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Runoff Calculations Curve Number Time of Concentration Rainfall Overland Flow Land-Use Reporting

Click for complete self-training materialsSubcatchment FAQ

This document discusses general issues about the use of a "subcatchment" within HydroCAD.

What is a subcatchment?

A subcatchment is used to model the runoff from a given area of land.  Each subcatchment generates a runoff hydrograph, that is typically routed into a downstream reach or pond.   A subcatchment can also be used to account for the rain falling directly on the surface of a pond.  A subcatchment cannot be used to route an inflow hydrograph. Instead, use a subcatchment to calculate the runoff and a separate reach to perform the routing.

How is the runoff calculated?

HydroCAD provides a choice of runoff techniques, including the SCS/NRCS Unit Hydrograph procedure (aka TR-20), the Santa Barbara Urban Hydrograph (SBUH).

Click for complete self-training materialsHow is the rainfall defined?

For the SCS and SBUH runoff procedures, a predefined rainfall distribution is selected for the project area.  When using the Rational method, a custom IDF file can be created.  For details, read about rainfall options.

How is the subcatchment characterized?

Each subcatchment must have a curve number (or c-value) that characterizes the soil type and ground cover.  HydroCAD also provides for special handling of impervious areas.

Each subcatchment also requires a Time-of-Concentration.  HydroCAD provides a range of techniques for calculating the Tc.  Details here.

What is the maximum subcatchment size?

The SCS runoff procedure is suitable for subcatchment areas up to 20 square miles (12,800 acres) and a total watershed area up to 300 square miles.  As the size increases, routing effects and spatial rainfall variations become more significant and a statistical (regression) analysis becomes necessary using the appropriate modeling software.

Why can't I route an inflow through my subcatchment?

A subcatchment is used only to generate a runoff hydrograph.  To route a separate inflow hydrograph you may want to use a reach.  Also read about overland flow.


For further information please read about runoff calculations.

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