Unveiling the Climatic Drivers of Multi-Year Droughts in Sardinia: A Study of Reservoir Storage and Precipitation Dynamics

Sardinia Island, situated in the Mediterranean Sea, is a water-scarce region frequently affected by severe multi-year droughts. This study investigates the dynamics of two distinct reservoir systems on the island—Bau Pressiu, a single reservoir with a small basin and limited storage capacity, and Flumendosa, a complex system of four interconnected reservoirs. By analyzing their monthly reservoir storage dynamics alongside the basin’s average monthly precipitation, we aim to understand their response to drought and its propagation. We employed the n-month Standardized Precipitation Index (SPI) and 1-month Standardized Storage Dynamics Index (SSDI), calculated using non-parametric fitting methods, to characterize precipitation and storage variability. Correlation analyses using Pearson and Kendall’s tau identified the precipitation accumulation period (propagation time) strongly correlated with storage dynamics. Contrasting operational rules and societal demands led to markedly different responses during droughts between the two systems. Continuous Wavelet Transform (CWT) and Cross Wavelet Transform (XWT) analyses revealed multiscale correlations between precipitation and reservoir storage. While precipitation exhibited independent multiscale power, reservoir signals displayed consistent annual-scale power linked to societal demand during summers and broader-scale patterns during severe droughts. Additionally, cross-wavelet analyses between SPI and large-scale climatic indicators, such as the Niño 3.4 index and Atlantic Multidecadal Oscillation (AMO), highlighted their significant but contrasting influences during multiyear droughts. Our findings confirm that both systems effectively mitigate short-term drought impacts. However, multiyear droughts, driven predominantly by large-scale climatic oscillations, severely strain reservoir systems and societal resilience, underscoring the so-called "reservoir effects". These insights are critical for improving water resource management strategies in drought-prone regions like Sardinia.

Keywords: multiyear drought, storage dynamics, wavelet analysis, climatic drivers, reservoir effect