Climate Assessment and the Development of a Blended Drought Index for Water Crisis Management in Northwestern Italy

The Piedmont region (NW Italy), situated between the Mediterranean and Alpine climate hotspots, has recently experienced amplified climate change impacts on water resources. Indicators for 1958–2023 reveal increasing mean annual temperatures and reduced snow cover, as well as increasing drought frequency over the last five decades. The resulting conflicts over water allocation highlight the need for improved monitoring and forecasting. Most existing indices focus on agricultural impacts, failing to capture the phenomenon's multifaceted nature.

To address this gap, we present a new monthly Blended Drought Index (BDI) that integrates the Standardized Precipitation Evapotranspiration Index (SPEI), the Standardized Snowpack Index (SSPI), and the Standardized Runoff Index (SRI), thereby reflecting multiple components of the hydrological cycle, including the critical contribution of snow cover.

The BDI was validated qualitatively by comparing its values with newspaper records of drought events in Piedmont for the period 2000–2023. A further semi-quantitative validation correlated the BDI with water emergency interventions managed by SMAT, the water utility responsible for the integrated water service in the Metropolitan City of Turin. Both validations confirmed the ability of the BDI to describe water availability conditions and its capacity to predict drinking water shortages up to two months in advance.

Future projections for the 21st century indicate a progressive worsening of drought conditions, particularly under the RCP8.5 scenario. An increasing frequency and duration of severe droughts in the second half of the century are expected, especially at longer aggregation scales (24 and 48 months).

By incorporating snow cover data—a key component of water availability in Alpine areas—the BDI advances drought assessment and mitigation strategies in hydrologically complex regions. Once calibrated with local climatological and hydrological data, the index can be adapted to diverse geographical areas, enhancing drought monitoring and management.