Observed Stable Water Isotope Variability Across a Mountainous Watershed

Elevational gradients determine isotopic fluxes into the watershed.

Stable water isotope sample collection for (a) snowpack with Jeffrey Deems and Lauren Foster and (inset) snowmelt. A comparison of annually averaged isotopic values for winter snowfall and snowpack at sample locations during peak snow water equivalent for (b) δ18O and (c) d-excess. ×=mean value.

[Image in (a) courtesy Desert Research Institute and images in (inset), (b), and (c) reprinted with permission from Carroll, R. W. H., et al. "Variability in Observed Stable Water Isotopes in Snowpack Across a Mountainous Watershed in Colorado." Hydrological Processes 36 (8), e14653 (2022). DOI:10.1002/hyp.14653. © 2023 John Wiley & Sons Ltd.]

The Science

Isotopes are elements with a different number of neutrons than protons, which allows them to be used as tracers to understand how different materials move throughout an environment. Scientists collected stable water isotopic information over five years in a large Colorado watershed, with data spanning different elevations, vegetation types, and seasonal climates. The data was combined with a land-surface model for daily estimates of snowfall and climate at sample locations. The study showed how landscape position and annual climate control snow water isotopic inputs across the watershed.

The Impact

Stable isotopes of water are used as tracers to better understand how water moves through ecosystems. In mountain systems dependent on snow, it is difficult to obtain adequate data to understand how snow accumulation and melt affect isotopic inputs. Using a large stable water isotope dataset across a large mountain basin, this study found that elevation is the dominant control on snowmelt isotopic inputs. Elevation controls snow presence and absence, change in precipitation’s isotopic signature with altitude, and precipitation phase changes from snow to rain. Transformations to snowpack isotopic signature due to melt-freeze cycles and vapor loss were found significant at lower elevations where temperatures are warmer and snowpack accumulation is shallow.

Summary

Stable water isotopes are used as natural tracers to assess water sourcing to vegetation water use, groundwater replenishment, and stream water export. Mountainous watersheds have strong variability in snowpack accumulation and snowmelt, which may affect the accuracy of using water isotopes as tracers. However, studies that assess how water isotopes vary in the snowpack and snowmelt are limited in mountain environments. Over a five-year period, researchers collected the largest known snow water isotope dataset within a mountainous watershed. Isotopic inputs in snowfall adjusted for altitude described most of the snowpack isotopic variability. North- and east-facing slopes act as a secondary control through vapor loss of persistent snowpack in the early winter. Melt-freeze cycles and vapor loss back to the atmosphere altered the isotopic signature of snowpack. This occurred where and when air temperatures were high and snow accumulation was low. Overall, observed data indicate that elevation is the dominant control on snow water isotopic inputs to mountainous basins. Elevation dictates timing of snow accumulation and melt, rate of isotopic change in precipitation with altitude, and effect of vapor loss on snowpack isotopes. Studies in mountain environments will require adjustment for elevational controls to properly understand water sourcing of stable water isotopes from snowmelt.

Principal Investigator

Rosemary Carroll
Desert Research Institute
[email protected]

Co-Principal Investigator

Eoin Brodie
Desert Research Institute
[email protected]

Funding

This work was supported by the Watershed Function Science Focus Area at Lawrence Berkeley National Laboratory, which is funded by the Biological and Environmental Research (BER) Program within the U.S. Department of Energy’s (DOE) Office of Science under contract DE-AC02-05CH11231.

References

Carroll, R. W. H., et al. "Variability in Observed Stable Water Isotopes in Snowpack Across a Mountainous Watershed in Colorado." Hydrological Processes 36 (8), e14653  (2022). https://doi.org/10.1002/hyp.14653.