A 60-year drought analysis of meteorological data in the western Po River basin

Abstract

Since the start of the 21st century, increasing focus has been put on drought and its wide range of environmental and socioeconomic effects, particularly in the context of climate change. The identification of changes in drought occurrence has been done at different spatiotemporal scales and using different approaches, with results that may not be fully comparable. This study aims to analyse drought trends in the northwestern region of Italy, encompassing the Piedmont and Aosta Valley regions, characterized by diverse topography and warming rates. The analysis is carried out over the last 60 $\mathrm{years}$ using the Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration Index (SPEI) at 3- and 12-month timescales and deriving drought events at the local and regional spatial scales. By leveraging on a continuous and spatially coherent precipitation and temperature dataset, we explore the temporal and spatial variability of drought conditions and compare results obtained with different approaches.

Our results reveal widespread drying trends in the region, with temperature playing a crucial role. The SPEI indicates more extensive and steeper negative trends than the SPI due to temperature increases. However, the onset and cessation of drought events are predominantly driven by precipitation anomalies, while temperature plays a key role in longer-term drought conditions. Both the SPI and SPEI consistently identify local and regional drought events. In the 1990–2020 period, drought event severity, duration, and intensity generally increased compared to during the 1960–1990 period, even though this increase is less significant than the one shown by the SPI and SPEI. Nevertheless, the spatial scale of the analysis plays a significant role in interpreting these trends. Local-drought characteristics are more influenced by temperature increases in the SPEI, whereas regional droughts are more affected by precipitation patterns, as seen in the SPI, with more frequent short-term droughts aggregating into longer-term deficits. Drying trends are more pronounced in lower, less rugged areas, while alpine regions show fewer drought trends. Interestingly, drought characteristics and trends are found to be more correlated with terrain ruggedness than with mean elevation. In fact, a clear drying trend is not found at a region-wide level but is instead found when considering homogeneous areas defined by terrain ruggedness. Furthermore, changes in the number of drought episodes and in their severity, duration, and intensity are found to be correlated with terrain ruggedness at all timescales.

These findings emphasize the need for high-resolution, region-specific studies to better understand how droughts evolve in complex terrains like the northwestern Italian Alps. Future research should investigate whether similar outcomes are found in other regions and what the potential causes are as this is instrumental for evaluating how these trends may continue to evolve under projected climate change scenarios.