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The origin and evolution of the USDA SCS curve number method for estimating runoff from small ungauged rural catchments is traced, and the characteristics of the method are examined. When the method is expressed as an infiltration equation, the infiltration rate becomes dependent on both total storm rainfall and rainfall intensity. When expressed as a spatially varied saturation overland flow model, the method implies that some part of any catchment has infinite surface storage capacity. The lack of physical reality in the formulation of the method is an inherent limitation to any further development. A major weakness is the sensitivity of estimated runoff to errors in the selection of the curve number. Changes of about 15-20% in the curve number doubles or halves the total estimated runoff. The results of some Australian studies where curve numbers have been calibrated against actual runoff data are collated.
WC Boughton (Fri,) studied this question.