Curve Number: Empirical Evaluation and Comparison with Curve Number Handbook Tables in Sicily

The curve number (CN) method is widely used for estimating direct runoff depth from rainstorms. The procedure is on the basis of the parameter CN, a lumped expression of basin absorption and runoff potential, and a second parameter, initial abstraction (IA), which represents the interception, infiltration, and surface depression during the early part of a storm. The evaluation of CN in Sicily at a basin scale from rainfall-runoff multiday events is done using rainfall-runoff observations during the period 1940–1997 (mean record length of 20 years) in 61 Sicilian watersheds using three different methods: (1) the national engineering handbook, section 4 hydrology (NEH4) method (NEH4M) (the median CN for the annual flood events); (2) asymptotic fitting of ordered and natural data; and (3) least-squares method using rainfall-runoff using both ordered and natural data. Asymptotic fitting showed a major occurrence of the standard CN response (43 basins), with a lesser complacent response (10 basins), and a few cases of violent behavior (three basins). For five basins, the data quality and/or small sample size did not allow the identification of a CN. The least-squares and asymptotic fittings furnished similar results but showed that the original assumption of the initial abstraction ratio (IA/S or λ) of 0.20 in watersheds with a standard and violent CN response is unusually high. Using both natural and ordered rainfall-runoff data, a median λ value of 0 and 0.05 was found for natural data and ordered data, respectively. The CNs found by the recommended handbook method (NEH4 method) were higher than those found from the handbook tables. The asymptotic fitting and least-squares fitting methods were all lower than the table values. Comparisons were made between the CNs estimated using hydrologic soil-cover complexes on the basis of Soil Conservation Service (SCS)–National Resources Conservation Service (NRCS) handbook tables and the CN evaluated from rainfall-runoff multiday events in 36 of the basins. Overall, although comparisons were poor, the NEH4 method and asymptotic fitting method that used all of the ordered data were closest to the CNs from the handbook tables. The link between the CN estimated with the asymptotic fitting method (AFM) (T-series, ordered data) and the watersheds’ morphoclimatic and hydrologic characteristics was also investigated, and a useful relationship for basins with a larger drainage area (SDA>160  km2) was found.