poster - International Conference of Agricultural Engineering

data P (mm), gauged in 130 pluviographs placed inside the 61 watersheds published by
Dipartimento dell’Acqua e dei Rifiuti of Sicilian Region in the observation period 1940-1997
(mean record length equal to 20 years).
The authors evaluated the daily rainfall event P for each watershed using the Thiessen
Polygon method, while the correspondent direct runoff event Q i (mm) at day i was calculated
as
Q i = Qt i – Qb i (5)
where Qt i is total runoff at day i (mm, obtained dividing volumetric runoff by catchment area),
Qb i is daily baseflow at the day i (mm), evaluated by means of a single-parameter digital
filter:
Qbi 1 Qbi
1min Qt i;Qti4
(6)
where Qb i-1 is the baseflow at day i-1, Qt i-4 is the total runoff at day i-4 and (1-) is the
recession constant equal to 0.93 for South Italy (Manfreda et al. 1993).
Digital filter expressed by (6) is different from the original filter proposed by Chapman and
Maxwell (1996) because of the use of min(Qt i ;Qt i-4 ) instead of Q i . The latter assumption
allows to avoid the unrealistic sharp peak of baseflow right under the measured hydrograph
peak found in the Chapman digital filter (Tan et al. 2009).
Once obtained rainfall-runoff P i ,Q i data as mentioned above, D’Asaro & Grillone (2010, 2012)
evaluated CN emp at basin scale from rainfall-runoff multi-daily events (Mockus 1964),
computing about 35,000 events, using AFM and NEH4M (Table 2).
Results indicated that the NEH4M detects improbably high median CN values in a narrow
range for each watershed (Table 2), caused by the process of computing CN for small events
that biases the CN toward high values (Hawkins et al. 2009).
This evidence shows how the NEH4M leads to an overall unlikely rainfall-runoff response of
the Sicilian watersheds: most of P is transformed into Q, as if all basins were almost totally
composed of impervious areas. This unrealistic result underlines that the NEH4M is not able
to give a correct CN emp for Sicilian watersheds.
The results obtained for the AFM indicated that 43 out of 61 basins (about 75%) of the
studied watersheds have a standard behavior and 3 out of 61 (5%) have a violent response
(Table 2). Thus, the runoff curve number method can be correctly used and CN emp can be
defined in 80% of the Sicilian watersheds studied.
Using tabulated CNs, Viola et al. (2011) estimated CN w in 36 out of 46 watersheds classified
by D’Asaro & Grillone (2012) as basins where the curve number method can be correctly
used, basing on land cover and hydrologic soil properties Sicilian maps.
3. Results
The comparison between CN emp estimated using NEH4M (D’Asaro & Grillone, 2012) and
CN w (estimated by handbook CN tables, Viola et al. 2011) shows that CN emp are in a narrow
range (about 90) and there is no link with CN w (Fig. 2).
On the contrary, comparison between CN emp estimated using AFM (D’Asaro & Grillone,
2012) with CN w shows (Fig. 2) that CN w values are generally higher than corresponding
CN emp and their range is narrower than CN emp range.
Thus, it is clear that CN w does not match CN emp in a semi-arid climate, such as
Mediterranean area and in particular in Sicily, tending to overestimate watershed CN and
thus runoff Q.
Thus, in order to estimate the right CN L (lumped CN) in ungaged watersheds, it is preferable:
1. to estimate CN w using the tabulated CN
2. to “adjust” the CN w estimation using the following simple linear relationship:
CNL
0.8064CNw
7.95
(7)
obtained in this study for Sicilian watersheds (Fig.2). In this way the CN w is brought to a more
reliable value, close to the CN emp estimated by means of AFM, nowadays considered the
best way to evaluate the watershed CN (Woodward et al. 2010, D’Asaro & Grillone 2012).