HydroCAD® Stormwater Modeling - Since 1986

HydroCAD, TR-20, TR-55 Comparison

The following table summarizes the technical similarities and differences between these three programs.  HydroCAD combines the most-used capabilities of TR-20 and TR-55, plus many other techniques and features not provided by either program.   We also have a step-by-step guide for comparing the results from these programs.






Runoff methodology

Unit hydrograph procedure (unlimited points), Santa Barbara Urban Hydrograph, Rational Method

Unit hydrograph procedure (301 points)

Tabular method derived from TR-20

Rainfall types

Extensive rainfall library, user-defined storms, & synthetic rainfall distributions generated from local precipitation frequency data.

SCS type I, IA, II & III

SCS type I, IA, II & III

Unit hydrograph

Extensive UH library including SCS, Delmarva, and others.

SCS Unit Hydrograph or user defined curve

SCS Unit Hydrograph only

Curve number entry

Automatic curve number lookup and weighting, with adjustment for unconnected impervious areas

Requires direct entry of composite curve number

Automatic curve number lookup and weighting, with adjustment for unconnected impervious areas.

Curve number weighting Area-weighted, or separate runoff for each CN with weighted flow. Requires direct entry of composite curve number Area-weighted curve number

Curve number limitations

No restrictions

No restrictions

Reduced accuracy as CN differs from 75

Time of concentration calculations

All TR-55 methods plus common channel shapes, upland method, CN method, and Kerby/Kirpich method

Contains no Tc calculations: Requires direct entry of Tc

Calculates Tc for sheet, shallow concentrated, & channel flow

Tc limitations

No restrictions

Must be greater than zero

Limited to .1 to 2 hrs

Tc precision

1/10 minute or 1/600 hour

1/100 hour

Rounded to nearest step

Subarea limitations



Must not differ by more than factor of 5

Runoff accuracy

Within 1% of 1982 TR-20 release

“The standard”

Within +25% of TR-20

Reach routing

Muskingum-Cunge or Storage-indication method with optional routing delay (translation)

Modified Att-Kin procedure

No routing, separate tables for travel times of 0 to 3 hours

Reach hydraulics

Automatic calculation for common shapes or custom cross-sections

None.  Requires entry of routing coefficients.

n/a, no reach routing

Pond routing

Storage-indication method, dynamic storage-indication method, or simultaneous routing


None, only estimates ponding effects

Pond hydraulics

Automatic calculations for orifice, weir, culvert, etc, used alone or combined for outlet structures

Must be entered directly, no calculations provided

n/a, no pond routing

Tailwater Dynamic tailwater handling with all standard outlet devices none - uses static stage-discharge curves none - uses static stage-discharge curves
Dam Breach Progressive dam breach triggered at specified time or WSE none none
Pumps Full pump simulations including pipe loses and tailwater effects none none

Pond Sizing Estimate

Using actual inflow hydrograph


Using assumed hydrograph shape

Detention Time

By plug-flow and center-of-mass methods

not calculated

not calculated

Pond storage calculations

Automatic calculation from pond dimensions or surface areas, plus direct entry

Direct entry only

n/a, no pond routing

Underground Storage Automatic chamber layout and modeling, including embedded storage calculations requires external calculations requires external calculations

Flow diversions

(split flows)

Automatic diversion of outflow from specified pond outlet(s)

Separate “Divert” procedure based on user-defined curve


Pollutant Loading Automatic land-use reporting and pollutant loading calculations. none none

Routing diagram

Interactive, on-screen, with labels and background image



Calculation procedure

Automatically calculated as required

“Batch mode” calculation of entire watershed

Manual initiation of each calculation

Units system

US/English, Metric, or custom, plus split input/output units

US/English only

US/English only

Graphics capabilities

Full graphics to screen, printer, or file




Automatic reports with headings, graphics, etc.



Data Storage

Automatic by job name




Note: This table reflects the earlier DOS versions of TR-20 and TR-55.

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