Isotopic Evidence from the Po River Under Prolonged Drought Conditions (Northern Italy, 2022–2023)

The Po River, the largest watercourse in northern Italy, represents a fundamental resource for the socio-economic system of the Padanian Plain. Between February 2022 and February 2023, the basin was affected by exceptional climatic anomalies, with unprecedented high temperatures, marked precipitation deficits, and the most severe hydrological drought documented in the instrumental record. Po river waters sampled during this period showed variable increases (Na+, K+, Mg2+, HCO3−, Cl−, SO42−) or decreases (Ca2+, NO3−) in the geochemical composition of major ions compared to data from previous decades collected under various climatic and hydrological conditions In contrast, the water stable isotope composition (δ2H and δ18O) of the period 2022–2023 displayed distinct and peculiar signatures, ranging from −64.1 to −53.5‰ for δ2H and from −9.4 to −5.7‰ for δ18O, compared to historical averages for 1998–2014 (−71.3 to −58.0‰ and −10.0 to −8.7‰, respectively). These values indicate a strong enrichment in heavy isotopes, reflecting warmer and drier climatic conditions, comparable only to those observed during the severe drought of 2015. Two groups of data were identified: Group 1, showing affinities with Eastern Mediterranean precipitation, and Group 2, characterized by pronounced evaporative isotopic enrichment due to prolonged drought, as evidenced by strongly negative d-excess and LC-excess values, consistent with those from arid and semi-arid regions worldwide. This study demonstrates how climate change and increasing hydrological stress are altering the isotopic composition of one of Europe’s most important river systems. Stable isotopes provide a sensitive tool for tracing moisture sources, quantifying evaporative processes, and assessing drought impacts, confirming their role as Essential Climate Variables (ECVs) in climate and water-resource studies.