SCS Curve Number

The method builds in an initial abstraction (Ia), typically 20% of the potential maximum retention S, that rainfall must exceed before any runoff begins — physically, this represents the rain intercepted by leaves and grass, ponded in small surface depressions, and infiltrated into dry soil before the ground becomes saturated enough to start shedding water. For a low curve number (well-drained, vegetated ground), S is large, so Ia is large, and a light storm can be entirely absorbed with zero calculated runoff — this is the method correctly reflecting that healthy, permeable land soaks up modest rainfall rather than a bug in the formula.

NRCS publishes detailed curve number tables organized by land cover type (forest, pasture, row crops, residential lots at various densities, pavement) crossed with hydrologic soil group (A through D, from well-drained sand to poorly-drained clay) — you look up the combination that matches your site rather than picking a number from memory. Real sites are rarely one uniform surface, so the standard practice is to compute an area-weighted composite CN across all the distinct land cover and soil combinations present, weighted by the fraction of total watershed area each covers.

Runoff depth (Q) is the portion of total rainfall that becomes surface flow leaving the site, while infiltration is the portion that soaks into the ground — together with the initial abstraction, these three quantities add up to the total precipitation you entered. A low infiltration figure relative to precipitation signals a site with limited capacity to absorb the storm, which is exactly the situation stormwater detention basins, permeable pavement, and other mitigation measures are designed to address.

Yes — the curve number method scales to any drainage area, from a single parcel to a large regional watershed, as long as you use a curve number and precipitation depth appropriate to that specific area. Many local stormwater ordinances require exactly this kind of lot-scale runoff calculation to size an individual site's detention or infiltration requirements, using a design storm depth specified by the local jurisdiction rather than an arbitrary rainfall figure.