HydroCAD Stormwater Modeling - Since 1986


Comparing Results to Other Programs

HydroCAD runoff calculations have been extensively cross-checked against TR-20, and generally match the TR-20 runoff volumes and peaks to within 1%.  This is demonstrated by the "TR-20 Sample Job #1" that is installed with HydroCAD.  See the comments at the top of each summary report for full details on the comparison.  Significant differences will occur only at short Tc values, where the coefficient-based rainfall tables used in HydroCAD will generally produce more accurate (higher) peak runoff.  However, runoff volumes will remain the same under all conditions.

Differences in results between HydroCAD and other programs are usually the result of different input data.  This might be the result of a misplaced decimal point, failure to observe required units (such as acres vs. square-feet), or a simple typo.

The best way to check your HydroCAD input is to read the applicable summary report.  The report shows all your input data, and tells you how these values have been interpreted by the software. Read the entire summary carefully. Check all units, especially areas and slopes. About 99% of the time this will reveal an input error that is responsible for the discrepancy.

Do not use the input screens to check your data. The summary report shows everything in one place and is much more readable.  Read the report from top to bottom and make sure you understand everything.  With HydroCAD 7.1 (and later), you can click any item in the on-screen summary for detailed help on that specific part of the report.

If a review of the summary report doesn't reveal the problem, then a more systematic review will be required, as outlined below.  Start with a single subcatchment at the head of the watershed, and then work downstream until the difference is resolved.  Do not jump ahead! You need to identify the first (upstream) place where the results are different.

Start by comparing your runoff results:

bulletSCS Method (also applies to SBUH method)
 
bulletRational Method (also applies to modified rational method)

If your runoff volume and peak are in agreement, you can proceed to compare your routing results:

bullet Pond Routing
 
bulletReach Routing

 

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