Daily rain sampling in Manaus links the 2023-24 Amazon drought to shifted moisture sources

Heavy water molecules condense first, and lighter ones evaporate more readily ^[6]. That fractionation is what turns rain into a record of its own route. An air mass that has already dropped rain several times on the way inland arrives depleted in heavy isotopes, and the ratio in the rain that finally falls carries that history. "The ratios between light and heavy isotopes are altered by the evaporation and condensation of water throughout the hydrological cycle," said Rafaela Rodrigues Gomes, the geographer and doctoral candidate at UNESP in Rio Claro who is first author of the paper, published in June in Hydrological Processes and based on her master's thesis ^{[7] [8]}. The band she found in control of the wet-season ratios, the Intertropical Convergence Zone, delivers 30% of the world's precipitation ^[16].

Two hundred and seven samples across the 24 months from March 2023 to February 2025 is one sample for about 28% of the 731 days in that window, or one every three and a half days on average ^[1]. Samples come only when it rains, so the series thins in the dry season and thins most in a drought, which is the condition the method is being offered to anticipate ^[4].

According to phys.org, the study suggests isotopes can provide an early warning of prolonged droughts ^[4]. The result underneath that suggestion is a seasonal climatology: which atmospheric system sets the ratio in which months ^{[15] [17]}. How much warning is a separate matter. No lead time in days appears in the account, and no test against the river flows that fell in 2023 and 2024 ^[1]. The window covers one drought episode, so the denominator for any claim about detecting onset is one.

Monthly rainwater sampling in Manaus ran until 1990, and then nothing replaced it for 30 years ^{[10] [2]}. "Only now, starting in 2020, thanks to the efforts of our group, the National Institute for Amazonian Research (INPA), the Geological Survey of Brazil and other institutions, we're implementing a new monitoring network in the region," said Didier Gastmans, the geologist who coordinates UNESP's isotope laboratory and advised Gomes ^{[11] [9]}. Monthly totals smooth out the day-to-day swings the new series is built on. There is no matching earlier record at daily resolution against which 2023-24 can be called anomalous.

Gomes checked the isotope series against a weather station installed beside the rain collector, then against global data and models from the European Centre for Medium-Range Weather Forecasts and NOAA ^[14]. The samples themselves were split between three laboratories, one of them the Tracer Hydrology Group at the University of Texas at Arlington ^[13]. The published account carries no cost figures for collection or analysis.