Valle della Gallina basin
Location
The small basin has drainage divide belonging to the Alpine hydrographic network of the river Sesia, a tributary of the Po, and forms a foothill region or buffer zone between the Alps and the upper Po Valley.
Rainfall events range between 1300 and 1400 mm isohyets (1921-1970) and the pluviometric regime is of a continental Mediterranean-type, with minimum average pluviometric coefficients of 0.4 and 0.5 (in January and December respectively) and maximum average pluviometric coefficients of 1.2 and 1.5 (November and May respectively) (Caroni, 1979).
The pluviometric trend of the small basin is comparable to the regional trend. Instrument data (1982-2006) show an average value equal to 1300 mm/year for rainfall amounts, with a first peak in spring and a second peak in autumn. The runoff shows an average of 750 mm/year, with an average discharge of 0.02 m³/s and a runoff coefficient of 0.57. The maximum flow was 6.44 m³/s in 1995.
The annual sediment transport at the basin outlet has varied between 1m3 (1985) and 73 m³(2002), depending on the availability of sediment in transit in the stream network and all effective discharge, mobilization and transport conditions being equal.
As regards the sediment yield and erosive rainfalls, a survey carried out between 1991-2000 showed 280 erosive rainfall events selected at the standard threshold of 12.6 mm/event and their number varied between 20 and 37 per year. On an annual scale, the correlation between accumulated sediment volumes and erosive rainfall events and their duration is almost equivalent to the total annual rainfall events, with correlation indexes of about 8 (Maggi et al., 2003).
The basin, located at an average 417 m. a. s. l., with a surface area of 1.08 km² and a length of 1.35 Km along the basin axis, has an average slope of 43% (Caroni, 1982).
Sediment yield in the streams is favoured by the wide dendritic drainage network (52 km/km2) which has developed all the way from the first-order heads. The yield derives from erosion of the breaking up and meteoric alteration products of the bedrock, which discontinuously generate regolithic soil with a maximum thickness of 2 meters.
In 1975 the basin had wooded vegetation cover (coppice) over about 75% of the surface, mostly distributed from the streams to just over half way up the slopes. This study has revealed a reduction since 1992 in sediment yield due to a progressive expansion of the vegetation towards the heads causing a reduction in the soil erosion which fed the sediment transport to the stream channels.
