HR: 0800h
AN: H51D-1178    [Abstracts]
TI: Diurnal variations in the \delta$^{18}$O of water vapor observed in the Pacific Northwest coniferous forests
AU: * Schauer, A J
EM: schauer@biology.utah.edu
AF: Department of Biology University of Utah, 257 S. 1400 E., Salt Lake City, UT 84112 United States
AU: Lai, C
EM: lai@biology.utah.edu
AF: Department of Biology University of Utah, 257 S. 1400 E., Salt Lake City, UT 84112 United States
AU: Ehleringer, J R
EM: ehleringer@biology.utah.edu
AF: Department of Biology University of Utah, 257 S. 1400 E., Salt Lake City, UT 84112 United States
AU: Bond, B
EM: barbara.bond@orst.edu
AF: Department of Forest Science Oregon State University, 321 Richardson Hall, Corvallis, OR 97331 United States
AU: Paw U, K
EM: ktpawu@ucdavis.edu
AF: Department of Land, Air and Water Resources, University of California, Davis, CA 95616-8627 United States
AB: Changes in the $^{2}$H and $^{18}$O of atmospheric water vapor provide information for integrating aspects of gas exchange within forest canopies. In this study we showed that overstory transpiration dominated \delta$^{18}$O values of canopy water vapor (\delta$^{18}O$_{v}$) in an old-growth coniferous forest in the Pacific Northwest of United States. Diurnal fluctuations nearly 5 $\permil$ in \delta$^{18}O$_{v}$ were observed at 3 heights both above and within the canopy. Values of \delta$^{18}O$_{v}$ were $\sim$ -18 $\permil$ (VSMOW scale) before dawn, and gradually decreased to $\sim$ -23 $\permil$ at noon before slowly becoming more enriched again in the late afternoon. This diurnal pattern reflected a balance of \delta$^{18}$O signatures associated with early-transpired water and the water transpired at later hours when isotopic steady state was established. Small transpiration flux relative to leaf water contents in the early morning prevents leaf transpiration quickly approaching steady state. Using a non-steady state model, we compared the measured and predicted \delta$^{18}$O values of leaf water over the course of a day and showed a lagged response of leaf water enrichment. Using an isotopic approach, we determined that canopy transpiration accounted for $\>$ 80 $%$ of total evapotranspiration for two summer days in this old-growth forest, which agreed closely with above- and within-canopy eddy covariance measurements. Partitioning evapotranspiration into transpiration and evaporation fluxes using \delta$^{18}$O as a tracer needs to carefully assess whether steady state assumption is satisfied.
DE: 1818 Evapotranspiration
DE: 1833 Hydroclimatology
DE: 1851 Plant ecology
DE: 0315 Biosphere/atmosphere interactions
DE: 0400 Biogeosciences
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting