HydroCN
SCS Curve Number calculator: SSURGO soils + NLCD land cover fetched automatically from federal web services, with slope-resolved dual hydrologic groups, AMC adjustment, and NOAA Atlas 14 design-storm runoff.
Free and open source, from the maker of HydroComplete (https://hydrocomplete.com). HydroCN computes the area-weighted SCS Curve Number for a watershed or site (CONUS only) with zero manual data prep. It fetches SSURGO soil polygons and hydrologic groups (USDA SDA), NLCD 2021 land cover and imperviousness (MRLC), and a DEM (USGS 3DEP or state LiDAR) for the area of interest, resolves dual hydrologic groups (A/D, B/D, C/D) via a slope threshold, assigns TR-55 curve numbers (Good/Fair/Poor condition, or your own CSV table), and reports composite CN, per-class breakdown, mean imperviousness, and SCS runoff depths/volumes for NOAA Atlas 14 design storms. Outputs a CN polygon layer, a per-cell CN raster on the NLCD grid, and CSV/summary artifacts. Ported from the author's HydroCN Builder toolbox for ArcGIS Pro; the math and data sources are identical. Requires an internet connection.