Time‐Varying Storage–Water Age Relationships in a Catchment With a Mediterranean Climate

Recent studies on the relationships between catchment storage and water ages using Travel Time Distributions (TTDs), Residence Time Distributions (RTDs), and StorAge Selection (SAS) functions have led to the hypothesis that streamflow preferentially mobilizes younger water when catchment storage is high. This so‐called “Inverse Storage Effect” (ISE) needs further evaluation in more catchments with diverse climates and physiographical features. In this work, we assessed the validity of the ISE in WS10 (H. J. Andrews forest, Oregon, USA), a forested headwater catchment in a Mediterranean climate. A conceptual model of the catchment, developed based on experimental observations of water flow paths in WS10, was calibrated to streamflow and δ18O in streamflow. Based on the calibrated model results, we determined RTDs, and streamflow TTDs and SAS functions by assuming that the soil reservoir and the groundwater reservoir act as well‐mixed systems. The streamflow SAS functions and travel time dynamics showed that the ISE generally applies in WS10. Yet, during transitions from dry summer periods to wet winter periods and vice versa, the marked seasonal climate caused rapid and strong storage variations in the catchment, which led to deviations from the ISE. The seasonality of streamflow travel times in WS10 is the result of the seasonal contributions of younger water from the hillslopes added to the rather constant groundwater contributions of older water. The streamflow SAS functions were able to capture the relative importance of contrasting flow paths in the soils and in the bedrock highlighted by previous studies in WS10. Plain Language Summary Water age, which is the time water spends in a catchment after entering it, is a fundamental descriptor of how water flows through catchments. Recently, it was hypothesized that in many catchments, streamflow is fed by the youngest water present in the catchment when it is wet. In this work, we determined how the water of various ages feeds streamflow in the catchment WS10 (H. J. Andrews forest, Oregon, USA), a forested headwater catchment in a Mediterranean climate. A model of the catchment WS10 was used to simulate streamflow and water isotopic composition (i.e., oxygen‐18, 18O) as a tracer, which is naturally present in precipitation. Once calibrated to the observed data, the model allowed an examination of the water ages in the catchment and in streamflow. The model results revealed that in WS10, streamflow is also