Climatic data and spatial approach

Rainfall and temperature monthly time series of the Italian hydrological service (Servizio Idrografico e Mareografico Nazionale, SIMN), have been considered (SIMN 1916-2000). A total of 126 rainfall gauges were finally selected from among 817 gauges (SIMN 1976) (Fig. 1). The gauges were selected to obtain a sufficient gauge density and spatial continuity mainly of rainfall and secondly of temperature, covering the maximum monitoring period with the minimum of data gaps; 41 of the selected gauges were also temperature gauges. The unpublished data are available courtesy of the Naples, Bari and Catanzaro SIMN departments. Data before 1915 were collected by Eredia (1918). The time series can be considered almost complete only from 1921 to 2001, the so- called main study period (MSP). Climatic time series utilized by previous authors (Polemio & Casarano 2004; Polemio et al. 2004a) have been enlarged, reducing gap percentages, and have been improved afterwards by homogeneity evaluation.

Residual gaps of time series are filled using multiple regressions based on a selection of the best correlated data series of the nearest gauges. The multiple regressions were performed on the normalized deviation of the considered value from the annual mean of the same time series, considering the best-correlated time series (r > 0.7) of the nearest gauges (up to six).

The spatial analysis of each variable is carried out by interpolation of point values (gauges) in a Geographical Information System (GIS) environment, operating with a 1-km spaced grid. The mean annual precipitation (MAP) plot, as in any case of variables related to rainfall, has been calculated in the MSP for each cell by weighting data from the 12 nearest gauges, with weights proportional to the inverse square of the distances (Fig. 1).

The climatic homogeneity of the study area has been evaluated on the basis of principal component analysis applied to the deviation of annual rainfall from the average normalized by the respective standard deviation; each time series or gauge is considered an individual while each year is a character or component.

Two main homogeneous climatic areas (HCAs) can be recognized, located along the Tyrrhenian and the Ionian-Adriatic coasts respectively (Fig. 2). The former can be divided into two sub-areas, Campania and the Tyrrhenian Basilicata-Calabria. The latter includes the Ionian Basilicata-Calabria and all of Apulia. Transitional sub-areas between the Tyrrhenian and Ionian-Adriatic influenced areas can be considered sub-areas of the SW portion of Calabria and Inner Basilicata.

The principal component analysis result is coherent with the spatial rainfall distribution in southern Italy (Fig. 1), mainly influenced by humid air circulation and by its arrival along the Italian coasts across the Mediterranean sea, moved by winds of the third and fourth quadrants and secondly by altitude and by the Apennine range which is located along the Tyrrhenian coast in Calabria and Basilicata and inland in Campania.

The statistical and spatial calculations (Table 1) have been performed considering both administrative regions (Fig. 1), where local governments are entirely in charge of water cycle management, and homogeneous climatic areas (Fig. 2). The results are quite similar; they are described here preferring the administrative zonation in order to enhance the message to water cycle managers.

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