Isotopes improve estimates of precipitation recycling in tropics

Using stable isotopes as tracers for evaporated moisture helps researchers understand local rainfall patterns across globe

January 12, 2022

Precipitation recycling happens when rain falls on land, then the water evaporates back into the atmosphere to form clouds that rain again in the same local region. Los Alamos researchers showed that using stable isotopes of rainfall as tracers helps overcome challenges to quantifying this ecologically important phenomenon around the globe. Credit: Dreamstime

Precipitation recycling, whereby rain falling on the Earth’s surface evaporates to fall again locally, is crucial to many regional ecosystems and water supplies, but is poorly represented in large-scale earth-system models because it is difficult to directly quantify. Recent research using stable isotopes in rainfall as naturally occurring tracers for recycled water suggests a new method for improving models of precipitation recycling across the global tropics.

In a new paper, a research team led by Los Alamos National Laboratory studied how deuterium excess (d-excess), a stable-isotope quantity sensitive to recycling effects, acts as an observational proxy for recycling. While previous studies have used this method at local or regional scales, the Los Alamos research compared results from more than 3,000 precipitation isotope samples from across the global tropics against three contemporary recycling models.

“We found strong agreement between d-excess measurements and model-generated recycling ratios for a wide range of models and tropical regions,” said Kurt Solander, a coauthor of the paper and scientist at Los Alamos National Laboratory, where he studies water and vegetation dynamics. “This suggests the importance of incorporating d-excess into next-generation recycling models to evaluate model performance.”

As temperatures on Earth rise, precipitation recycling becomes even more essential to sustaining regional ecological conditions and water supplies, particularly in the tropics. Precipitation recycling is also important for tracking moisture as it moves through the water cycle, which is central to modeling the climate accurately because of its interaction with heat and energy in the atmosphere.

“Precipitation recycling is challenging to precisely quantify,” Solander said. “Yet it’s essential to be able to evaluate recycling models especially since precipitation recycling cannot be directly measured at global scales.”

Solander and his colleagues studied extensive data related to the regional recycling ratio, the fraction of locally sourced precipitation occurring over a region. The team also looked at two physically based models used to calculate land recycling ratios. The land recycling ratio is the percentage of precipitation in each region contributed by evaporation from the land surface. Precipitation d-excess correlates with land recycling because of the differences in how different stable isotopes in water separate when evaporation occurs over land.

The Los Alamos team of Stephen Cropper (a recent Los Alamos student), Kurt Solander, Brent Newman, and Chonggang Xu  analyzed the isotope data in the context of rainfall recycling in collaboration with precipitation modelers Obbe A. Tuinenburg, Arie Staal, and Jolanda J. E. Theeuwen.

Paper:Comparing deuterium excess to large-scale precipitation recycling models in the tropics,“ by Stephen Cropper, Kurt Solander, Brent D. Newman, Obbe A. Tuinenburg, Arie Staal, Jolanda J. E. Theeuwen, and Chonggang Xu, npj Climate and Atmospheric Science, DOI: 10.1038/s41612-021-00217-3.

Funding: This work was supported by the Department of Energy (DOE), Office of Science, Next Generation Ecosystem Experiments­­–Tropics project, and the Office of Workforce Development for Teachers and Scientists under the Science Undergraduate Laboratory Internships (SULI) Program.