Declining Summer Streamflow and Weakening PDO Teleconnections in Oregon

Climate change influences on streamflow are globally variable and difficult to quantify due to the confounding effects of climate oscillations and water management activities. Within the Pacific Northwest, Oregon supports extensive irrigated agriculture, globally important aquatic ecosystems, and a high prevalence of snow-dominated watersheds, making the region socioeconomically and ecohydrologically vulnerable to climate change. Using an information-theoretic framework, streamflow stationarity was assessed at 51 low-disturbance reference stations across Oregon. After accounting for Pacific Decadal Oscillation (PDO) and El Niño-Southern Oscillation variability, 24% of stations showed strong evidence of declining annual streamflow. Monthly trends were strongly seasonal, with June-September exhibiting strong evidence of declines at 22-51% of stations, consistent with earlier snowmelt. Power analysis indicated that these percentages may underestimate the actual extent of change. Subperiod analysis of 30 longer-record stations suggested trends were most prevalent before ~1980, aligning with documented global regime shifts. Consistent with this pattern, a step-change model with a breakpoint near 1980 was strongly preferred for precipitation-PDO relationships. PDO-precipitation correlations weakened substantially over time, declining from a median r = -0.32 (40% of stations significant) in early periods to r = -0.09 (2% significant) in recent decades, while precipitation-streamflow correlations remained strong (r > 0.8). These results suggest that Oregon's water resources are experiencing non-stationarity driven by both long-term hydrologic change and evolving teleconnections, challenging reliance on historical conditions to predict future water availability.