Hydrologic modeling estimates how rainfall and watershed conditions produce runoff for defined project scenarios.
Definition
Hydrologic modeling uses rainfall, land cover, soil, topography, and drainage information to estimate runoff volumes, flow rates, or timing for a defined area and scenario. A firm may include data review, watershed delineation, model setup, calibration where suitable observations are available, scenario runs, and documentation. Scope depends on the decision the model must support and the requirements set by the client or reviewing agency. Firms calculate results differently when they use different methods, data sources, time steps, or assumptions, so the basis should be stated. Outputs are estimates for the modeled conditions, not measurements or forecasts of exactly what will happen in a storm. Hydrologic modeling estimates runoff generation and flow inputs; it does not by itself describe water movement through a pipe, channel, or floodplain, which is hydraulic modeling. EPA stormwater resources provide context for stormwater planning and permitting.
How owners use it
- Compare estimated runoff for existing and proposed site conditions to inform drainage planning.
- A model estimates 12 acre-feet of runoff before a site change and 15 after it, so the increase is 15 - 12 = 3 acre-feet.
- Support stormwater reports by documenting rainfall inputs, watershed assumptions, methods, and modeled scenarios.
- Coordinate runoff estimates with hydraulic analysis when evaluating a connected drainage system.
