Curve Numbers (CN) & Runoff

This command calculates the weighted curve number (CN) as used by the SCS Method of runoff calculation. It will also calculate total, potential runoff from an area. The curve number is used by routines based on the TR-55 program. The weighted curve number is a weighted average of the curve numbers for each subarea of the watershed. The weights are based on the areas. The Description and Soil Type fields are used in the report. Shown here is the Curve Number Library from which to select curve numbers. The initial Curve Number Library is taken from the SCS TR-55, Urban Hydrology for Small Watersheds. You can modify the library using the New and Edit buttons and you can use the Load and SaveAs buttons to store the library to separate .rcn files.

First highlight a row on the spreadsheet, and then select the curve number from the library using the Select CN button and click on the Select Subarea button to select all the subarea closed polylines. These polylines can be generated by the Sub-Watershed by Land Use command. The program will sum the polylines that are selected for a total area. The Subarea By Interior Point button allows you to select subarea closed polylines by picking inside the polylines. If you have the runoff layers defined for the watershed beforehand, you can simplify the process by selecting the closed polyline of the whole watershed, the subareas and their curve numbers, soil types, cover descriptions as well as area values will be filled into the spreadsheet automatically.

When all the land-use curve numbers and areas are entered, enter the rainfall for the storm in question and then click on the Calc button to calculate the weighted curve number and the runoff given the weighted curve number. This curve number can then be used in the Time of Concentration and Peak Flow commands. The Runoff Volume equals the Runoff Q times the total area. You can also save the table entries to a curve number (.cn) file and reload these values later.