Snowpack and stream water isotope sampling demonstrate the importance of high-elevation snowmelt for the water supply of the Colorado River.
Image courtesy of authors
In years with relatively low maximum snow water equivalent (SWEmax, y-axis) and high air temperature during the snowmelt period (Tair, x-axis) the high-elevation contribution to streamflow is highest (blue colored area).
The Science
The isotopic signal of water, a natural tracer, was measured in the snowpack and stream water for over seven years in the mountainous headwaters of the Colorado River. Data from these measurements provided insights on the share of the water in the headwaters that was sourced from the highest elevations during the snowmelt period. In years with relatively little snowfall and warm air temperature, the share of high-elevation snowmelt contributions to the headwaters was highest. The observed variations of high-elevation snowpack contributions during snowmelt were detected in both small mountainous catchments and large watersheds in the Upper Colorado River.
The Impact
The Colorado River, providing the water supply to 40 million people, is mainly sourced by the snowmelt in the Rocky Mountains. To understand the potential of water availability changes, knowledge about the consequences of changes in snowpack and air temperature on the river’s headwaters is crucial. Data from the past seven years demonstrate that an increase in the relative contributions from high-elevation snowmelt underlines the critical role mountains play in sustaining the water supply. Because the snowpack at lower elevations is going to be impacted most by climate change, the water of the snowmelt from snowpack at the highest elevations will become more important to sustain ample water flow throughout the summer.
Summary
Measurements of stable isotopes of water in the snowpack and headwater rivers in the Upper Colorado River basin by the LBL-lead Watershed Science Focus Area (SFA) over the seven eight years enabled this multi-institutional team to relate the spatial variation in the snowpack isotope ratio along an elevation gradient with the snowmelt stream discharge and its isotopic composition based on mixing analyses. The results of this tracer-based method highlight the importance of the snowpack in the highest elevations of the Rocky Mountains for streamflow generation. Connecting the DOE-funded SFA efforts with the stream/river monitoring led by the U.S. Geological Survey, allowed the team to scale up from the intensely measured headwaters to larger watersheds. The results suggest that the temporal variation of high-elevation snowmelt contributions is transferrable to other snow-dominated mountainous regions. Changes in the stream water isotope dynamics during the snowmelt period could therefore be used to identify changes in the snow water equivalent (SWE) of the snowpack that would be challenging to observe with ground-based instrumentation or remote sensing.
Contact
Matthias Sprenger
Lawrence Berkeley National Laboratory
msprenger@lbl.gov
Funding
This work was supported by the US Department of Energy Office of Science under contract DE-AC02- 05CH11231 as part of the Lawrence Berkeley National Laboratory Watershed Function Science Focus Area.
Publications
Sprenger M, Carroll RWH, Marchetti D, Bern C, et al. “Stream water sourcing from high elevation snowpack inferred from stable isotopes of water: A novel application of d-excess values”, Hydrology and Earth System Science (2024). [DOI: 10.5194/hess-28-1711-2024]