Empirical Investigation of Curve Number Method Parameters in the Mediterranean Area

The curve number (CN) method is widely used as a technique for estimating surface runoff depth from rainstorms. This simply lumped method is based on the main parameter CN, which represents the lumped expression of basin absorption, and on a parameter that represents interception, infiltration during the early part of a storm, and surface depression storage, called initial abstraction. In this paper, CN is evaluated at the basin scale from rainfall-runoff multiday events, in the observation period 1940-1997 (recorded length mean equal to 20 years) for 61 Sicilian basins with three different methods: NEH4 method, asymptotic fitting method, and a least-squares method. A first analysis of Sicilian watershed behavior indicates a major occurrence of standard CN response (43 basins), as opposed to a complacent response (10 basins), and a few cases of violent behavior (3 basins). For basins with complacent behavior a modified formula of a runoff CN equation is proposed. The original assumption of the initial abstraction ratio (Ia/S or lambda) equal to 0.20 is investigated for watersheds with standard and violent CN response, using "natural" and "ordered" rainfall-runoff data. Results indicate a median lambda value of 0 for natural data and 0.05 for ordered data, according to recent worldwide studies. CN seasonal preliminary analysis indicates higher CNs in the dormant season versus in the growing season, whereas lambda seasonal analysis indicates values close to 0 in both the dormant and growing seasons. DOI: 10.1061/(ASCE)HE.1943-5584.0000570. (C) 2012 American Society of Civil Engineers.