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 λ) 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 λ 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 λ seasonal analysis indicates values close to 0 in both the dormant and growing seasons.