HR: 10:35h
AN: B22B-02    [Abstracts]
TI: Stable Isotope Composition of Water Vapor in Coniferous Forests of the Pacific Northwest, USA.
AU: * Lai, C
EM: lai@biology.utah.edu
AF: Department of Biology, University of Utah, 257 S, 1400 E, Salt Lake City, UT 84112-0840 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 84112-0840 United States
AB: Measurements of stable isotope composition of water vapor (δ²H and δ¹⁸O) were made every 3 hours for 3 consecutive days above and within an old-growth coniferous forest in the Pacific Northwest of the United States. The objective of these measurements was to examine the impact of atmospheric mixing, transpiration, and evaporation on the diel variation in δ²H and δ¹⁸O values of water vapor in forests. Atmospheric vapor samples were collected simultaneously at 3 different heights, i.e. 0.5, 10 and 60 m above ground. Significant vertical gradients were consistently observed in the morning for all 3 days, with more positive values at 60 m (20-30 ‰ for δ²H and 3-4 ‰ for δ¹⁸O; SMOW scale). This vertical separation in both δ²H and δ¹⁸O signals suggests that a one-dimensional budget approach may be applicable to explain relative contributions of atmospheric mixing, transpiration and evaporation on the diurnal δ²H and δ¹⁸O values of canopy vapor. When δ²H and δ¹⁸O values of water vapor were plotted against each other, in conjunction with values from other ecosystem waters (i.e. rain, stem and soil water), we found that rain, stem and soil waters and the majority of nighttime vapor formed a straight line (δ²H = 6.89*δ¹⁸O - 17.2; R² = 0.98, n = 47). This relationship, however, was different from the local meteoric water line (δ²H = 8.2*δ¹⁸O + 12.39), ad hoc the remarkable difference in the intercept (D-excess). δ²H and δ¹⁸O values of leaf water also showed a significant linear relationship, but reside off the source water-vapor regression line with a smaller slope resulting from transpiration. Daytime vapor appears to be more difficult to interpret for its lack of consistency between δ²H and δ¹⁸O values. We suggest that excessive evaporation might have occurred during rainfall, but evaporation becomes negligible once rainwater enters the soil. This suggestion supports the strong linear relationship between rainwater and stem and soil waters, and it may also explain why such a relationship deviates from the local meteoric water line.
UR: http://ecophys.biology.utah.edu/Research/DOE_TCP/index.html
DE: 0315 Biosphere/atmosphere interactions (0426, 1610)
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 1631 Land/atmosphere interactions (1218, 1843, 3322)
DE: 1813 Eco-hydrology
DE: 1818 Evapotranspiration
SC: Biogeosciences [B]
MN: Fall Meeting 2005