Enhancing Urban Water Balance Representation in EPA SWMM Using a Two-Layer Infiltration Approach

In EPA SWMM, infiltration is commonly represented using a single soil layer, typically parameterized as native soil with relatively low hydraulic conductivity and no soil evapotranspiration. This simplified approach does not adequately represent real soil systems, which often include a more permeable upper soil layer, vertical exchanges with underlying native soil, and soil evapotranspiration, leading to a poor representation of the soil water balance. A two-layer infiltration approach provides a more physically based approach by representing infiltration into the upper soil layer, percolation to the native soil, interflow, and subsurface evapotranspiration. In the City of Calgary, this approach is required for continuous simulation to demonstrate compliance with flow duration curves. It is implemented using SWMM LID Controls rather than the groundwater routine, which requires additional calibration parameters and provides less transparent control over soil moisture fluxes. LID Controls are also needed to represent landscape-based LID approaches, such as directing runoff to deeper topsoil, including rain gardens.

This study implements, calibrates, and validates a two-layer infiltration framework in PCSWMM using the Bioretention Cell LID Control for the Nose Creek watershed and compares its simulated water balance components with those from the single-layer approach to evaluate realism and consistency with expected values. Model parameters were calibrated against observed stormwater pond levels for May-October 2020 and validated using an independent May-October 2019 period to assess robustness and parameter transferability.

Under the single-layer approach, seepage dominates the simulated water balance (66%), greatly exceeding evapotranspiration (12%) and runoff (22%), which is inconsistent with Nose Creek’s evapotranspirative hydrologic regime and clay-rich soils. In contrast, the two-layer approach reduces seepage to approximately 4% and increases evapotranspiration to about 73%, closely aligning with predevelopment conditions when accounting for urban imperviousness and altered vegetation. These results demonstrate the practical value of a physically meaningful two-layer infiltration framework in PCSWMM

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