ABSTRACT
In mountainous river basins of the Pacific Northwest, climate models predict that winter warming will result in increased precipitation falling as rain and decreased snowpack. A detailed understanding of the spatial and temporal dynamics of water sources across river networks will help illuminate climate change impacts on river flow regimes. Because the stable isotopic composition of precipitation varies geographically, variation in surface water isotope ratios indicates the volume-weighted integration of upstream source water. We measured the stable isotope ratios of surface water samples collected in the Snoqualmie River basin in western Washington over June and September 2017 and the 2018 water year. We used ordinary least squares regression and geostatistical Spatial Stream Network models to relate surface water isotope ratios to mean watershed elevation (MWE) across seasons. Geologic and discharge data was integrated with water isotopes to create a conceptual model of streamflow generation for the Snoqualmie River. We found that surface water stable isotope ratios were lowest in the spring and highest in the dry, Mediterranean summer, but related strongly to MWE throughout the year. Low isotope ratios in spring reflect the input of snowmelt into high elevation tributaries. High summer isotope ratios suggest that groundwater is sourced from low elevation areas and recharged by winter precipitation. Overall, our results suggest that baseflow in the Snoqualmie River may be resilient to predicted warming and subsequent changes to snowpack in the Pacific Northwest.
ABSTRACT
The stable isotope ratios of stream water can be used to trace water sources within river basins; however, drivers of variation in water isotopic spatial patterns across basins must be understood before ecologically relevant and isotopically distinct water sources can be identified and this tool efficiently applied. We measured the isotope ratios of surface-water samples collected during summer low-flow across five basins in Washington and southeast Alaska (Snoqualmie, Green, Skagit, and Wenatchee Rivers, and Cowee Creek) and compared models (isoscapes) describing the spatial variation in surface-water isotope ratios across a range of hydraulic and climatic conditions. We found strong correlations between mean watershed (MWE) elevation and surface-water isotopic ratios on the windward west side of the Cascades and in Alaska, explaining 48-90% of variation in δ18O values. Conversely, in the Wenatchee basin, located leeward of the Cascade Range, MWE alone had no predicative power. The elevation relationship and predictive isoscapes varied between basins, even those adjacent to each other. Applying spatial stream network models (SSNMs) to the Snoqualmie and Wenatchee Rivers, we found incorporating Euclidean and flow-connected spatial autocovariance improved explanatory power. SSNMs improved the accuracy of river water isoscapes in all cases; however, their utility was greater for the Wenatchee basin, where covariates explained only a small proportion of total variation. Our study provides insights into why basinscale surface-water isoscapes may vary even in adjacent basins and the importance of incorporating spatial autocorrelation in isoscapes. For determining source water contributions to downstream waters, our results indicate that surface water isoscapes should be developed for each basin of interest.
