In Bentley StormCAD , the sub-basin runoff is defined solely by the CN input for each watershed. Bentley StormCAD features built-in spreadsheet forms that aid you by automatically computing weighted CN values as a function of soil hydrologic class and cover characteristics.

The USDA has classified its soil types into four hydrologic soil groups. The CN values for various land uses and cover characteristics for each soil classification are described below. To describe a sub-basin using CN, you must overlay a land cover layer over a hydrologic soil mapping overlay and a delineated drainage basin mapping overlay. You then determine the component CN areas that comprise each sub-basin, and enter these into Bentley StormCAD , which develops the actual weighted CN for use in hydrograph generation.