Climate change impact on water resources and forest sustainability of two Sardinian basins

Over the past century, climate change has been affecting precipitation regimes across the world (Giorgi et al., 2008). In the Mediterranean regions, there is a persistent declining trend of precipitation and runoff decreases (Martinez-Fernandez et al.2013), contributing to a desertification process with dramatic consequences for agricultural and water resources sustainability.

The position of the island of Sardinia, in the center of the western Mediterranean basin, with its low level of both urbanization and human activity, its complex orography with many mountains and alluvial valleys, and its strong correlation with North Atlantic Oscillation index makes Sardinia a primary reference for the investigation of climate change effects on Mediterranean ecosystems.

Two contrasting basins are investigated, the Rio Fluminimaggiore basin and Rio Flumendosa basin, which are different for position in the island (west side vs. east side), size (83km2 vs. 934  km2) and land covers (Rio Fluminimaggiore is mainly covered by forest while Rio Flumendosa land cover is mixed). These are basin are crucial for the water resources system of Sardinia, because include dams. In particular, two large dams, the Flumendosa Dam at Nuraghe and the Mulargia Dam at Monte Su Rei, are in the Flumendosa basin with a total reservoir of 600 million of cubic meter. Long database of hydrologic data (runoff, precipitation, temperature) are available for both basins from 1925, and in both basins eddy covariance towers are installed in representative field sites.

A distributed hydrological model at basin scale has been developed, which predicts runoff, soil water storage, evapotranspiration and grass and tree leaf area index (LAI). The model has been successfully calibrated for runoff estimation. An alarming historical decreasing trend of runoff and winter precipitation has been detected in both basins. We used the future climate scenarios predicted by Global climate models (GCM) in the Fifth Assessment report of the Intergovernmental Panel on Climate Change (IPCC). Hydro-meteorological scenarios are generated using a weather stochastic generator that allows simulation of hydrometeorological variables from GCM future scenarios. The use of the model allowed to predict soil water balance and vegetation dynamics for the generated hydrometeorological scenarios. Results demonstrated that tree dynamics are strongly influenced by the inter-annual variability of atmospheric forcing, with tree density changing according to seasonal rainfall. At the same time the tree dynamics affected the soil water balance. We demonstrated that future warmer scenarios will impact forest, which could be not able to adapt to the increasing droughts. The decrease of tree cover will affect water resources of the Sardinian basins. In the Flumendosa basin future scenarios predict a reduction of the runoff, which is crucial for the dam reservoir recharge. The water resources system planning needs to carefully takes into account the effect of future climate change on water resources and vegetation dynamics.