Il ruolo della vegetazione sui principali processi idrologici in un tipico ecosistema mediterraneo soggetto a condizioni idriche limitanti

In the last decades, arid and semi-arid ecosystems, as the Mediterranean climate, are affected by a reduction in water source availability. Therefore, the need to understand hydrological processes emerges in order to assess the effects of climate change and the anthropic activities on the hydrological balance. The knowledge progresses are related to field measurements using advanced tools that allow to estimate the hydrological variables. This work focuses on the study of surface runoff and evapotranspiration processes. The study has been performed at the Orroli site, Sardinia (Italy). The landscape is covered by patchy vegetation: wild olives trees in clumps, herbaceous species, drying to bare soil in late spring. The climate is Mediterranean maritime with long droughts from May to October, and an historical mean yearly rain of about 670 mm concentrated in the autumn and winter months. Soil depth varies from 10 to 50 cm, with underlying fractured rocky layer of basalt. From 2003, a 10 m micrometeorological tower equipped with eddy-covariance system has been used for measuring water and energy surface fluxes, as well as key state variables (e.g. leaf and soil skin temperature, radiations, air humidity and wind velocity) An innovative experimental system, consisting of a rainfall simulator, allowed to study the surface runoff generation in relation to the grass vegetation cover status. Eight rainfall simulation tests have been performed in four different period, characterized by different vegetation status. Results highlight that vegetation increases the infiltration capacity of the soil both directly and indirectly, modifying the soil pore network through its radical apparatus. This change the hydraulic proprieties of the soil: in particular the saturated hydraulic conductivity shows a strong relationship with the height of the herbaceous cover. The combined data of sap flow, soil water content, and eddy covariance, revealed hydraulic redistribution system through the plant and the soil at different layers, allowing to quantify the reliance of the system on different horizontally and vertically differentiated soil compartments. Results shows that during light hours, until transpiration decreases in midday, shallow roots uptake deplete the water content in the upper layer. As transpiration decreases, hydraulically redistributed water provides for both transpiration of wild olives and recharge of shallow soil layers. This buffering, attained by long recharge time of shallow soil, allow woody vegetation to remain physiologically active during very dry conditions. The hydraulically redistributed water is the main source of water for evapotranspiration in the dry summer, and its relevance increases with decreasing water availability.