A Curious Caper of Missing Moisture: Nature Hangs in the Balance

While statistically derived design storms are widely used for flood analysis and infrastructure design, other stormwater management objectives (i.e., water quality treatment, onsite retention, and environmental flow maintenance) require long-term continuous simulation to represent the full range of hydrologic loading conditions. Only continuous simulation is capable of explicitly representing dry-weather processes, particularly evapotranspiration (ET), which is the dominant component of a long-term water budget. Accurate simulation of ET requires explicit accounting of soilwater content in surficial soils; however, hydrologic models that limit ET to surface water processes, including the regular EPA SWMM5 Hydrology module, have fundamental limitations for water balance applications. In SWMM5, soil moisture dynamics must therefore be represented using the Groundwater or Low Impact Development modules to augment the surface hydrology solution.

This presentation provides practical guidance for developing credible and physically defensible water balance models using SWMM5. Concepts are illustrated using a suite of standardized example models distributed across North America, including 7 locations in Canada and 27 in the continental United States, grouped into distinct climate classes based on average annual temperature and precipitation. Each model reflects identical hydrologic response units and hydraulic network: an 8.9-ha (20-acre) pervious subcatchment discharging to a 0.9-m (3-ft) diameter culvert, and a 0.4-ha (1-acre) impervious subcatchment discharging to a 0.6-m (2-ft) diameter pipe. Local hydrometeorological inputs (hourly precipitation, daily temperature, and average monthly wind speed) are combined with site geographic parameters (elevation, latitude, and longitude relative to the standard meridian). A common set of groundwater/aquifer properties is applied to pervious areas, with stratigraphic elevation parameters aligned to the vertical configuration of the hydraulic network. Seasonal variability is represented through monthly adjustment factors for pervious depression storage, surface roughness, and saturated hydraulic conductivity. All models are calibrated to match the expected long-term evapotranspiration and observed snowpack depth in cold regions.

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