Minimum Rainfall for Applying the Curve-Number Method

Abstract

In engineering practice the curve-number method is sometimes inappropriately applied to low-rainfall events, thereby ignoring the dependence of the curve number on the rainfall amount. Lack of adjustment of the curve number for low rainfall can lead to substantial underestimation of runoff, and is particularly problematic in the design of water-quality components of stormwater-management systems. An expression is derived for the minimum rainfall required for the applicable curve number to be within 10% of its commonly cited handbook value, and a procedure is presented for adjusting the handbook curve number to account for low-design rainfall. A realistic example is presented to illustrate the procedure.