Inferring the source of evaporated waters using stable H and O isotopes.
Citation:
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. OECOLOGIA. Springer, New York, NY, 187(4):1025-1039, (2018). https://doi.org/10.1007/s00442-018-4192-5
Impact/Purpose:
Stable isotope ratios of H and O are widely used identify the source of water entering lakes or other water systems. In this manuscript we introduce an approach to calculating the source of water to a system where the system water has been evaporated (i.e. lakes). Evaporation alters the isotopic signature of water in a predictable way; however, some of the approaches used to correct for this alteration violate critical assumptions. We offer an approach based on theoretical principles and will allow for rigorous uncertainty estimates. We test this system on lake water isotopes from EPAs 2007 National Lakes Assessment. 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.
Description:
Stable isotope ratios of H and O are widely used 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 evaporated 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. 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.
