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HydroCAD Stormwater Modeling - Since 1986


TR-20:  Project Formulation - Hydrology

Author: USDA Soil Conservation Service (SCS)
            Now Natural Resources Conservation Service (NRCS)

"The TR-20 program provides a hydrologic analyses of a watershed under present conditions....  Output consists of peaks and/or flood hydrographs....  Subarea surface runoff hydrographs are developed from storm rainfall using the dimensionless unit hydrograph, drainage areas, times of concentration, and SCS runoff curve numbers.   Instructions to develop, route, add, store, divert, or divide hydrographs are established to convey floodwater from the headwaters to the watershed outlet."


1)  TR-20 is a computer program for the generation and routing of runoff hydrographs.  Unlike TR-55, which was developed for manual use, the calculations in TR-20 are far too complex and numerous to be of practical use without appropriate computer software.

2) While the TR-20 program remains the benchmark for runoff calculations using the SCS methods, it has limitations as a practical engineering tool.  Written in Fortran, and employing input forms dating from its punched-card ancestry, TR-20 takes considerable time to master and use.

3) TR-20 does not provide any procedures for calculating time-of-concentration, for deriving stage-storage tables, or for calculating stage-discharge relationships for hydraulic devices.  All such calculations must be performed by other means and the final results entered into TR-20.

4) The TR-20 manual is primarily an operating manual for the TR-20 program.  As such, it is of limited use as a general hydrology reference.  Other sources, such as TR-55 and NEH-4 provide better background on many of the calculations employed.


While TR-20 may still be used for some specific requirements, it lacks the ease-of-use or full range of tools required my most engineers.

How to get a copy:

You can download the software and documentation from the NRCS Tech Tools.

How it compares to HydroCAD:

HydroCAD is widely used for projects that might otherwise have been performed with TR-20.  HydroCAD provides several key features of TR-20, such as the unit-hydrograph runoff procedure, in a more user-friendly graphical format.  HydroCAD also adds other capabilities that have no equivalent in TR-20, such as time-of-concentration calculations, curve-number lookup, outlet hydraulics, exfiltration calculations, pond storage calculations, and a range of other features.  See the feature comparison table for details.

For most applications, HydroCAD is a preferred alternative to TR-20, and is widely accepted by reviewing agencies.  Results from the two programs are usually very close (within 1%), with any significant differences generally due to differences in input data.  For a sample comparison of runoff and pond routing results, see the "TR-20 Sample Job" installed with HydroCAD, as well as the TR-20 comparison procedure.

HydroCAD 8.5 can read TR-20 data files directly and convert them to an equivalent HydroCAD project.  Details here.

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