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.
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.