The isotopic composition of atmospheric water vapor during dry season in a central Amazon rainforest: Insights into local moisture recycling

The isotopic composition of water vapor (e.g., δ¹⁸O, δ²H) can be a powerful tracer to disentangle water vapor transport, mixing, and phase-changes (e.g., evaporation and condensation) that govern processes of the atmospheric hydrological cycle. The development and improvement of commercial laser-based spectrometers has expanded in situ continuous observations of water vapor isotope composition in a variety of sites worldwide. Nevertheless, until recently, no observations exist from the Amazon basin, a region influenced by the largest tropical rain forest that recycles significant fraction of precipitation as evapotranspiration (ET), thereby influencing regional and global atmospheric water cycling. Continuous water vapor isotope observation combined with meteorological and flux tower measurements in the Amazon rainforest are of high importance to better understand how rainforest ET contributes to regional atmospheric moisture cycles.

We report initial observations of water vapor isotope compositions at the Amazon Tall Tower Observatory (ATTO) site, located in an intact upland forest in the central Amazon, during August-September (dry-season) in 2022. A commercial cavity-ring down (CRDS) analyzer (L2140-i model, Picarro, Inc., USA) continuously measured water vapor concentration and isotope composition at four tower heights (79, 38, 24, and 4 m above ground) in and above the canopy (canopy height ~30 m). We assessed δ¹⁸O and δ²H relationships (i.e., local meteoric water line, LMWL) of different water sources (e.g., water vapor, soil water, leaf water) and deuterium excess (D-excess; D-excess = δ²H − 8 × δ¹⁸O) to trace processes that contribute to atmospheric moisture variations inside and above the canopy. For assessing the contribution of local ET to the total atmospheric moisture (i.e., local moisture recycling), the Keeling plot and intersection point methods were applied to estimate the isotope signatures of ET and background vapor, respectively.  

The LMWL of water vapor at ATTO site was δ²H = 5.2 × δ¹⁸O − 12.6, with a considerably lower slope than the Global Meteoric Water Line (δ²H = 8 × δ¹⁸O + 10). This indicates that rainforest ET significantly influences local atmospheric moisture signals. D-excess in water vapor generally increased from the early morning towards the afternoon, and reached maximum values between 12 pm and 4 pm, indicating that local processes in evaporation and transpiration contribute to local atmospheric moisture signals during daytime. In addition, the diel D-excess variation showed the positive logarithmic relationship with VPD, which indicates that VPD is the key factor for regulating diel moisture isotope signals. Based on isotope mixing models, the estimated contribution fraction of rainforest ET to the total atmospheric moisture showed maximum values (c.a., 20 % to 60 %) in the afternoon, indicating the significant contribution of rainforest ET to regional atmospheric moisture.