HR: 10:35h
AN: B12C-02    [Abstracts]
TI: Oxygen Isotopes of Water in Evapotranspiration and at the Sites of Leaf Evaporation in a Soybean Canopy
AU: * Welp, L R
EM: lisa.welp@yale.edu
AF: Forestry and Environmental Studies, Yale University, 21 Sachem St., New Haven, CT 06511, United States
AU: Lee, X
EM: xuhui.lee@yale.edu
AF: Forestry and Environmental Studies, Yale University, 21 Sachem St., New Haven, CT 06511, United States
AU: Kim, K
EM: kyounghee.kim@yale.edu
AF: Forestry and Environmental Studies, Yale University, 21 Sachem St., New Haven, CT 06511, United States
AU: Griffis, T J
EM: tgriffis@umn.edu
AF: Department of Soil, Water, and Climate, University of Minnesota, 1991 Upper Buford Circle, St. Paul, MN 55108, United States
AU: Billmark, K A
EM: kaycie@umn.edu
AF: Department of Soil, Water, and Climate, University of Minnesota, 1991 Upper Buford Circle, St. Paul, MN 55108, United States
AU: Baker, J M
EM: jbaker@umn.edu
AF: Department of Soil, Water, and Climate, University of Minnesota, 1991 Upper Buford Circle, St. Paul, MN 55108, United States
AU: Baker, J M
EM: jbaker@umn.edu
AF: Agricultural Research Service, United States Department of Agriculture, St. Paul, MN 55108, United States
AB: Stable isotopes in water have the potential to diagnose changes in the Earth's hydrologic budget in response to climate change and land use change. While the isotopic composition of the liquid water phase has been monitored for over four decades, there have been far fewer measurements of the isotopic composition of water in the vapor phase. The recent development of tunable diode laser technology (TDL) now makes it possible to monitor ambient water vapor isotopolgues with high temporal frequency. Combining this technology with micrometeorological techniques, it is also possible to determine the isotopic composition of evapotranspiration. We will present an unprecedented time series of the oxygen isotopic compositions of water vapor (δ_v) and evapotranspiration (δ_ET) above a soybean canopy for the entire 2006-growing season. We observed large variability in surface δ_v from the daily to seasonal timescales that can be largely explained by Rayleigh processes but was also increasingly influenced by local evapotranspiration (ET) in the evenings. We used δ_ET measurements to calculate the isotopic composition at the sites of evaporative enrichment in leaves (δ_L,e) and compared that to the commonly used steady state prediction (δ_L,s). During mid-day there was fair agreement. In the evening, non-steady state conditions caused δ_L,s to underestimate δ_L,e by nearly 2‰. Several new canopy scale properties emerged from this study. The formation of dew caused a sudden change in the sign of δ_ET providing unique evidence in support of nighttime transpiration from the lower canopy even in saturated atmospheric conditions. Isotopic equilibrium was approximated between dew water, water vapor and bulk leaf water suggesting that δ_v controlled the δ¹⁸O of ecosystem water pools during very humid nights. We also found that vertical humidity and temperature variability associated with canopy structure must have affected vertical gradients in the δ¹⁸O of bulk leaf water (δ_L,b). Finally, we examined this dataset for direct evidence of the Peclet effect and found that the existing theory did not fully explain the observed variability in δ_L,e and δ_L,b. These and other observations provide excellent tests for canopy-scale water transfer models.
DE: 0402 Agricultural systems
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0426 Biosphere/atmosphere interactions (0315)
DE: 0438 Diel, seasonal, and annual cycles (4227)
DE: 0454 Isotopic composition and chemistry (1041, 4870)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting