UPDATE 2014-01-15: The current version of this Mathcad Prime 3.0 worksheet (along with a .pdf copy for users of older versions of Mathcad) can be found in the Civil Engineering section. This version (i.e. in the main documents section) will not be updated further.--FEL
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.