Influence of the geological setting on the response to climatic change and to increasing prolonged drought periods of fractured limestones system.

It is widely known that, in Southern Europe and the Mediterranean area, in the last hundred years, the temperature has risen by about 1°C, and the average annual precipitation has increased. The analysis at different time scales of historical thermo-pluviometric series recorded in South/Central Italy shows a higher frequency and duration of droughts in the last few decades: the number of three years lasting dry periods recorded since 1990 are as many as those recorded in the interval 1921-1989. The hydrogeological processes regulating both ground and surface water availability are greatly influenced by climate change. If the present trend should continue, a reduction of total yield of 10-20% with respect to present should be expected in the next 50 years. This work shows that the response of a specific hydrogeological system to climate change closely depends on the geological and lithological characteristics of the system. The aquifers hosted in the karst/fractured limestones extensively outcropping in the mountain areas of Central Italy, which supply several mountain springs with high-quality water, are taken as examples. The recharge areas of the springs fed by these aquifers are unaffected by human activity: the analysis of spring discharges is therefore useful to understand the effect of climate change on groundwater regimes. Some of the springs of the Umbria-Marche Apennines, (e.g., Bagnara, Scirca, Pescara di Arquata del Tronto) have long, continuous discharge data sets which show statistically significant negative trends of mean annual and monthly discharges. The Bagnara spring, which is of great importance on the water supplying point of view, is an example of “local system” representing the “overflow” of a deeper regional groundwater circulation, connected with larger “base springs”, the quality of which is often poor due to interactions with evaporitic sediments of Triassic age. Springs of this kind are quite frequent in the Umbria-Marche region. A dynamic groundwater divide, the position of which depends on the amount of recharge, separates the recharge areas of the regional flow from those of local springs as Bagnara: low recharge periods correspond to a low level of the piezometric surface, with the piezometric divide shifted towards the systems located at higher elevations and a consequent reduction of their recharge areas. On the contrary, the other system taken as an example in this work, the Lupa spring, although similar to Bagnara for lithological, topographical and climate characteristics and for mean annual discharge (about 120 l/s), has a geologically defined recharge area the extension of which does not depend on the recharge. The analysis of the recession curves of these two springs showed that the response to prolonged drought periods is more severe for the Bagnara than for the Lupa spring; this result can be extrapolated to all the springs connected to a deep regional groundwater circulation, which are more vulnerable to climate change than those with invariable geologically defined recharge areas.