From drought to systemic shortage: a storage-aware diagnostic (SPI/SPEI–SRSI) for the interconnected Flumendosa system, Sardinia (1950–2024)

In regulated basins, drought impacts emerge when a meteorological signal is accumulated by storage and transformed by operations. We analyze the 2022 to 2024 episode in the Flumendosa system in Sardinia by coupling the climate signal from multi-scale SPI and SPEI for 1950 to 2024 at 9, 12, 24, and 36 months with a monthly Standardized Reservoir Storage Index (SRSI, 2006 to 2024) that normalizes reservoir volumes by season. The 1950 to 2021 baseline provides context for the recent evolution, and a scale and lag analysis links storage dynamics to antecedent climate at decision-relevant horizons.

Three features stand out. In 2022, SPI and SPEI at 12 and 24 months remained close to normal, yet SRSI declined through the year, indicating erosion of carryover despite the absence of a strong multi-season meteorological deficit. In 2023, short-horizon deficits at 9 to 12 months propagated into storage, with SRSI entering stressed classes for extended periods. By 2024, the system behaved as storage-limited, and intermittent climatic relief at short scales did not rebuild capacity because multi-season memory and operations had locked in a low-storage state.

The diagnostics are consistent with this progression. Coupling between SRSI and SPEI is strongest and most stable at 24 to 36 months with short lags of about zero to two months, reflecting the multi-season integration of reservoir systems, while 9 to 12 months best capture onset timing. Framed as onset at 9 to 12 months, operations and carryover at 12 to 24 months with SRSI, and persistence at about 36 months, the workflow explains why territories under similar meteorology can exhibit markedly different service outcomes. The method yields decision-ready outputs, including SRSI thresholds for restriction staging and carryover targets to protect next-season resilience, and it is reproducible and transferable to other Mediterranean, reservoir-dominated basins.