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.
Rainfall IDF (Intensity-Duration-Frequency) Curves are used in hydrology calculations to convert the Time of Concentration (Tc) to the appropriate Rainfall Intensity (i). IDF curves are available from a variety of sources, including state and local agencies, in electronic or paper formats. Most published IDF curves are in the form i=k*Tc^m, where k is positive and m is negative.
The purpose of this worksheet is to "reverse engineer" the equations for up to three IDF curves from scaled measurements made on paper IDF charts (= graphs). Logarithmic interpolation is used to determine the values for two points on each curve, then power functions are fitted to each pair of points. See Lusk_Logarithmic Interpolation.mcdx for the basic technique. Finally, using the calculated coefficients for these power functions as a guide, the user can select rounded off coefficients to employ in future calculations.