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Inferring the source of evaporated waters using stable H and O isotopes
Bowen, G., A. Putman, J. Renee Brooks, D. Bowling, E. Oerter, AND S. Good. Inferring the source of evaporated waters using stable H and O isotopes. American Geophsical Union Fall Meeting, New Orleans, LA, December 11 - 15, 2017.
Stable isotope ratios of H and O are widely used to identify the source of water, e.g., in aquifers, rivers, lakes, streams, soils, plant xylem, and beverages. We introduce a mathematical framework estimate source water for samples that have been evaporatively enriched, and apply this approach to data from EPA's 2007 National Lakes Assessment. The new analytical framework should improve the rigor of using isotopes for source water determination in ecohydrology and related sciences. This abstract contributes to SSWR 3.01A.
Stable isotope ratios of H and O are widely used to identify the source of water, e.g., in aquifers, river runoff, soils, plant xylem, and plant-based beverages. In situations where the sampled water is partially evaporated, its isotope values will have evolved along an evaporation line (EL) in 2H/18O space, and back-correction along the EL to its intersection with a meteoric water line (MWL) has been used to estimate the source water’s isotope ratios. Here we review the theory underlying isotopic estimation of source water for evaporatively enriched samples (iSWE). We note potential for bias from a commonly used regression-based approach for EL slope estimation and suggest that a model-based approach may be preferable if assumptions of the regression approach are not valid. We then introduce a mathematical framework that eliminates the need to explicitly estimate the EL-MWL intersection, simplifying iSWE analysis and facilitating more rigorous uncertainty estimation than the traditional approach. We apply this approach to data from EPAs 2007 National Lakes Assessment. We find that data for most lakes is consistent with a water source similar to annual runoff, estimated from monthly precipitation and evaporation within the lake basin. Strong evidence for both summer- and winter-biased sources exists, however, with winter bias pervasive in most snow-prone regions. The new analytical framework should improve the rigor of iSWE in ecohydrology and related sciences, and our initial results from U.S. lakes suggest that previous interpretations of lakes as unbiased isotope integrators may only be valid in certain climate regimes.