Isotopic fingerprint of precipitation in NW Argentina

The isotopic composition of rainfall varies spatially and temporarily, depending on the climatic phenomena that originate the movement of air masses, their moisture content and the isotope fractionation processes that occur until precipitation falls isotopic composition of rainfall varies spatially and temporarily, depending on the climatic phenomena that originate the movement of air masses, their moisture content and the isotope fractionation processes that occur until precipitation falls.

One passive precipitation collector was installed in the lower part of Los Pozuelos basin, is located in the extreme northwest of Argentina, in the province of Jujuy. 19 accumulated precipitation samples were collected in the installed passive precipitation collector from 26 January, 2020 to 27 March, 2023. The objectives of this study are: (I) characterizing climatic variables; (II) Characterizing the isotopic composition of precipitation; (III) Establishing relationships between stable isotopes and the identified variables; and (IV) Identifying the trajectories of the air masses responsible for precipitation.

The time series of air temperature, relative humidity, precipitation, wind speed, solar radiation, OLR and SST exhibit a clear seasonal pattern, with the exception of the SST anomaly. The variables generally show a parametric distribution, except for daily rain. The δ¹⁸O and δ²H values of the 19 precipitation samples collected show interannual variation. The summer precipitation is depleted in heavy isotopes, has a high d-excess value and corresponds to the highest precipitation rates. This is due to the fact that the moisture masses have a greater continental extent in summer and convective precipitation dominates, both in the Amazon region and in the central and northern mountain ranges of the Andes. This is reflected in the high values of convective precipitation rate (CPR) and the lowest ORL values in the Los Pozuelos basin during the year. In winter, the heavy isotope enrichment in precipitation is due to the colder sea surface temperatures and lower evaporation associated with the Pacific Ocean compared to the Atlantic. Finally, the relationship of the isotopic composition of precipitation to the identified variables was determined. δ¹⁸O and δ²H show a high and direct correlation to each other, but inversely to precipitation amount and relative humidity. D-excess shows a moderate degree of correlation and the same tendency to increase as OLR.

The back trajectories of the HYSPLIT model air masses indicates that in summer, the dominant source of humidity was the Atlantic Ocean, which crossed both the Amazon basin and the Río de la Plata and Gran Chaco basins. A smaller percentage of the air masses is blown over by westerly winds from the Pacific Ocean. This is because the period of study is influenced by La Niña events, which cause an intensification of the westerly winds. In spring, all air masses come from the Pacific Ocean. In winter, the dominant source of humidity was the Pacific Ocean. The application of this methodology in the Los Pozuelos basin validated the appropriateness of our methods, contributing positively to the overall comprehension of water resource dynamics in the region.