December 02, 2021

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Using Isotopes to Constrain Modeled Estimates of Local Water Availability

Observations of precipitation isotopes are used to determine the spatial and temporal performance of modeled precipitation recycling in the tropics.

Comparison of mean monthly recycling ratios obtained from models (blue) to precipitation isotopes (deuterium, D) (red). The Kendall-Tau statistic is used to track model performance where 1 and 0 denote a perfect or no relationship between the two quantities.

[Reprinted under a Creative Commons Attribution 4.0 International License (CC BY 4.0) from Cropper, S., et al. “Comparing deuterium excess to large-scale precipitation recycling models in the tropics.” NPJ Climate and Atmospheric Science 4, 60 (2021). DOI: 10.1038/s41612-021-00217-3]

The Science

Precipitation recycling represents the amount of rainfall whose water originated from local plant transpiration or evaporation to the atmosphere. Heavily forested areas like the Amazon rainforest are known to get one-third to one-half of its water from precipitation recycling. Because field sampling can be challenging over large scales, modeled precipitation recycling estimates are often used, whereas precipitation isotopes are primarily used for local measurements. By accumulating isotopic observations through space and time, this study provided the first global-scale assessment of modeled precipitation recycling estimates over the humid tropics.

The Impact

Variability in precipitation recycling ratios has important implications for water availability of plants as well as tracking water movement through the water cycle. Thus, changes in this quantity are important to understand, as they may provide direct indications of health and sustainability of forest ecosystems. The new approach presented in this study can be used to check and improve model performance. Such efforts will be critical to understand and predict how plants and the water cycle will be impacted by climate change at local to regional scales.


The amount of precipitable water derived locally through evaporation from the land surface or transpiration from plants is known as precipitation recycling. By transpiring water that recently fell as rain, plants are effectively recycling water back to the atmosphere, so it can fall as rain again. A multi-international team of scientists comprised of both modelers and experimentalists developed a new approach that uses observed isotopes in the precipitation record (a known proxy of precipitation recycling) to constrain estimates from models, which had largely been used to evaluate these quantities over larger scales. Two types of models were assessed in this study – the mass balance and particle tracking models – the latter of which were only made possible very recently through advancements in computational power required to perform such simulations. This research highlights which of these models tend to perform better over different times of year based on comparisons to the isotopic observations. In addition, the models were assessed over different climate zones of the tropics to see how these might be playing a role in model performance. This new approach can be used to improve future modeled estimates of precipitation recycling so that scientists may better understand its variability and potential impact on plants in response to climate change.

Principal Investigator

Kurt Solander
Los Alamos National Laboratory

Program Manager

Brian Benscoter
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science


This research was supported as part of the Next Generation Ecosystem Experiments–Tropics (NGEE–Tropics), funded by the Office of Biological and Environmental Research (BER) within the U.S. Department of Energy’s (DOE) Office of Science.


Cropper, S., et al. "Comparing Deuterium Excess to Large-Scale Precipitation Recycling Models in the Tropics." NPJ Climate and Atmospheric Science 4 60  (2021).