Hydrological processes in the semi-arid small island of Pianosa: a multidisciplinary approach to increase knowledge, awareness and education on a highly climate-sensitive environment (HYDRO-ISLAND project UNESCO’s program)

Understanding and quantifying hydrology processes represent a mandatory step in semi-arid and arid regions for defining the vulnerability of these environments to climate change and human pressure, as well as for providing useful data to steer mitigation and resilience strategies. This generally valid concept becomes even more stringent for highly sensitive ecosystems, such as small islands.

It is the case of Pianosa Island (Tuscan Archipelago) that extends a few more than 10 km² within the Tyrrhenian Sea and it is characterised by a flat morphology (maximum altitude 29 m a.s.l.) and semi-arid climate conditions (550 mm and 17 °C as mean annual precipitation and temperature).

Because of the morphology and the medium-high permeability of superficial bio-calcarenite rocks, superficial water are absent. Nevertheless, the peculiar geological-hydrogeological setting guarantee a storage of groundwater in a phreatic aquifer and semi-confined/confined system, hitherto able to satisfy the local human water demand, mainly tied to seasonal tourism (thousands of visitors/year) and domestic exigencies (less than 30 permanent people). Evapotranspiration represents the most important voice of the water budget, given the windy and relative high temperature conditions.

In the precarious hydro-equilibrium for biosphere and human communities, and considering sea-level rise and climate regime trends that the Mediterranean is experiencing, HYDRO-ISLAND project (UNESCO’s program) intends to deploy a multi-disciplinary approach (geology, hydrogeology, geochemistry, geophysics, remote sensing-smart technology) for better understanding and quantifying the hydrological processes affecting the water availability and for sharing data and transfer knowledge to the community and younger generations, possibly suggesting best practices for water sustainability.

First results pointed out as over the last decade the annual rainfall weakly tended to increase, but at the same time such increasing resulted concentrated in summer and autumn seasons, whereas during winter and spring a decreasing tendency is even observed. This precipitation regime has led to a major rate of evapotranspiration and minor effective infiltration that caused a decreasing of piezometric level over several years. Quantity and chemical-isotopic features of rainfall and effective infiltration water measured/collected by a raingauge and a high precision lysimeter describe the hydrological processes at soil level and characterize the rate and seasonality of groundwater recharge in an experimental site. Using multispectral data by drone, we are trying to extend the experimental site information to a wider area in order to understand the general behaviour at island scale. Measurements, water sampling and analyses for shallow and deep wells, together with the study of geological constraints, are highlighting the distribution and relationship among different groundwater components, including the seawater that intrudes the aquifer from the SE side of the island. Furthermore, the comparative analyses of continuative data monitoring in wells and weather station showed the presence of possible concentrated water infiltration processes during rainfall extreme events that induce a quick response of groundwater systems in terms of water level rise and decrease of electrical conductivity. Thus, elements of vulnerability of the aquifer to pollution are pointed out, as well as the possibility to provide technical solutions for enhancing water infiltration and groundwater availability.