poster - International Conference of Agricultural Engineering

In this approach, called asymptotic fitting method (AFM), the CN of the watershed is defined
in standard and violent cases as the constant CN value at higher rainfalls (CN ∞ ). If a clear
CN ∞ is not identified, an asymptotic equation suggested by Hawkins et al. 2009 allows to
determinate CN ∞ .
AFM uses all events, but of course the results are mostly influenced by the largest event,
which is in keeping with the usual intended applications of the method. For this reason this
procedure is now suggested in literature for the determination of watershed CN from rainfallrunoff
data, CN emp (Van Mullem et al. 2002; Hawkins et al. 2009).
As already mentioned, in ungaged sites, watershed CN is obtained by the handbook tables
developed by SCS-NRCS according to the hydrologic soil-cover complexes (USDA NRCS
2004, 2009), as area-weighted-average CN w .
2.2 Study area
The mountain ranges of Sicily (Fig. 1), the largest island of Italy and of the Mediterranean
Sea with its 25,700 km 2 , split up the island into three versants: the first one in the north, the
second one in the south and the last one in the oriental part.
Sicilian territory is 62% hilly, principally in the inland areas of the island, 24% mountainous,
mainly in the north and 14% plains in the coastal areas.
The 1,636,690 ha of agricultural land is mainly made up of: sown land (50%), olive groves
(15%), orchards (10%, with prevalence of citrus orchards), vineyards (10%).
The mean annual rainfall P m varies in the mountain ranges from 600 mm to 1,600 mm, while
in the rest of island P m varies from 300 mm to 800 mm.
The mean annual temperature T m is about 14-15 °C, with lower T m in the mountain ranges
(8-13 °C) and higher T m in the costal areas (18-19 °C).
Using P e T data from about 150 stations located all over the island, the Sicilian Agrometeorological
Informative Service (SIAS) obtained the De Martonne’s aridity index (2002),
which indicates a major presence of semi-arid and semi-humid areas rather than humid
zones.
According to the well-known Köppen climate classification, many authors (McKnight & Hess
2000; SIAS 2002; Peel et al. 2007) report that Sicily has a typical Mediterranean climate
(Group C -Temperate/mesothermal climates), with hot and dry summers.
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FIGURE 1: Location of 61 Sicilian watersheds studied and three Sicilian versants.
2.3 Data and previous studies
D’Asaro & Grillone (2010, 2012) collected daily total runoff data Qt (mm), measured in 61
Sicilian streamflow gauging stations (Fig. 1, Table1), and the correspondent daily rainfall