Elevation and spatial structure explain most surface-water isotopic variation across five Pacific Coast basins.

Area of Science:

• Hydrology
• Geochemistry
• Environmental Science

Background:

• Stable isotope ratios in stream water serve as tracers for water sources within river basins.
• Understanding spatial variations in water isotopes is crucial for identifying distinct water sources and applying tracing tools effectively.

Purpose of the Study:

• To compare models of surface-water isotope ratios across diverse hydraulic and climatic conditions in five river basins.
• To investigate the influence of mean watershed elevation and spatial autocorrelation on surface-water isoscapes.

Main Methods:

• Surface-water samples were collected during summer low-flow across five basins in Washington and southeast Alaska.
• Isotope ratios (δ18O) were measured and compared with models (isoscapes) incorporating hydraulic and climatic variables.
• Spatial stream network models (SSNMs) were applied to assess the impact of spatial autocovariance.

Main Results:

• Strong correlations between mean watershed elevation and surface-water isotopic ratios were observed on the windward west side of the Cascades and in Alaska (explaining 48-90% of δ18O variation).
• In the leeward Wenatchee basin, mean watershed elevation alone had no predictive power.
• Spatial stream network models improved the accuracy of river water isoscapes, particularly in basins where covariates had limited explanatory power.

Conclusions:

• Basin-scale surface-water isoscapes can vary significantly, even between adjacent basins, due to differing drivers.
• Incorporating spatial autocorrelation is important for developing accurate isoscapes.
• Surface water isoscapes should be developed independently for each basin to accurately determine source water contributions to downstream waters.