City, UT, as shown on the map.

Define the Stream Network

Map the stream or conveyance network leading to the outfall. In the absence of existing GIS stream data, the network can be created based on topographic information. GeoSWMM provides tools to generate stream networks using Digital Elevation Model (DEM) data, ensuring that the network correctly reflects terrain-driven drainage direction and connectivity.

Add Outlets or Nodes

For the stream network to receive runoff from the land surface, all entry points must be defined. Each entry point is represented as an outlet of a contributing drainage area and is modeled as a node or junction. GeoSWMM provides tools to import outlets and to add them automatically or manually. To improve model accuracy, the watershed is subdivided into smaller subcatchments based on the locations of these outlets.

Delineate Subcatchments

A watershed represented by multiple smaller subcatchments provides a more accurate hydrologic representation than a single large unit. Therefore, the watershed is divided into multiple subcatchments, each draining to a defined outlet. GeoSWMM allows automatic delineation of subcatchment boundaries using DEM data and outlet locations.

Populate Attributes and Parameters

The preceding steps establish the physical structure of the model and populate some default characteristics. However, the model is not complete until all required attributes and parameters are specified. These include subcatchment properties such as imperviousness and infiltration parameters, as well as hydraulic characteristics of links such as cross sections and roughness coefficients. GeoSWMM provides tools to estimate some parameters, while others can be entered through tables or dedicated input interfaces.

Specify Weather Data and Boundary Conditions

Weather data such as rainfall, evaporation, and snowmelt are primary drivers of hydrologic response and must be specified for the model. Appropriate weather data sources must be assigned to each subcatchment.

GeoSWMM also requires specification of downstream boundary conditions, such as normal depth or fixed water surface elevation, at the outfall.

Review Data and Run the Simulation

Review connectivity among subcatchments, nodes or junctions, and links. GeoSWMM provides a Network Analysis tool to identify missing, disconnected, or improperly connected elements.

After resolving any issues, save the project and run the simulation using the Run Model command.

View Results

Model results may be examined through hydrographs, depth and velocity profiles, flood‑inundation mapping, and tabulated output metrics. GeoSWMM offers built‑in visualization tools and full interoperability with ArcGIS Pro to support advanced analysis and interpretation.

Expand the Model (Advanced Features)

For more complex systems, GeoSWMM supports additional hydraulic components, including:

• Storage units (ponds, detention basins)
• Pumps (for controlled flow transfer)
• Orifices, weirs, and outlets (hydraulic control structures)
• Inlets and manholes (urban stormwater systems)

These elements are typically represented and analyzed within the hydraulic portion of the model, where flow routing and hydraulic interactions govern system behavior.

Junctions may also be placed at locations such as flow gages or points where channel characteristics change, for example changes in cross section, slope, or roughness. These junctions allow stream segments to be split without subdividing the contributing subcatchment and enable generation of modeled hydrographs at intermediate locations.

Detailed guidance on these features is provided in the User’s Manual and Tutorials.

Calibrate and Validate the Model

Calibration involves comparing modeled hydrographs with observed data at flow gages and adjusting parameters such as infiltration and roughness to make the model results match the observed data better.

Validation is performed by comparing model results against an independent dataset to confirm model reliability.

Tips for Beginners

• Start small by building a simple model with one or two subcatchments.
• Check elevations carefully, as flow direction errors often result from DEM issues.
• Maintain consistency in units and avoid mixing SI and US customary units.
• Save incremental versions of the model after major edits.
• Use Model Check tools to verify network connectivity before running simulations.

Resources

• GeoSWMM User’s Manual – Detailed descriptions of model components, parameters, and workflows.
• Tutorial Projects – Step-by-step example models for hands-on learning.
• Support Portal – FAQs, troubleshooting guidance, and user community resources.