AquaSWMM

Migration guide

Moving a PCSWMM model to AquaSWMM

A PCSWMM model is an EPA SWMM 5 model: the network, hydrology and options live in a standard .inp file. AquaSWMM opens that file as it is and runs it on EPA’s own runswmm.exe. It adds storm-sewer design review, run comparison and its own DEM-based 2D overland flow, for $490 a seat per year, or $1,200 for five seats.

For comparison, PCSWMM lists Professional at $1,600 and Professional 2D at $2,400 per user per year (pcswmm.com/Pricing, October 2026).

This page covers what comes across, what doesn’t, and how to prove the results match before you switch a project.

1. Open the .inp

File → Open, and pick the model’s .inp. There is no conversion step.

  • Nothing in the file is lost. AquaSWMM keeps every byte it did not change: comments, blank lines, column spacing, section order, and sections it has no editor for. Saving a model you only looked at writes the identical file. This is tested byte for byte against EPA’s sample models (manual §1.6).
  • 1D-2D models open as they are. PCSWMM 2D represents the surface as “a network of junctions and open conduits” (Abdelrahman et al., JWMM 26: C454, 2018), and those are ordinary SWMM objects in the .inp. AquaSWMM opens and runs them like any other model, and its run comparison can split the differences into 1D, 2D and coupling objects.

2. Bring in ARM subcatchments

PCSWMM keeps ARM subcatchments outside the .inp, in its project database (model.db), and feeds their flows to the engine through a [FILES] USE INFLOWS interface file. Opened anywhere else, the model has none of those subcatchments.

AquaSWMM reads them. When a model has a model.db with ARM subcatchments, it offers Import PCSWMM ARM, which turns each row into an ordinary subcatchment with its outlet, rain gage, area, slope, polygon and curve number, and removes the USE INFLOWS line so the flows aren’t counted twice. It is one undo step. What it reads is listed in manual chapter 24. Check the imported subcatchments against your PCSWMM project before relying on them.

3. Prove the results match

Don’t take our word for it. Run the .inp in AquaSWMM, then open Results → Compare Runs and press Add results from file… to load the .out PCSWMM wrote. AquaSWMM tabulates the differences in node depths, inflows and flooding, and link flows, velocities and fullness, and names the largest. From a script, aquaswmm-swmm compare theirs.out ours.out --inp model.inp does the same and splits the differences into 1D, 2D and coupling objects (manual §9.7).

Expect small differences, and know why. PCSWMM runs its own build of the SWMM 5 engine, not EPA’s runswmm.exe, so the order of floating-point operations differs. A healthy model shrugs that off: peaks and totals agree to the reported precision. A model near its limits, with time steps that do not converge, may agree in every total and still differ in local peaks. That spread tells you how far those local peaks can be trusted, in either program.

Every AquaSWMM run is stamped with the engine version and the SHA-256 of the executable that ran it, so a reviewer can tie any number to the binary that produced it.

4. What doesn’t come across

  • Project data kept outside the .inp, other than ARM subcatchments. AquaSWMM reads the model file, not PCSWMM’s project database or its extra map layers. Re-add background layers with File → Import GIS Layer, or a map backdrop.
  • PCSWMM’s own .arm binary hydrograph file. AquaSWMM rebuilds ARM flows from the imported subcatchments instead.

You don’t have to switch everything at once. Keep PCSWMM for the projects already in it, and compare runs on the next one.

Try it on your own model

The 30-day trial is the full app. If you’re moving a firm’s models and want help, email michael@hydrocomplete.com. I’ll send a 90-day evaluation key and look at any model that doesn’t come across cleanly.

PCSWMM is a trademark of Computational Hydraulics International (CHI). AquaSWMM is independent software from HydroComplete LLC and is not affiliated with or endorsed by CHI. Facts about PCSWMM are from the linked public sources as of October 2026; check current terms with CHI.