Il ruolo della vegetazione sui processi idrologici di ecosistemi Mediterranei in condizioni idriche limitanti

Climate change, human influences (deforestation, overgrazing, urbanization and pollution) and increased pressures on water supplies by civil and industrial utilizations are affecting the water availability in semi-arid Mediterranean regions. For instance, in Sardinia water resources supply decreased significantly in the last decades, such as shown by annual input from stream discharge into the reservoir system, which are the major source of water for the Sardinia Region. This happens for example in the Flumendosa reservoir system which constitutes the water supply for much of southern Sardinia, including the island's largest city, Cagliari. For this reason, in this study the fundamental processes (e.g., evapotranspiration and surface runoff) of the hydrologic balance have been studied extensively throughout the use of modern techniques for field site monitoring and the support of numerical models. In the first part of this work the evapotranspiration process in a typical Mediterranean ecosystem has been studied through the coupled use of sap flow sensor observations and eddy covariance measurements. Results show the key role of sap flow technique for the estimate of ET in such dry conditions, typical of Mediterranean ecosystems. In the next step the contrasting influences of vegetation differences (grassland versus mixed ecosystems) and soil differences (deeper alluvial valley soils versus shallow upland soils) on evapotranspiration (ET) and CO2 exchange dynamics have been examined. Data from two representative case study sites within the Flumendosa river basin on Sardinia were obtained. At both sites, land-surface and CO2 fluxes were estimated by eddy covariance instruments on micrometeorological towers. Fluxes at the two ecosystems were compared, and the effect of the vegetation cover was examined with the help of an ecohydrologic model to control for the different soil influences. The results show that the water and carbon fluxes in these ecosystems are more controlled by soil differences during the late spring, when the deeper soil depth leads to a doubling of the available moisture and an increase of 48% in the mixed natural vegetation transpiration. The system then switches to vegetation control in the summer as the presence or absence of drought-tolerant trees is the dominant imprint on continued transpiration and photosynthesis. In fact, total grassland ET in the summer is only 20% as large as the mixed vegetation ET in the summer. Finally the study of the surface runoff process have been developed experimentally in the Orroli field site to assess the runoff response of the land surface with varying vegetation states to ultimately predict how changes in the climate of Mediterranean watersheds may affect the needs of water resource management. For this purpose a 4 m by 4 m rainfall simulator was designed, constructed, and tested as the first stage of this research. The rainfall simulator consisted of four independent lines of low-cost pressure washing nozzles operated at a pressure of 80 mbar, with the number of nozzles determining the rainfall intensity delivered to the plot. The rainfall intensity of the simulator varies from approximately 31 to 62 mm/h with a coefficient of uniformity ranging from 0.62 to 0.75. Measurements taken include surface runoff using a tipping bucket flow meter and soil moisture throughout the plot. The simulator was used to monitor changes in the surface runoff throughout the seasons (July 2010, August 2010, June 2011, July 2011, December 2011, January 2012, May 2012) as the vegetation changes. Results shows the great impact of the changes in vegetation cover on soil runoff process: the vegetation cover is an important factor governing the infiltration process and consequently the surface runoff. The increase of vegetation cover induces large infiltration rates and it drastically reduces the surface runoff amount.