Snowmelt plays a crucial role in the global water cycle, acting as a buffer between winter precipitation and summer water demand. This buffering effect is particularly significant in snow-dominated regions like the western United States and the Italian Alps, where snowmelt contributes substantially to streamflow. The increasing frequency and intensity of snow droughts, driven by climate change, threaten water security for billions of people who rely on mountain water resources. Understanding the mechanisms linking snow droughts in headwater basins to socio-hydrologic impacts in lowlands is critical for developing effective adaptation and mitigation strategies. This study focuses on the 2021-2022 snow drought in the Po basin, Italy's largest river basin, which supports a significant portion of the Italian population and economy. The research questions are: (i) To what extent do snow droughts affect water storage anomalies beyond streamflow deficits? (ii) What is the role of snowmelt deficit in driving water supply anomalies? (iii) How do the spatial distribution and timing of snowmelt deficits relate to escalating socio-hydrologic drought impacts? The Po basin's diverse climates and the parallels between the 2021-2022 snow drought and future climate projections make it an ideal case study for understanding the broader implications of snow droughts in warming mountain regions worldwide. The study employs a multi-faceted approach combining data from various sources to comprehensively analyze the event.

Existing research highlights the increasing intensity of snow droughts in the Northern Hemisphere since 1980, with projections suggesting a rise in frequency under warming climates. Studies have shown that snow droughts can escalate impacts, starting from snowmelt deficits and culminating in water scarcity and emergency water restrictions. However, the mechanisms linking snow droughts in headwaters to socio-hydrologic impacts in lowlands remain largely unexplored. Previous work has primarily focused on snow accumulation and melt, but there is a need to understand the role of snowmelt deficit beyond peak snow water equivalent (SWE) in driving water-supply anomalies and socio-hydrologic impacts. The connection between the spatial and temporal patterns of snowmelt deficit and the severity of downstream impacts is also under-researched.

This study leverages multiple data sources to analyze the 2021-2022 snow drought in the Po basin. Synoptic weather patterns were characterized using the ECMWF Reanalysis v5 (ERA5) dataset, which provides hourly snapshots of atmospheric variables. Spatially distributed snow water equivalent (SWE) anomalies were assessed using the IT-SNOW model reanalysis, which incorporates in-situ snow depth measurements and remote sensing data. Terrestrial water storage anomalies were derived from NASA's Gravity Recovery and Climate Experiment (GRACE) data. Streamflow data from 38 river sections across the Po basin were used to examine streamflow deficits and their correlation with upstream SWE. Finally, an inventory of 886 municipal emergency water-use restrictions was compiled from various sources to quantify socio-hydrologic impacts. Statistical methods, including the Mann-Kendall test, were employed to analyze trends and correlations between different datasets. Anomaly calculations utilized both dimensional and percentage anomalies relative to the 1991-2020 reference period (with some adjustments for specific variables and comparisons).

The 2021-2022 winter drought in the Italian Alps stemmed from a persistent high-pressure ridge resulting in significant temperature and precipitation anomalies across Europe. The Po basin experienced a striking spatial homogeneity in SWE deficit, with an average deficit of -88% compared to the 2011-2021 reference period. The study observed a quasi-stationary SWE during mid-winter due to a balance between occasional snowfalls and frequent melt events caused by higher-than-usual temperatures. This, coupled with earlier-than-usual snowmelt, exacerbated the drought conditions. GRACE data revealed unprecedentedly low terrestrial water storage in both winter and summer 2022, underscoring the impact of the antecedent winter snow deficit. Analysis of streamflow anomalies across 38 river sections in the Po basin indicated a shift towards more intense combined snow and streamflow droughts in recent years, with 2022 showing the highest anomalies. The timing of emergency water-use restrictions coincided with the peak in snowmelt deficit (June 2022) rather than the peak precipitation deficit (January 2022), emphasizing the critical role of snowmelt in triggering socio-hydrologic impacts. Spatial analysis showed a progression of water restrictions from headwater regions in spring to lowland areas in early summer, further supporting the importance of snowmelt availability.

The findings demonstrate the significant contribution of the antecedent winter snow deficit to the 2022 socio-hydrologic drought in the Po basin. The study highlights the importance of considering snowmelt deficit in addition to peak SWE when assessing drought impacts. The concurrent precipitation and temperature anomalies, coupled with the multi-decadal decline in terrestrial water storage, further exacerbated the drought severity. The strong correlation between snowmelt deficit and the timing of emergency water-use restrictions underscores the need for seasonal early-warning systems to better manage socio-hydrologic drought risks in snow-dominated regions. The different responses of peak SWE and melt-out dates along elevation gradients suggest that mitigation/adaptation strategies need to be tailored to specific altitude ranges.

This research provides strong evidence for the crucial role of winter snow deficit in exacerbating the summer 2022 socio-hydrologic drought in the Po basin. The study's findings highlight the importance of integrating snowmelt dynamics into drought forecasting and management strategies. Future research should focus on developing improved early-warning systems for snow-related droughts and exploring the complex interplay between snowmelt, terrestrial water storage, and socio-hydrologic impacts across different spatial and temporal scales. Refining the understanding of the regional hydrological response to climate change is also crucial to guide effective water resource management.

The study relies on data from various sources with varying spatial and temporal resolutions. The GRACE data, while valuable for assessing terrestrial water storage, has limitations in spatial and temporal resolution. The inventory of water-use restrictions, though comprehensive, might not be entirely exhaustive. While the study attempted to account for potential biases and limitations in the data, these factors could impact the generalizability of the findings. Future work should incorporate high-resolution data on water abstractions and use to improve the accuracy of the analysis.