- hydrology
- stormwater
The runoff curve number is a single coefficient between 0 and 100 that captures how much of a rainfall a piece of land turns into runoff. A low CN (forest on sandy HSG A soil) absorbs most of a storm; a high CN (pavement, or clay HSG D soil) sheds nearly all of it. The NRCS method looks the number up from a land-cover × soil-group table.
Curve number is where land-use change becomes hydrology. Converting cropland or woods to the roofs and parking of a large campus raises the site CN, so the same storm yields more runoff, faster — the effect a post-construction stormwater design and its erosion and sediment control have to hold. Because the output is sensitive to the CN, a small error in the assumed soil group or cover class propagates straight into the modeled peak flow.