Climatic changes and their influence on hydrologic and hydrogeologic systems in Central Italy.

There are many reliable time series recording both rainfall and temperature since the end of the 19th century in western-central Italy. Up until now all of the time series with a statistically significant linear trend show a decrease in rainfall and an increase in temperature, with gradients up to -6 mm/yr (Terni station) and +0.004 °C/yr (Civitavecchia station). The causes of the detected climatic trends (as well as their persistence in the future) are far from being clear. They could be linked to the “anthropogenic greenhouse effect”, although there are many indications that these kind of variations have already occurred during the last 3000 years, due to unknown and natural causes. Some studies carried out recently indicate that, as expected, the observed climatic trends have influenced the discharge of some small hydrogeological systems (springs with recharge areas of up to 10 Km2) uninfluenced by the anthropic activity, which have been under observation for the last few decades. New data analyses carried out on large river basins, such as the Tiber (catchment area approx. 18,000 Km2), seem to indicate that large systems are also influenced by the observed trends, as they show a decrease in the average water yield. Although it is difficult to define quantitatively the influence of climate variability on large systems, due both to the complexity of the systems and to the considerable exploitation and anthropic pressure they are subject to, it appears that the climatic trend can no longer be neglected when dealing with the water resources management of large systems. In light of the current trends, and assuming that they will persist in the near future, it will be necessary to revise considerably the criteria used until now for managing water resources, including those for managing artificial reservoirs, storing water for hydroelectric, irrigation and drinking purposes.