Drivers of relative streamflow contribution in mountainous headwater streams

Characterizations of the spatial and temporal variability of runoff generation processes in headwater systems contribute to understanding of how physiography and meteorological factors affect flow paths and streamflow contributions to downstream systems. Water stable isotope ratios were used to evaluate runoff generation processes across headwater streams draining contrasting terrain in a forested headwater system in the H.J. Andrews Experimental Forest, OR. Eight years (2015–2023) of weekly precipitation and one year of stream grab samples at 10 catchments (0.7–62.4 km²) were used in this investigation. Results indicated a strong climate control in the annual variability of average precipitation isotopic signatures. As such, precipitation was the most depleted in 2017, a cold year with high precipitation (2598 mm) and snow water equivalent (SWE) above the long-term median. Conversely, 2020 a warm year with low precipitation (1721 mm) and SWE below the long-term median, exhibited the highest enrichment in a water year. Precipitation inputs were consistently more depleted in late fall, winter, and early spring and consistently, more enriched in late spring, summer, and early fall. Stream and precipitation d-excess signatures were similar in fall and winter and different for spring and summer, indicating that precipitation inputs on fall and winter contributed the most to streamflow. While for a given stream, the temporal variability in stream isotopic signatures was low, the spatial variability in stream isotopic signatures was high across catchments and driven in part by elevation. The stream with the most depleted isotopic signature was Cold Creek, a high elevation spring-fed stream in the snow zone and the stream with the most enriched isotopic signature was Nostoc Creek, located at lower elevation in the transient snow zone. End-member mixing analysis indicated that Cold Creek contributed between 7 and 90% of the Lookout Creek flow. In contrast, Nostoc and Longer Creeks were found to contribute less than 43% to the Lookout Creek streamflow at their confluences to the main stem. Overall, water contributions to the Lookout Creek at the gauging station (62.4 km2) were high for Cold, Longer, and Mack Creeks during low flows conditions ranging and from 31% to 63%. Considering drainage area, Cold and Longer Creeks supplied more water per unit area than the other tributary. Collectively, our findings demonstrated strong variability across headwater with significant implication for understanding how meteorological and physiographic features impact downstream streamflow contributions. This underscores the complexity of environmental systems and highlights the need for integrated hydrological and chemical approaches to effectively manage and protect headwater streams in the face of ongoing environmental challenges.