The Po River Water Isotopes during the Drought Condition of the Year 2017

Abstract

The year 2017 was anomalously warm and dry across the whole Italian Peninsula, and the paucity of precipitation was emphasized during the extreme summer drought of the main Italian river, i.e., the Po river, which was characterized by a discharge persistently below 600 m 3 /s (in spite of the average discharge of 1500 m 3 /s). During these extreme conditions, the Po river oxygen and hydrogen stable isotopes (δD, δ 18 O) displayed a relation (δD = 6.1*δ 18 O − 6.6) significantly different from that recorded in the previous investigation of the years 2012-2014 (δD = 7.5*δ 18 O + 6.5). The lowering of the slope and the negative intercept clearly reflect the transition toward arid conditions that characterized the investigated period. The difference is expressed by the derivative parameter Line-Conditioned excess (LC-exc), which better describes the compositional difference of Po river water in the year 2017 with respect to that of the period 2012-2014, when the system was less affected by warm or dry conditions and the river discharge was more similar to the historical trends. The isotopic anomaly observed in 2017 throughout the river is even greater in the terminal part of the river, where in the meanders of the deltaic branches, the river flow progressively slows down, suffering significant evaporation. The isotopic signature of the water appears, therefore, an appropriate tool to monitor the watershed response to evolving environmental conditions. These sensitive isotopic parameters could be interpreted as "essential climate variables" (ECV) that are physical, chemical, or biological geo-referenced parameters that critically contribute to the characterization of Earth's climate. Future research needs to find relationships between ECV (including the water stable isotopes) and the evolution of ecosystems, which especially in the Mediterranean area, appear to be fragile and severely affected by natural and anthropogenic processes.

Headwaters’ Isotopic Signature as a Tracer of Stream Origins and Climatic Anomalies: Evidence from the Italian Alps in Summer 2018

“…The study will provide additional information for the climate change debate, as by proposing water isotopes as essential climate variables (ECVs,[38]), it will help corroborate studies that have already proposed relationships between climatic conditions and isotopic fingerprints [39][40][41][42][43].…”

“…), which is +2.3 • C with respect to the historical series (Supplementary Figure S1); in doing so, we obtained gradients of 0.5% / • C and 8% / • C for δ 18 O and δ 2 H, respectively. In our view, this observation implies that river water isotopes can be ascribed as ECVs [38,41] that are useful in monitoring sensitive ecosystems threatened by global warming. On the other hand, it is important to take also into account that different sub-basins are included in the Noce river basin and in the Sarca river basin.…”

“…For instance, they can be used to separate the contribution of each streamflow component [25], although evaporation processes may complicate this framework [25,36,37]. Within this context, we tested the reliability of water stable isotopes, in addition to hydrochemical parameters, as spatio-temporal tracers in order to identify (i) water origin, being the isotopic signature of stream water inherited from snowmelt, ice melt, groundwater, and rainfall, that fed headwaters in different proportions; and (ii) regional (year-specific) meteorological conditions, being the isotopic signature of precipitations affected by air temperature, humidity, and aqueous vapor origin.For this purpose, we investigated the ionic and isotopic composition (δ 18 O and δ 2 H) of high-altitude streams in the Italian Alps during the ablation season, in early and late summer 2018.The study will provide additional information for the climate change debate, as by proposing water isotopes as essential climate variables (ECVs,[38]), it will help corroborate studies that have already proposed relationships between climatic conditions and isotopic fingerprints [39][40][41][42][43].…”

“…These standards, obtained from the Los Gatos Research Company, were calibrated according to international IAEA (International, Atomic Energy Agency) standards. Analytical precision and accuracy, based on replicate analyses of standards, were better than 0.3% and 1.0% for δ 18 O and δ 2 H, respectively [39][40][41][42].…”

“…The regression line between δ 2 H and δ 18 O can also be calculated for water samples of different types, such as river waters, soil waters or groundwater. In the case of river water, regression lines can be defined as the river water line (RWL; 41,42,59). In this paper, we propose a regression line relative to Alpine headwater, using the isotopic data included in this study as a tool to characterize hydrological conditions in periglacial systems.…”