Creating your own Rainfall Distribution

There is nothing magical about the SCS rainfall distributions. They are simply templates that the program uses to determine what the hydrograph is shaped like. Looking at one of the above distributions, it is apparent that it starts at zero (0) and ends and one (1). StormShed3G™ stores the rainfall distributions in a subdirectory under the program installation directory labeled RAC. The RAC subdirectory contains lots of files with the .rac extension (read “dot rac”). The format for each RAC file is similar to what is shown for the SCS rainfall distributions. Each file consists of:

Line 1: A description line.
Line 2: The interval in hours. Note that the interval appears first, followed by other text. Remainder of lines: The hyetograph points, starting from zero and progressing to one (1). The number of points times the interval gives the duration of the hyetograph.

Typically, how does one create a RAC file from rainfall data? Assume that you want to create a RAC six (6) hour hyetograph given the following hourly data:

Hour	Precip (in)	Accum (in)	hyetograph
0	0	0	0
1	0.15	0.15	0.1829
2	0.25	0.4	0.4878
3	0.20	0.6	0.7317
4	0.15	0.75	0.9146
5	0.05	0.80	0.9756
6	0.02	0.82	1.00

The values to enter in the table are simply each value in the accum column divided by the total precipitation, 0.82 inches. The RAC table does not require that the values are entered four (4) per row. Entering the values one per row is perfectly acceptable to the program. Typically the values are cut and paste into the RAC file.

Note: The RAC file is an ASCII file, i.e., it is a text file and cannot be a Microsoft Word document. However, it can be created in a program like Microsoft Word, but must be saved as a text document.