poster - International Conference of Agricultural Engineering

100
90
AFM
NEH4M
80
CNemp
70
60
50
CNemp = 0.8064 CNw + 7.95
40
40 50 60 70 CNw 80 90 100
FIGURE 2: Pairs (CN w ,CN emp ) with CN emp estimated using AFM and NEH4M and CN w
derived from CN tables (watersheds n. 31 and 57 are not represented because outliers).
Finally, in order to estimate CN in ungaged watersheds without using handbook CN tables, a
relationship between CN emp and watershed morpho-climatic and hydrologic characteristics
was investigated in Sicilian basins.
Using the following continuous variables: drainage area, river length, mean slope, mean
annual rainfall, mean annual temperature, average normalized difference vegetation index
(NDVI), mean basin altitude, gauging station altitude and the three versants of Sicily as a
categorical variable, a unique regression relationship was sought for the 46 watersheds with
standard and violent responses (Table 2). The following linear equations for watershed CN
estimation are obtained by stepwise regression:
CN = 96,52 - 0.034 P m (9a) for the north side of Sicily (versant 1 in Fig.1)
CN = - 2,33 + 0.117 P m (9b) for the south side of Sicily (versant 2 in Fig.1)
CN = 70,34 - 0.0025 P m (9c) for the oriental side of Sicily (versant 3 in Fig.1)
where P m is the mean annual rainfall in mm and the value of the regression coefficient R,
equal to 0.68, shows that equations (9) do not allow a good CN estimation. It is noteworthy
that only P m climatic variable resulted significant, mostly affecting CN estimation for versant
2. Thus, further investigations are necessary to explain watershed CN estimation in Sicily by
means of other variables.
4. Conclusions
The validity of handbook CN tables was investigated in Sicily (mid-Mediterranean area)
comparing the watershed CN w obtained using the hydrologic soil-cover complexes as
suggested by the SCS-NRCS tables, with curve numbers evaluated at basin scale from
rainfall-runoff multi-daily events in 36 Sicilian basins, CN emp , using the asymptotic fitting
method (AFM) and the NEH4M (NEH4 method as introduced by the SCS-NRCS).
The comparison between CN emp estimated using NEH4M (D’Asaro & Grillone, 2012) and
CN w (estimated by the handbook CN tables, Viola et al. 2011) shows that CN emp values are in
a narrow range (about 90) and there is no link with CN w , while the comparison between
CN emp estimated using AFM (D’Asaro & Grillone, 2012) and CN w shows that CN tab values are
generally higher than corresponding CN emp .
Finally, as far as the determination of an empirical relationship between CN L and some
morpho-climatic basin variables, only a link between CN L and the climatic variable mean
annual rainfall P m was found.