The Hydrologic regime over the last 90 years in the Flumendosa basin, Sardinia: the effect of climate change.

Abstract:

Dry periods increased in frequency over the Mediterranean climates with dramatic effects on the management of ecosystems, agriculture, and drinking water supplies. In the semi-arid Mediterranean regions, such as Sardinian island, dam reservoirs are key elements of the water resources system, because they accumulate autumn and winter runoff, which is then available for the dry months. In this sense, also the seasonal precipitation distribution and its time and spatial variability can be considered hydrologically important. In the Flumendosa basin (Sardinia), which is characterized by a reservoir system that supplies water to the main city of Sardinia, Cagliari, a drastic runoff reduction has been observed in last thirty years. Using data from 41 rain gauges stations (1922-2007 period) and runoff data at the outlet section, we show that: 1) the annual runoff in the latter part of the 20th century was less than half the historic average rate, 2) while the precipitation over the Flumendosa basin has decreased, but not at such a drastic rate as the discharge, suggesting a marked non-linear response of discharge to precipitation changes. Indeed runoff coefficient anomaly are strictly related to winter precipitation (correlation coefficient of 0.5). Trends in precipitation series, in the number of wet days and temperature were examined using the Mann-Kendall non parametric trend test, showing negative trends (-0.25 in winter for both precipitation and runoff). The relationship between winter precipitation and large scale pressure indices, such as the North Atlantic Oscillation (NAO), has been also investigated. High correlation between NAO and precipitation and runoff, -0.48 and -0.42 respectively, during the winter season is estimated for the whole basin homogenously, highlighting the large impact of NAO on the hydrologic dynamics of this Sardinian basin located in the east coast of the island.