Identifying Sources of Stream Water Based on Stable Water Isotopes and Hydrologic Models
Matthias Sprenger1* (email@example.com), Rosemary Carroll2, Erica Siirila-Woodburn1, P. James Dennedy-Frank1,3, Kenneth Williams1, Eoin Brodie1
1Lawrence Berkeley National Laboratory, Berkeley, CA; 2Desert Research Institute, Reno, NV; 3Northeastern University, Boston, MA
Identifying dynamic stream water contributions from the snowpack versus monsoonal rainfall is crucial to understanding potential changes in water resources in a changing climate with reduced snowpack. Using data from 81 snow pits collected over 5 years across the East River watershed, scientists investigated the temporal and spatial variation of the snowpack stable isotope ratios. These data showed that snow sublimation and melt-freeze redistribution led to isotopic enrichment with more intense fractionation at lower elevation. The snowpack isotopes were used together with LiDAR snowpack observations to couple a hydrologic and a snowpack isotope model to assess controls on isotopic inputs across the East River catchment.
Results showed that the most depleted isotope conditions occurred in the upper subalpine where snow accumulation was high, and rainfall was low. Energy and snow-availability determined the snowpack’s isotopic fractionation, which was highest where vegetation shading was low (i.e., above treeline and grass and Aspen-dominated portions of upper montane). Scientists used the precipitation isotope inputs to the East River catchment to investigate the partitioning of snow and rainfall into evapotranspiration (ET) and summer and nonsummer discharge via isotope mixing analyses. Results showed that one third of the snow partitioned to ET and 13% of the snowmelt sustained summer streamflow. Only 8% of the rainfall contributed to the summer streamflow, because most of the rain (67%) partitioned to ET. Catchments with higher tree cover demonstrated a higher share of snow becoming ET and less summer streamflow. Finally, physically based integrated hydrologic model simulations with dynamic-flux particle tracking showed a strong linear trend with the isotope mixing analysis for the fraction of ET coming from rain versus snow (R2=0.88), suggesting the simulation captures key functional dynamics of water partitioning to ET. However, simulated discharge from snow exceeds that of the amount estimated from the isotope observations, suggesting either an underestimation of simulated ET and/or a limitation of the endmember analysis to account for groundwater storage changes.
The isotope data further enabled scientists to relate isotopically distinct high elevation snowmelt to stream water isotope dynamics. Results showed that stream water isotopes can serve as a catchment integrated signal of high elevation snowmelt contributions to stream discharge. Interannual variation of the stream water isotopes could serve as an early warning signal for a shift towards reduced high elevation snowmelt contribution to the catchment runoff.