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Rainfall-Runoff Modeling: The SCS-CN (NRCS) Method for Surface Runoff

 The Soil Conservation Service Curve Number (SCS-CN) Method estimates direct surface runoff (Q) from storm rainfall (P) based on land use, hydrologic soil group, and antecedent moisture condition (AMC). The governing water balance relation assumes that the ratio of actual retention to maximum potential retention (S) equals the ratio of actual runoff to potential runoff: $$Q = \frac{(P - I_a)^2}{(P - I_a) + S}$$ ​Where $I_a$ is initial abstraction (typically assumed as $I_a = 0.2 \cdot S).$ The potential maximum retention S (in mm) is directly linked to the non-dimensional Curve Number ($CN,$ ranging from 0 to 100) via: $$S = \frac{25400}{CN} - 254$$ ​Applying standard SCS-CN tables in Indian watersheds often over-estimates runoff due to static $I_a/S$ ratios that do not reflect monsoon dry-spell variations. ​Modern hydrological research across Indian catchments utilizes modified multi-factor SCS-CN models integrated with remote-sensing-derived land-use cover (LULC). By continuously...