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Understanding Water Isotopes in Hydrology: Variability, Processes, and Modeling

Explore the hydrological cycle, isotopic variations in precipitation, fractionation processes, and modeling techniques for d18Oprecip. Understand the factors contributing to isotopic variations in water bodies like oceans, lakes, and seas. Learn about the global meteoric water line and the concept of deuterium excess. Unravel the mysteries behind temperature effects on d18O and dD, salinity relationships, and the intricacies of the planetary boundary layer in water isotopic studies.

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Understanding Water Isotopes in Hydrology: Variability, Processes, and Modeling

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  1. Water Isotopes in the hydrology 10/03/18 Dana Mawlood • Lecture outline: • the hydrological cycle • dD and d18O variability • 3) fractionation processes • d18O, dD of precipitation • 5) modeling d18Oprecip spectrometer light intake

  2. Water Isotopic Variations NOTE: water isotopes are always reported with respect to SMOW Ocean d18O = 0 ± 2‰ dD = 0 ± 16‰ Lake Michigan d18O = -7‰ dD = -54‰ Lake Chad d18O = -20‰ dD = -110‰ Dead Sea d18O = +4.4‰ dD = 0‰ What processes explain these variations?

  3. Temperature effect on the d18O of precipitation holds for both spatial T variability and temporal variability Rozanski, 1993

  4. Surface Water Salinity-d18O relationship - tropics Fairbanks et al., 1997 Slope of d18O-salinity relationship is 0.273 in the deep tropics (<5 N and S), vs. 0.45 elsewhere. Why?

  5. The “Global Meteoric Water Line” – what happens to d18O happens to dD, but with a different a annual mean dD vs. d18O of precipitation But month-to-month variations at a given site fall off this line – “deuterium excess” Craig, 1961 Rozanski, 1993

  6. 1-3km Water Why don’t all waters fall on the GMWL? Or…. why do different “source” waters have different ‘deuterium excess’ values? Fact: water vapor above the ocean is -13‰ in d18O, not the -9.2‰ expected from equilibrium fractionation. Why? • Planetary boundary layer • the layer where exchange occurs • between the surface and the free • atmosphere • evaporation not purely • equilibrium process • what other type of fractionation • is involved? Given the potential for complicated boundary layer physics, it’s a wonder that the GMWL exists at all!

  7. Deuterium excess Humid regions will show smaller departures from GMWL than arid regions. Generally interpreted as a proxy for the “source” of the moisture.

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