Calculate C-Factor

The C-Factor is the C in the Q=CIA (quantity of flow = C * Intensity of Runoff * Area). This is known as the Rational Method of peak flow calculation, and is often used in smaller, urban areas, as opposed to the SCS Method which involves curve numbers (CN), and typically applies to agricultural and rural settings. However, both methods are used for flow calculations for all varieties of applications. The C factor is a maximum of 1 if all the water runs off (e.g. from a non-porous surface). C factors are very low for wooded, leafy, flat terrain (water is absorbed into the ground). For a site of mixed use, with roads, houses, driveways, lawns and woods, it is necessary to compute the net C factor as a weighted C factor based on the respective areas of distinct surface types. This routine calculates the weighted C factor by permitting selections of C-Factors and polylines, as shown in the dialog here:

C-Factor Calculation dialog

Referring to the subdivision drawing shown here, first select a C factor for a category from the C-Factor library by clicking on the Select C-Factor button, the C-Factor and subarea description will be filled automatically, and then click on the Select Subarea button to select all closed polylines of the areas for this category, to complete the highlighted entry. Repeat the process for other categories of the subareas. You can also enter all values manually. If you have the runoff layer defined for the watershed beforehand, you can simplify the process by selecting the closed polyline for the whole watershed, the subareas and their C-Factor as well as area values will be filled into the spreadsheet automatically. After entering all information, click on the Calc CF button to see the result. Clear button allows you to clear the spreadsheet for entering new data. Report button reports the subarea information and the C-Factor result.

Subdivision Drawing

C-Factor Report

Pulldown Menu Location: Watershed > Calculate C-Factor
Keyboard Command: calc_cfactor
Prerequisite: None