UPDATE 2014-01-15: The attached .zip file now contains a Mathcad Prime 3.0 worksheet (.mcdx) and—for those of you who are still using earlier verisions of Mathcad—an Adobe Acrobat printout (.pdf) of the worksheet so can see how it is put together.
This worksheet performs a simple Rational Method calculation to estimate peak storm runoff from a single watershed. The Rational Method applies (very approximately) only to SMALL watersheds, though how small the watershed needs to be is an open question. A detailed description of the Rational Method and its uses and limitations is beyond the scope of this worksheet, but can be found in several of the references as well as many other sources.
Two methods are included for estimating the Time of Concentration: the Kirpich Formula and the FAA Formula. Both formulas, and others, can be found in Applied Hydrology [Ref. 1]. It is instructive to compare the different formulas for Tc because the results vary widely. Such a comparison, though, is beyond the scope of this worksheet. The selection of an appropriate formula for Tc is one of the many judgment calls required with the Rational Method.
This document is populated with sample data for a hypothetical watershed in Fresno, California. The development of coefficients for the IDF (Intensity-Duration-Frequency) Curves is covered in another worksheet (Lusk_IDF Curves.mcdx).