Understanding the Basis of the Curve Number Method for Watershed Models and TMDLs

The curve number (CN) rainfall-runoff method is described, with inferred extension to daily time-step models, which are the bases for most total maximum daily load (TMDL) assessments. Although originally developed for engineering design for extreme rainfall events, its flexibility, its basis in soils and land use, and its history of accepted use have encouraged adaptation and adjustment to the continuous daily time-step models needed for TMDL evaluations. The original event-based CN model itself, background assumptions, improvements, problems, and evolved usage are described. Included are the roles of the initial abstraction ratio (Ia : S), antecedent conditions (ARC), the limits of application with different land types, model sensitivity, effects of land slope, and seasonality. Inconsistencies and recent improvements are stressed, including (1) the adjustment of the abstraction coefficient Ia : S to 0.05 from the traditional value of 0.20; (2) awareness of the nonapplicability of the CN method in all cases and the preferred use of distributed/weighted runoffs from an array of CN source areas over the use of runoffs from averages CNs; and (3) use of local data and on-site inspections for calibration and perspective. The application of CN-based watershed hydrology models for TMDL studies should be conducted cautiously because it can produce a biased estimation of hydrograph components in certain soils and landscapes. Model parameter calibration in the application of CN method for continuous simulations of TMDLs is strongly suggested.