Water Budget Analyses: Lucid Liquid Ledgers for Stormwater Modeling

Mike Gregory, Computational Hydraulics (CHI), ON, Canada


Tracking the soilwater content in surficial soils is critical when conducting water budget and flow duration analyses using results from a hydrologic/hydraulic stormwater model. In this presentation, the terms one-layer and two-layer hydrology are used. One-layer hydrology refers to the traditional hydrologic model solution where surface water input (i.e., rainfall, snowmelt, irrigation, etc.) either fills up available depression storage, evaporates from depression storage, infiltrates into the soil, or runs off the surface. In this case, the water budget only includes infiltration as a flux term, no attempt is made at accounting for the change in soilwater or groundwater storage volumes. With two-layer hydrology, surficial soils are subdivided into upper/lower layers and the water content is accounted in each layer accordingly.

One-layer hydrology is used in the original SWMM5 solution for Subcatchment entities. Two-layer hydrology is available in SWMM5 by using either the Groundwater or LID module. Two layers are distinguished in the Groundwater module, an upper unsaturated and lower saturated zone, and only one soil type can be specified in a single Aquifer entity. Two layers are distinguished in the LID module, an upper soil layer and a lower storage layer, and these can represent two different soil types in a single LID Control entity. One-layer hydrology models do not properly account for two important components of the water budget: evapotranspiration from soils within the root zone as well as interflow from soils within the unsaturated zone. Whether conducting a water budget analysis in arid regions or designing facilities to protect or enhance hydrologically sensitive natural wetlands or watercourses in humid regions, two-layer hydrology solutions are required. In this presentation, two-layer hydrology applications of SWMM5 are demonstrated for a variety of case studies.

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