Choosing the Right Tool for Urban Flood Modeling in Arizona

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  AGWA Urban Tool

  Built on the KINEROS2 model and designed to work inside ArcMap, AGWA treats each parcel in a neighborhood — roof, driveway, yard — as its own modeling unit. This fine-grained approach makes it well suited for evaluating green infrastructure like rain gardens or permeable pavement at the lot level.

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  EPA SWMM

  A free, widely used tool from the US EPA, SWMM excels at modeling stormwater networks — pipes, inlets, channels, and the flow between them. It has no built-in GIS connection, which means more manual data entry, but it offers detailed hydraulic simulation that practitioners rely on for drainage design.

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  HEC-HMS

  Developed by the US Army Corps of Engineers, HEC-HMS is the go-to tool for many flood control agencies across the country. It offers a broad range of hydrologic methods, some GIS capability, and a well-established track record in engineering practice.

• 14 ha Watershed Area
• 66 Residential Lots
• 1,780 m² Avg. Lot Size
• 2.89 in 100-yr / 2-hr Storm
• 2 Scenarios

La Terraza is a residential subdivision in southeastern Arizona — the kind of neighborhood you'd find in any growing Sunbelt city. Each model was set up to simulate the same design storm: a 100-year, 2-hour rainfall event. Two scenarios were run in each model — one representing conditions before the neighborhood was built, and one representing it as it exists today. Comparing those two scenarios reveals how much development has changed the way water moves through the landscape.

The AGWA Urban Tool used GIS tools in ArcMap to automatically extract parcel dimensions and soil parameters, making setup relatively straightforward. Results from AGWA have been validated against 47 observed precipitation events at this exact site, giving it a strong credibility baseline.

HEC-HMS allowed direct import of the parcel dataset but required manual connection of flow paths. SWMM had no GIS capability — parcel coordinates had to be exported from GIS, reformatted, and manually inserted into the input file. In all three models, soil and land cover parameters were derived from the same source data to ensure a fair comparison.

The differences between models come down to how each one calculates runoff. AGWA and HEC-HMS treat streets as part of each parcel's drainage calculation, while SWMM handles them separately — which helps explain why SWMM shows a much larger jump between pre and post development. No single model is definitively "right," but AGWA's results have been validated against observed data from this exact site, giving it a credibility edge for this particular watershed. Both HEC-HMS and SWMM would benefit from local calibration before being used for design decisions here.