ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-13-1607-2013 Determining water sources in the boundary layer from tall tower profiles of water vapor and surface water isotope ratios after a snowstorm in Colorado Noone D. 1 2 Risi C. 1 2 4 Bailey A. 1 2 Berkelhammer M. 1 2 Brown D. P. 1 2 Buenning N. 1 2 5 Gregory S. 1 2 Nusbaumer J. 1 2 Schneider D. 1 2 6 Sykes J. 1 2 Vanderwende B. 1 Wong J. 1 2 Meillier Y. 2 Wolfe D. 2 3 Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, CO, USA Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA Physical Sciences Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO, USA now at: Laboratoire de Météorologie Dynamique, Paris, France now at: Department of Earth Sciences, University of Southern California, Los Angeles, CA, USA now at: Climate and Global Dynamics Division, National Center for Atmospheric Research, Boulder, CO, USA 08 02 2013 13 3 1607 1623 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/13/1607/2013/acp-13-1607-2013.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/13/1607/2013/acp-13-1607-2013.pdf

The D/H isotope ratio is used to attribute boundary layer humidity changes to the set of contributing fluxes for a case following a snowstorm in which a snow pack of about 10 cm vanished. Profiles of H<sub>2</sub>O and CO<sub>2</sub> mixing ratio, D/H isotope ratio, and several thermodynamic properties were measured from the surface to 300 m every 15 min during four winter days near Boulder, Colorado. Coeval analysis of the D/H ratios and CO<sub>2</sub> concentrations find these two variables to be complementary with the former being sensitive to daytime surface fluxes and the latter particularly indicative of nocturnal surface sources. Together they capture evidence for strong vertical mixing during the day, weaker mixing by turbulent bursts and low level jets within the nocturnal stable boundary layer during the night, and frost formation in the morning. The profiles are generally not well described with a gradient mixing line analysis because D/H ratios of the end members (i.e., surface fluxes and the free troposphere) evolve throughout the day which leads to large uncertainties in the estimate of the D/H ratio of surface water flux. A mass balance model is constructed for the snow pack, and constrained with observations to provide an optimal estimate of the partitioning of the surface water flux into contributions from sublimation, evaporation of melt water in the snow and evaporation from ponds. Results show that while vapor measurements are important in constraining surface fluxes, measurements of the source reservoirs (soil water, snow pack and standing liquid) offer stronger constraint on the surface water balance. Measurements of surface water are therefore essential in developing observational programs that seek to use isotopic data for flux attribution.

 References Aemisegger, F., Sturm, P., Graf, P., Sodemann, H., Pfahl, S., Knohl, A., and Wernli, H.: Measuring variations of $\delta^18$O and $\delta^2$H in atmospheric water vapour using two commercial laser-based spectrometers: an instrument characterisation study, Atmos. Meas. Tech., 5, 1491–1511, http://dx.doi.org/10.5194/amt-5-1491-2012doi:10.5194/amt-5-1491-2012, 2012. Angert, A., Lee, J. E., and Yakir, D.: Seasonal variations in the isotopic composition of near-surface water vapour in the eastern Mediterranean, Tellus B, 60, 674–684, 2008. Araguas-Araguas, L., Froehlich, K., and Rozanski, K.: Deuterium and oxygen-18 isotope composition of precipitation and atmospheric moisture, Hydrol. Process., 14, 1341–1355, 2000. Avissar, R. and Werth, D.: Global hydroclimatological teleconnections resulting from tropical deforestation, J. Hydrometeorol., 6, 134–145, 2005. Balsley, B. B., Jensen, M. L., and Frehlich, R. G.: The use of state-of-the-art kites for profiling the lower atmosphere, Bound.-Lay. Meteorol., 87, 1–25, 1998. Bigeleisen, J.: Statistical mechanics of isotope effects on the thermodynamic properties of condensed systems, J. Chem. Phys., 34, 1485–1493, 1961. Blumen, W.: An Observational Study of Instability and Turbulence in Nighttime Drainage Winds, Bound.-Lay. Meteorol., 28, 245–269, 1984. Boe, J. and Terray, L.: Uncertainties in summer evapotranspiration changes over Europe and implications for regional climate change, Geophys. Res. Lett., 35, L05702, http://dx.doi.org/10.1029/2007GL032417doi:10.1029/2007GL032417, 2008. Boone, A., Habets, F., Noilhan, J., Clark, D., Dirmeyer, P., Fox, S., Gusev, Y., Haddeland, I., Koster, R., Lohmann, D., Mahanama, S., Mitchell, K., Nasonova, O., Niu, G. Y., Pitman, A., Polcher, J., Shmakin, A. B., Tanaka, K., van den Hurk, B., Verant, S., Verseghy, D., Viterbo, P., and Yang, Z. L.: The Rhone-aggregation land surface scheme intercomparison project: An overview, J. Climate, 17, 187–208, 2004. Bosilovich, M. G. and Chern, J. D.: Simulation of water sources and precipitation recycling for the MacKenzie, Mississippi, and Amazon River basins, J. Hydrometeorol., 7, 312–329, 2006. Brown, S. S., Dubé, W. P., Osthoff, H. D., Wolfe, D. E., Angevine, W. M., and Ravishankara, A. R.: High resolution vertical distributions of NO<sub>3</sub> and N<sub>2</sub>O$_5$ through the nocturnal boundary layer, Atmos. Chem. Phys., 7, 139–149, http://dx.doi.org/10.5194/acp-7-139-2007doi:10.5194/acp-7-139-2007, 2007. Businger, J. A.: Turbulent Transfer in the Atmospheric Surface Layer, Workshop on Micrometerology, Boston, MA, 67–100, 1973. Cohn, S. A., Brown, W. O. J., Martin, C. L., Susedik, M. E., Maclean, G. D., and Parsons, D. B.: Clear air boundary layer spaced antenna wind measurement with the Multiple Antenna Profiler (MAPR), Ann. Geophys., 19, 845–854, http://dx.doi.org/10.5194/angeo-19-845-2001doi:10.5194/angeo-19-845-2001, 2001. Crossley, J. F., Polcher, J., Cox, P. M., Gedney, N., and Planton, S.: Uncertainties linked to land-surface processes in climate change simulations, Clim. Dyn., 16, 949–961, 2000. Dansgaard, W.: Stable Isotopes in Precipitation, Tellus, 16, 436–468, 1964. Ehhalt, D. H.: Vertical profiles of HTO, HDO and H<sub>2</sub>O in the troposphere, National Center for Atmospheric Research, Boulder, ColaradoNCAR-TN/STR-100, 1974. Ehhalt, D. H., Rohrer, F., and Fried, A.: Vertical profiles of HDO/H2O in the troposphere, J. Geophys. Res.-Atmos., 110, D13301, http://dx.doi.org/10.1029/2004JD005569doi:10.1029/2004JD005569, 2005. Einaudi, F. and Finnigan, J. J.: Wave-Turbulence Dynamics in the Stably Stratified Boundary-Layer, J. Atmos. Sci., 50, 1841–1864, 1993. Einaudi, F., Bedard, A. J., and Finnigan, J. J.: A Climatology of Gravity-Waves and Other Coherent Disturbances at the Boulder-Atmospheric-Observatory during March–April 1984, J. Atmos. Sci., 46, 303–329, 1989. Ekaykin, A. A., Hondoh, T., Lipenkov, V. Y., and Miyamoto, A.: Post-depositional changes in snow isotope content: preliminary results of laboratory experiments, Clim. Past Discuss., 5, 2239–2267, http://dx.doi.org/10.5194/cpd-5-2239-2009doi:10.5194/cpd-5-2239-2009, 2009. Frehlich, R., Meillier, Y., Jensen, M. L., and Balsley, B.: Turbulence measurements with the CIRES tethered lifting system during CASES-99: Calibration and spectral analysis of temperature and velocity, J. Atmos. Sci., 60, 2487–2495, 2003. Gat, J. R. and Matsui, E.: Atmospheric Water-Balance in the Amazon Basin – an Isotopic Evapotranspiration Model, J. Geophys. Res., 96, 13179–13188, 1991. Gedney, N., Cox, P. M., Douville, H., Polcher, J., and Valdes, P. J.: Characterizing GCM land surface schemes to understand their responses to climate change, J. Climate, 13, 3066–3079, 2000. Good, S. P., Soderberg, K., Wang, L. X., and Caylor, K. K.: Uncertainties in the assessment of the isotopic composition of surface fluxes: A direct comparison of techniques using laser-based water vapor isotope analyzers, J. Geophys. Res., 117, D15301, http://dx.doi.org/10.1029/2011jd017168doi:10.1029/2011jd017168, 2012. Gossard, E. E., Gaynor, J. E., Zamora, R. J., and Neff, W. D.: Fine-Structure of Elevated Stable Layers Observed by Sounder and Insitu Tower Sensors, J. Atmos. Sci., 42, 2156–2169, 1985. Griffis, T. J., Zhang, J., Baker, J. M., Kljun, N., and Billmark, K.: Determining carbon isotope signatures from micrometeorological measurements: Implications for studying biosphere-atmosphere exchange processes, Bound.-Lay. Meteorol., 123, 295–316, http://dx.doi.org/10.1007/s10546-006-9143-8doi:10.1007/s10546-006-9143-8, 2007. Guo, Z. C., Dirmeyer, P. A., Koster, R. D., Bonan, G., Chan, E., Cox, P., Gordon, C. T., Kanae, S., Kowalczyk, E., Lawrence, D., Liu, P., Lu, C. H., Malyshev, S., McAvaney, B., McGregor, J. L., Mitchell, K., Mocko, D., Oki, T., Oleson, K. W., Pitman, A., Sud, Y. C., Taylor, C. M., Verseghy, D., Vasic, R., Xue, Y. K., and Yamada, T.: GLACE: The Global Land-Atmosphere Coupling Experiment. Part II: Analysis, J. Hydrometeorol., 7, 611–625, 2006. Gupta, P., Noone, D., Galewsky, J., Sweeney, C., and Vaughn, B. H.: Demonstration of high-precision continuous measurements of water vapor isotopologues in laboratory and remote field deployments using wavelength-scanned cavity ring-down spectroscopy (WS-CRDS) technology, Rapid Commun Mass Sp, 23, 2534–2542, http://dx.doi.org/10.1002/rcm.4100doi:10.1002/rcm.4100, 2009. Gurney, S. D. and Lawrence, D. S. L.: Seasonal trends in the stable isotopic composition of snow and meltwater runoff in a subarctic catchment at Okstindan, Norway, Nordic Hydrology, 35, 119–137, 2004. He, H. and Smith, R. B.: Stable isotope composition of water vapor in the atmospheric boundary layer above the forests of New England, J. Geophys. Res.-Atmos., 104, 11657–11673, 1999. Henderson-Sellers, A.: Improving land-surface parameterization schemes using stable water isotopes: Introducing the 'iPILPS' initiative, Global Planet. Change, 51, 3–24, 2006. Henderson-Sellers, A., McGuffie, K., Noone, D., and Irannejad, P.: Using stable water isotopes to evaluate basin-scale simulations of surface water budgets, J. Hydrometeorol., 5, 805–822, 2004. Hoffmann, G., Werner, M., and Heimann, M.: Water isotope module of the ECHAM atmospheric general circulation model: A study on timescales from days to several years, J. Geophys. Res., 103, 16871–16896, 1998. Holtslag, B.: Preface – GEWEX atmospheric boundary-layer study (GABLS) on stable boundary layers, Bound.-Lay. Meteorol., 118, 243–246, http://dx.doi.org/10.1007/s10546-005-9008-6doi:10.1007/s10546-005-9008-6, 2006. Hurley, J. V., Galewsky, J., Worden, J., and Noone, D.: A test of the advection-condensation model for subtropical water vapor using stable isotopologue observations from Mauna Loa Observatory, Hawaii, J. Geophys. Res., 117, D19118, http://dx.doi.org/10.1029/2012jd018029doi:10.1029/2012jd018029, 2012. Jouzel, J. and Merlivat, L.: Deuterium and O-18 in Precipitation – Modeling of the Isotopic Effects during Snow Formation, J. Geophys. Res., 89, 1749–1757, 1984. Jouzel, J., Russell, G. L., Suozzo, R. J., Koster, R. F., White, J. W. C., and Broecker, W. S.: Simulation of the HDO and H218O atmospheric cycles using the NASA GISS general circulation model: the seasonal cycle for present-day conditions, J. Geophys. Res., 92, 14739–14760, 1987. Kaimal, J. C. and Gaynor, J. E.: The Boulder Atmospheric Observatory, J. Clim. Appl. Meteorol., 22, 863–880, 1983. Keeling, C. D.: The concentration and isotopic abundances of atmospheric carbon dioxide in rural areas, Geochim. Cosmochim. Acta, 13, 322–334, 1958. Koster, R. D. and Milly, P. C. D.: The interplay between transpiration and runoff formulations in land surface schemes used with atmospheric models, J. Climate, 10, 1578–1591, 1997. Koster, R. D., Guo, Z. C., Dirmeyer, P. A., Bonan, G., Chan, E., Cox, P., Davies, H., Gordon, C. T., Kanae, S., Kowalczyk, E., Lawrence, D., Liu, P., Lu, C. H., Malyshev, S., McAvaney, B., Mitchell, K., Mocko, D., Oki, T., Oleson, K. W., Pitman, A., Sud, Y. C., Taylor, C. M., Verseghy, D., Vasic, R., Xue, Y. K., and Yamada, T.: GLACE: The Global Land-Atmosphere Coupling Experiment. Part I: Overview, J. Hydrometeorol., 7, 590–610, 2006. Kurita, N., Newman, B. D., Araguas-Araguas, L. J., and Aggarwal, P.: Evaluation of continuous water vapor δD and $\delta^18$O measurements by off-axis integrated cavity output spectroscopy, Atmos. Meas. Tech., 5, 2069–2080, http://dx.doi.org/10.5194/amt-5-2069-2012doi:10.5194/amt-5-2069-2012, 2012. Lee, H., Smith, R., and Williams, J.: Water vapour O-18/O-16 isotope ratio in surface air in New England, USA, Tellus B, 58, 293–304, 2006. Lee, J., Feng, X., Faiia, A. M., Posmentier, E. S., Kirchner, J. W., Osterhuber, R., and Taylor, S.: Isotopic evolution of a seasonal snowcover and its melt by isotopic exchange between liquid water and ice, Chem. Geol., 270, 126–134, http://dx.doi.org/10.1016/j.chemgeo.2009.11.011doi:10.1016/j.chemgeo.2009.11.011, 2010. Lee, X. H., Kim, K., and Smith, R.: Temporal variations of the O-18/O-16 signal of the whole-canopy transpiration in a temperate forest, Glob. Biogeochem. Cy., 21, GB3013, http://dx.doi.org/10.1029/2006GB002871doi:10.1029/2006GB002871, 2007. Lee, X., Huang, J., and Patton, E.: A large-eddy simulation study of water vapour and carbon dioxide isotopes in the atmospheric boundary layer, Bound.-Lay. Meteorol., 145, 229–248, 2012. Majoube, M.: Fractionation in O-18 between Ice and Water Vapor, Journal De Chimie Physique Et De Physico-Chimie Biologique, 68, 625–636, 1971. Mathieu, R. and Bariac, T.: A numerical model for the simulation of stable isotope profiles in drying soils, J. Geophys. Res., 101, 12685–12696, 1996. Merlivat, L.: Molecular Diffusivities of (H2o)-O-1 6 Hd16o, and (H2o)-O-18 in Gases, J. Chem. Phys., 69, 2864–2871, 1978. Miller, J. B. and Tans, P. P.: Calculating isotopic fractionation from atmospheric measurements at various scales, Tellus B, 55, 207–214, 2003. Milly, P. C. D., Dunne, K. A., and Vecchia, A. V.: Global pattern of trends in streamflow and water availability in a changing climate, Nature, 438, 347–350, http://dx.doi.org/10.1038/nature04312doi:10.1038/nature04312, 2005. Moreira, M. Z., Sternberg, L. D. L., Martinelli, L. A., Victoria, R. L., Barbosa, E. M., Bonates, L. C. M., and Nepstad, D. C.: Contribution of transpiration to forest ambient vapour based on isotopic measurements, Glob. Change Biol., 3, 439–450, 1997. Noone, D.: The influence of midlatitude and tropical overturning circulation on the isotopic composition of atmospheric water vapor and Antarctic precipitation, J. Geophys. Res., 113, D04102, http://dx.doi.org/10.1029/2007JD008892doi:10.1029/2007JD008892, 2008. Noone, D.: Pairing measurements of the water vapor isotope ratio with humidity to deduce atmospheric moistening and dehydration in the tropical mid-troposphere J. Climate, 25, 4476–4494, 2012. Noone, D., Galewsky, J., Sharp, Z. D., Worden, J., Barnes, J., Baer, D., Bailey, A., Brown, D. P., Christensen, L., Crosson, E., Dong, F., Hurley, J. V., Johnson, L. R., Strong, M., Toohey, D., Van Pelt, A., and Wright, J. S.: Properties of air mass mixing and humidity in the subtropics from measurements of the D/H isotope ratio of water vapor at the Mauna Loa Observatory, J. Geophys. Res., 116, D22113, http://dx.doi.org/10.1029/2011JD015773doi:10.1029/2011JD015773, 2011. O'Neil, J. R.: Hydrogen and Oxygen Isotope Fractionation between Ice and Water, J. Phys. Chem., 72, 3683–3684, 1968. Pataki, D. E., Ehleringer, J. R., Flanagan, L. B., Yakir, D., Bowling, D. R., Still, C. J., Buchmann, N., Kaplan, J. O., and Berry, J. A.: The application and interpretation of Keeling plots in terrestrial carbon cycle research, Glob. Biogeochem. Cy., 17, 1022, http://dx.doi.org/10.1029/2001GB001850doi:10.1029/2001GB001850, 2003. Pitman, A. J., de Noblet-Ducoudre, N., Cruz, F. T., Davin, E. L., Bonan, G. B., Brovkin, V., Claussen, M., Delire, C., Ganzeveld, L., Gayler, V., van den Hurk, B. J. J. M., Lawrence, P. J., van der Molen, M. K., Muller, C., Reick, C. H., Seneviratne, S. I., Strengers, B. J., and Voldoire, A.: Uncertainties in climate responses to past land cover change: First results from the LUCID intercomparison study, Geophys. Res. Lett., 36, L14814, http://dx.doi.org/10.1029/2009GL039076doi:10.1029/2009GL039076, 2009. Poulos, G. S., Blumen, W., Fritts, D. C., Lundquist, J. K., Sun, J., Burns, S. P., Nappo, C., Banta, R., Newsom, R., Cuxart, J., Terradellas, E., Balsley, B., and Jensen, M.: CASES-99: A comprehensive investigation of the stable nocturnal boundary layer, B. Am. Meteorol. Soc., 83, 555–581, 2002. Rambo, J., Lai, C. T., Farlin, J., Schroeder, M., and Bible, K.: On-Site Calibration for High Precision Measurements of Water Vapor Isotope Ratios Using Off-Axis Cavity-Enhanced Absorption Spectroscopy, J. Atmos. Ocean. Tech., 28, 1448–1457, http://dx.doi.org/10.1175/Jtech-D-11-00053.1doi:10.1175/Jtech-D-11-00053.1, 2011. Rutter, N., Essery, R., Pomeroy, J., Altimir, N., Andreadis, K., Baker, I., Barr, A., Bartlett, P., Boone, A., Deng, H. P., Douville, H., Dutra, E., Elder, K., Ellis, C., Feng, X., Gelfan, A., Goodbody, A., Gusev, Y., Gustafsson, D., Hellstrom, R., Hirabayashi, Y., Hirota, T., Jonas, T., Koren, V., Kuragina, A., Lettenmaier, D., Li, W. P., Luce, C., Martin, E., Nasonova, O., Pumpanen, J., Pyles, R. D., Samuelsson, P., Sandells, M., Schadler, G., Shmakin, A., Smirnova, T. G., Stahli, M., Stockli, R., Strasser, U., Su, H., Suzuki, K., Takata, K., Tanaka, K., Thompson, E., Vesala, T., Viterbo, P., Wiltshire, A., Xia, K., Xue, Y. K., and Yamazaki, T.: Evaluation of forest snow processes models (SnowMIP2), J. Geophys. Res., 114, D06111, http://dx.doi.org/10.1029/2008JD011063doi:10.1029/2008JD011063, 2009. Salati, E., Dallolio, A., Matsui, E., and Gat, J. R.: Recycling of Water in the Amazon Basin – Isotopic Study, Water Resour. Res., 15, 1250–1258, 1979. Salmond, J. A. and McKendry, I. G.: A review of turbulence tin the very stable nocturnal boundary layer and its implications for air quality, Prog. Phys. Geog., 29, 171–188, http://dx.doi.org/10.1191/0309133305pp442radoi:10.1191/0309133305pp442ra, 2005. Slater, A. G., Schlosser, C. A., Desborough, C. E., Pitman, A. J., Henderson-Sellers, A., Robock, A., Vinnikov, K. Y., Mitchell, K., Boone, A., Braden, H., Chen, F., Cox, P. M., de Rosnay, P., Dickinson, R. E., Dai, Y. J., Duan, Q., Entin, J., Etchevers, P., Gedney, N., Gusev, Y. M., Habets, F., Kim, J., Koren, V., Kowalczyk, E. A., Nasonova, O. N., Noilhan, J., Schaake, S., Shmakin, A. B., Smirnova, T. G., Verseghy, D., Wetzel, P., Yue, X., Yang, Z. L., and Zeng, Q.: The representation of snow in land surface schemes: Results from PILPS 2(d), J. Hydrometeorol., 2, 7–25, 2001. Stewart, M. K.: Stable Isotope Fractionation Due to Evaporation and Isotopic-Exchange of Falling Waterdrops - Applications to Atmospheric Processes and Evaporation of Lakes, J. Geophys. Res., 80, 1133–1146, 1975. Sturm, C., Zhang, Q., and Noone, D.: An introduction to stable water isotopes in climate models: benefits of forward proxy modelling for paleoclimatology, Clim. Past, 6, 115–129, http://dx.doi.org/10.5194/cp-6-115-2010doi:10.5194/cp-6-115-2010, 2010. Taylor, S., Feng, X. H., Kirchner, J. W., Osterhuber, R., Klaue, B., and Renshaw, C. E.: Isotopic evolution of a seasonal snowpack and its melt, Water Resour. Res., 37, 759–769, 2001. Tremoy, G., Vimeux, F., Mayaki, S., Souley, I., Cattani, O., Risi, C., Favreau, G., and Oi, M.: A 1-year long delta O-18 record of water vapor in Niamey (Niger) reveals insightful atmospheric processes at different timescales, Geophys. Res. Lett., 39, L08805 http://dx.doi.org/10.1029/2012gl051298doi:10.1029/2012gl051298, 2012. Wang, L. X., Caylor, K. K., Villegas, J. C., Barron-Gafford, G. A., Breshears, D. D., and Huxman, T. E.: Partitioning evapotranspiration across gradients of woody plant cover: Assessment of a stable isotope technique, Geophys. Res. Lett., 37, L09401, http://dx.doi.org/10.1029/2010GL043228doi:10.1029/2010GL043228, 2010. Wen, X. F., Lee, X. H., Sun, X. M., Wang, J. L., Tang, Y. K., Li, S. G., and Yu, G. R.: Intercomparison of Four Commercial Analyzers for Water Vapor Isotope Measurement, J. Atmos. Ocean. Tech., 29, 235–247, http://dx.doi.org/10.1175/Jtech-D-10-05037.1doi:10.1175/Jtech-D-10-05037.1, 2012. West, A. G., Patrickson, S. J., and Ehleringer, J. R.: Water extraction times for plant and soil materials used in stable isotope analysis, Rapid Commun. Mass Sp., 20, 1317–1321, 2006. Williams, D. G., Cable, W., Hultine, K., Hoedjes, J. C. B., Yepez, E. A., Simonneaux, V., Er-Raki, S., Boulet, G., de Bruin, H. A. R., Chehbouni, A., Hartogensis, O. K., and Timouk, F.: Evapotranspiration components determined by stable isotope, sap flow and eddy covariance techniques, Agr. Forest Meteorol., 125, 241–258, http://dx.doi.org/10.1016/j.agrformet.2004.04.008doi:10.1016/j.agrformet.2004.04.008, 2004. Yepez, E. A., Williams, D. G., Scott, R. L., and Lin, G. H.: Partitioning overstory and understory evapotranspiration in a semiarid savanna woodland from the isotopic composition of water vapor, Agr. Forest Meteorol., 119, 53–68, http://dx.doi.org/10.1016/S0168-1923(03)00116-3doi:10.1016/S0168-1923(03)00116-3, 2003.