RUNOFF CURVE NUMBER METHOD IN SICILY: CN DETERMINATION AND ANALYSIS OF THE INITIAL ABSTRACTION RATIO

The Curve Number method is widely used in hydrology because it’s simply based on a single parameter, CN, that represents the basin absorption. In this paper CN is evaluated at basin scale from rainfall-runoff multi-daily events (Mockus, 1964), in the observation period 1940-1997 (record length mean equal to 20 years), for 61 Sicilian basin with three different methods: NEH4 method, Asymptotic fitting method (Hawkins, 1990, Hawkins et al., 2002, Hawkins et al., 2009), Least squares method (Woodward et al., 2006, Hawkins et al., 2009). A first analysis of Sicilian watershed behavior indicates a major occurrence of standard CN response (42 basins), rather than complacent response (11 basins) and no violent behavior. The original assumption of Initial abstraction ratio (Ia/S or lambda) equal to 0.20, is investigated for watersheds with standard CN response, using “natural” and “ordered” rainfall-runoff data. Results indicate a median lambda value of 0, for natural data and 0.035, for ordered data, according to recently world-wide researches (Hawkins et al., 2010).