Conclusions

A widespread decreasing trend of annual rainfall is observed over 97% of the whole area from 1921 to 2001. The spatial average of trend value and MAP highlight the fact that the rainfall trend worsens or decreases as the MAP increases. This phenomenon is extremely worrying because high MAP areas are wide Apennine portions of the widest drainage basins of the artificial lakes which guarantee a relevant percentage of water supplies. The spatial mean of trend ranges from — 0.8 mm/a in Apulia to — 2.91 mm/a in Calabria.

The downward trend is mainly the effect of a succession of low-rain years; this succession is anomalous from about 1980, in terms of frequency and intensity of annual rainfall less than MAP; in this context, the droughts of 1988-1992 and 2000-2001, the worst since 1921, appear to be more important.

On a seasonal basis, the downward trend is concentrated in winter: the precipitation deficit of the last 20 years is mostly due to a reduced contribution of winter rainfall.

A Mann-Kendall test does not show a significant prevalence of negative or positive temperature trends. Although in some stations the highest temperatures have been recorded in the last ten years and a slight increase seems to prevail, especially from about 1980 onwards, this is not enough to determine a significant and generalized increasing temperature trend for the whole area.

The annual mean of net rainfall ranges from 52 to 1565 mm. The trend of net rainfall is everywhere strongly negative; in the whole study period the reduction of net rainfall can be roughly assessed from 27 to 33% of the annual mean.

The selected aquifers show high hydrogeologi-cal characteristics as confirmed by the consistent memory effect, which is not shorter than 4 months. This characteristic is of great importance during droughts or dry spells.

The cross-correlation with piezometric level shows that the variability of groundwater availability can be explained in terms of rainfall, temperature and, where it exists, river discharge variability. The significant lag or duration of this influence generally decreases from the first variable, rainfall, to the last, river discharge, and respectively from 3-5 to 1 -2 months. The intensity of this influence, in terms of maximum cross-correlation coefficient, is generally due to temperature, river discharge and rainfall in decreasing order.

The piezometric trend is on the decline everywhere, so widespread as to determine serious effects in terms of groundwater discharge sustain-ability. The worst trend in each aquifer or structure ranges from 0.06 to 0.41 m/a notwithstanding the limiting effect of sea level boundary condition. Detailed spatial studies show an average decrease of 7.93 m over the last 15 years in the Tavoliere, 1.1 m over 50 years in the Metaponto plain and 4.4 m over 70 years in the Sibari plain.

During 2002 the latest widespread and dramatic drought ended. On the basis of the data set available, the most likely piezometric trend, ending in the second half of 2002, was a very serious one over the entire area covered by porous aquifers, as in the case of the Tavoliere, the Metaponto plain and the Sibari plain. This situation is confirmed by sporadic data of 2003 in the case of the Murgia and the Salento, notwithstanding the effect of more than a year of abundant rainfall.

The whole piezometric downward trend appears to be due to the overlapping effects of natural recharge and of increasing well discharge. The effects of the latter phenomenon appear to be influenced by the progressive availability of surface water resources tapped by dams. The increasing role of these water resources to cover the water demand does not prevent a return to pumping water from aquifers during the recent and unusual droughts.

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