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 table from which to select curve numbers:
The most efficient approach is to first select the curve numbers from the table using the Select CN button, then click on the Select Areas button and 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. If you click on the Select Areas button first, you will be prompted to either enter the curve number or type T to select a curve number from the table. The areas and curve numbers selected in this procedure overwrite any previous entries.
When all the land-use curve numbers and areas are entered, click on the Calc CN button to calculate the weighted curve number. This curve number can then be used in the Time of Concentration and Peak Flow commands. You can also save the table entries to a curve number (.cn) file and reload these values later.
To calculate the runoff given the weighted curve number, enter the rainfall for the storm in question and then click on the Calc Runoff button. The Runoff Volume equals the Runoff Q times the total area. A typical Report is shown here:
Pulldown Menu Location: Watershed
Keyboard Command: curveno
Prerequisite: None
File Names: \lsp\cntr_grd.arx, \lsp\hydro.dcl