SHALLOW GROUNDWATER ISOSCAPE OF THE UPPER KLAMATH RIVER BASIN, OREGON

In the 9,772 km² upper Klamath River Basin (UKRB; Oregon-California, USA), declining streamflow and groundwater levels cannot meet water demands for irrigated agriculture, preservation of wetland habitat, and instream flows to support conservation of threatened and endangered species. Improved understanding of the temporal and spatial characteristics of groundwater recharge will help resource managers prioritize management efforts. We used spatial variability in water stable isotopes (δ²H, δ¹⁸O; WSI) to constrain the spatial characteristics of groundwater recharge and discharge areas contributing to stream base-flow throughout the UKRB, elucidating the basin’s shallow groundwater WSI isoscape. Samples for the analysis of WSI were collected from 45 streams and 10 springs during base-flow conditions in 2022 throughout the UKRB. Variations in shallow groundwater WSI values primarily reflect variations in orographic-uplift-driven atmospheric moisture fractionation south-to-north along the crest of the Cascade Range and west-to-east rainout fractionation across the UKRB. Superimposed on these primary signals, secondary evaporative signals reflect precipitation fractionation via pre-depositional subcloud evaporative losses that vary with topography and post-depositional evaporation from wetlands and open water. This shallow groundwater isoscape provides context for interpreting historic and new WSI data from groundwater, springs, and streams and provides improved understanding of recharge source areas, streamflow sources, and groundwater-surface water interactions.