Introduction of Seasonal Hydrologic Modeling Capability for SWMM

Many continuous modeling applications would benefit from being able to model physical changes in hydrology that occur throughout the year. Examples include the impacts of frozen soil in winter, and changes in runoff from agricultural fields due to the stages of plant growth and field management practices throughout the year.

Other than evaporation, hydrologic parameters used in the US EPA SWMM5 program do not consider seasonal variability. A number of these parameters can show significant changes: surface roughness, depression storage and infiltration can vary with the stage of vegetation growth. This seasonal variability may have a significant effect on computed model results when conducting a continuous simulation.

The authors attempt to account for these seasonality of hydrologic parame-ters by allowing monthly time patterns to be applied to various hydrological parameters. Since the SWMM5 engine only allows fixed parameters, the SWMM engine was modified to accommodate this change. Manning’s n and depression storage for pervious areas, and Green and Ampt infiltration parameters (initial moisture deficit, hydraulic conductivity, and soil suction at the wetting front) are candidates for monthly variation in this new engine.

The paper presents the effect of seasonal hydrologic modeling in computed model results and its applications. This paper also discusses the process and implications of changing the SWMM code to accommodate seasonal variation in hydrologic parameters. The testing of results with seasonality in comparison to official SWMM5 results will also be discussed.