The Curve Number (CN) method is a cornerstone in runoff estimation, developed by the USDA Soil Conservation Service (SCS, now NRCS) in the 1950s to provide a simple, empirical approach to predict direct runoff from rainfall events based on land use, soil type, and antecedent conditions.
Initial CN values were obtained through empirical watershed studies, calibration with observed rainfall and runoff events, and statistical analyses on hundreds of small watershed datasets. The original tables classified soils into hydrologic groups (A–D) by infiltration capacity and established hydrologic conditions (good, fair, poor) for land cover, further subdivided by AMC (I, II, III). Over decades, CN tables expanded with regional modifications as data accumulated, including adjustments for unique land covers, hydrologic management practices, and calibration for urban stormwater systems. Advanced variants now incorporate probabilistic and slope-adjusted CNs to address spatial heterogeneity.
From the 1980s to 2000s, GIS technologies transformed Curve Number methodology from simple lookup tables and hand calculations to spatially distributed watershed models. Software tools like ArcHydro, HEC-HMS with GIS/DEM input, and HEC-GeoHMS became standard, enabling automated CN mapping, landscape-scale analysis, and seamless linkage with digital elevation models (DEMs), stream networks, and rainfall grids. The adoption of land cover databases such as NLCD (US) and CORINE (EU), as well as high-resolution soil datasets (e.g., HYSOGs250m), allowed for rapid and automated assignment of CNs across large areas using remote sensing and GIS raster analysis. Remote sensing, especially Landsat imagery, facilitated dynamic land cover change detection, a critical driver for contemporary CN estimation and spatially distributed runoff modeling.
In this section, I’ll walk you through a Python-powered web app that calculates curve number values for any watershed in just seconds. Start by watching the video below, then give the app a try yourself.
You can access the app here: https://huggingface.co/spaces/mohsennasab/CN_Generator
ArcGIS Pro Tutorial- Learn how to calculate CN in ArcGIS Pro:
Fundamentals of CN Hydrology and Create CN Raster Using ArcHydro Tools - ArcGIS Pro