A Correlation Analysis of Extreme Climate Indices and River Flood Events in Northwestern Italy

Abstract:

The occurrence of floods is strongly related to specific climatic conditions
that favor extreme precipitation events. Although the impact of precipitation and
temperature patterns on river flows is a well discussed topic in hydrology, few studies
have focused on the rainfall and temperature distributions extremes in their relation
with peak discharges. This work presents a comparative analysis of Climate Change
Indices (ETCCDI) annual time series, calculated using the NorthWestern Italy Optimal
Interpolation (NWIOI) dataset, and maximum daily discharge time series in Piedmont
Region. The Spearman's rank correlation was used to determine which indices mostly
correlate with river peak flows, allowing to infer on the main physical processes involved
in the production of floods. The correlation hypothesis was verified with the Spearman's
rank correlation test, considering a Student's t distribution and a 5% significance level.
Moreover, the influence of climate variability on river flows was examined by correlating
the filtered time series and analysing trends. The methodology was first applied at
the catchment scale and then extended to the regional scale, by matching a spatially
averaged flood magnitude index with the spatially averaged ETCCDI indices. It is
suggested that the results obtained at the local scale can be effectively extended to
the regional scale, providing a distributed picture of the temporal correlation between
climate forcings and peak discharges in Northwestern Italy.