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Deuterium excess reveals diurnal sources of water vapor in forest air

Oecologia volume 165, pages 213–223 (2011)Cite this article

Abstract

An understanding of atmospheric water vapor content and its isotopic composition is important if we are to be able to model future water vapor dynamics and their potential feedback on future climate change. Here we present diurnal and vertical patterns of water isotope ratios in forest air (δ2Hv and δ18Ov) not observed previously. Water vapor observed at three heights over 3 consecutive days in a coniferous forest in the Pacific Northwest of the United States, shows a stratified nocturnal structure of δ2Hv and δ18Ov, with the most positive values consistently observed above the canopy (60 m). Differences between 0.5 m and 60 m range between 2–6‰ for δ18O and 20–40‰ for δ2H at night. Using a box model, we simulated H2O isotope fluxes and showed that the low to high δ2Hv and δ18Ov profiles can be explained by the vapor flux associated with evaporation from the forest floor and canopy transpiration. We used d-excess as a diagnostic tracer to identify processes that contribute to the diurnal variation in atmospheric moisture. Values of d-excess derived from water vapor measurements showed a repeated diel pattern, with the lowest values occurring in the early morning and the highest values occurring at midday. The isotopic composition of rain water, collected during a light rain event in the first morning of our experiment, suggested that considerable below-cloud secondary evaporation occurred during the descent of raindrops. We conclude that atmospheric entrainment appears to drive the isotopic variation of water vapor in the early morning when the convective boundary layer rapidly develops, while evapotranspiration becomes more important in the mid-afternoon as a primary moisture source of water vapor in this forest. Our results demonstrate the interplay between the effects of vegetation and boundary layer mixing under the influence of rain evaporation, which has implications for larger-scale predictions of precipitation across the terrestrial landscape.

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Acknowledgments

We thank the two anonymous reviewers for their comments that improved the quality of this work. The authors thank Wind River Canopy Crane Research Facility for providing logistical support during our field work, AmeriFlux network for providing the meteorological data, and the Department of Atmospheric Science, University of Wyoming for providing the sounding data. We thank Craig Cook and Mike Lott from SIRFER, University of Utah for assistance with stable isotope analysis. C.-T. Lai was supported in part, through the Terrestrial Carbon Processes (TCP) program by the office of Science (BER), US Department of Energy under Grant No. DE-FG02-06ER64309, the National Institute for Climatic Change Research (NICCR) program, US Department of Energy under Grant No. DE-FC02-06ER64156, and US National Science Foundation Grant No. AGS-0956425.

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Affiliations

  1. Department of Biology, San Diego State University, 5500 Campanile Drive, San Diego, CA, 92182, USA

    Chun-Ta Lai

  2. Department of Biology, University of Utah, 257S 1400E, Salt Lake City, UT, 84112-0840, USA

    James R. Ehleringer

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  1. Chun-Ta Lai
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  2. James R. Ehleringer
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Correspondence to Chun-Ta Lai.

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Communicated by Dan Yakir.

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Lai, CT., Ehleringer, J.R. Deuterium excess reveals diurnal sources of water vapor in forest air. Oecologia 165, 213–223 (2011). https://doi.org/10.1007/s00442-010-1721-2

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Keywords

  • Water cycle
  • Atmospheric entrainment
  • Precipitation
  • Oxygen isotopes
  • Evapotranspiration
  • Land–atmosphere interaction