Preferential flow paths (e.g., macropores or subsurface pipe networks) in hydrological systems facilitate the rapid transmission of precipitation and solutes to streams, resulting in streamflow responses characterized by the release of younger water (i.e., recent precipitation) from the catchment and correspondingly short transit times (on the order of days). While preferential flow paths are documented in both the unsaturated zone and groundwater aquifers, it remains uncertain whether catchment-scale isotope-based transport models can adequately represent preferential flow using tracer measurements in streamflow. In this study, we hypothesize that the preferential release of young water from both the unsaturated zone and groundwater aquifers can be isolated from the streamflow tracer signal. This can be studied with StorAge Selection (SAS) functions, which describe how young or old water leaves a storage. We systematically compared multiple parameterizations of SAS functions describing how water of different ages is released from the unsaturated zone and groundwater aquifer within a single catchment-scale transport model using long-term measurements of hydrogen isotopes in water ( ^δ2H) from two headwater catchments (the Hydrological Open Air Laboratory (HOAL) in Austria and the Wüstebach catchment in Germany). The results show that ^δ2H measurements in streamflow exhibited sufficient variability to isolate the preferential release of younger water through preferential flow paths in the unsaturated zone. In contrast, the variability of ^δ2H in streamflow was insufficient to isolate the preferential release of younger water from the groundwater aquifer, as any seasonal variations in pore water ^δ2H were largely damped by substantial passive groundwater storage (water that mixes with the tracer signal of the active groundwater volume). Consistent with this interpretation, the degree of attenuation in the simulated streamflow isotope signal increased with increasing passive groundwater storage volumes and became pronounced when passive storage was orders of magnitude larger than active groundwater storage. The size of passive groundwater storage, in combination with groundwater SAS function parametrizations, regulated the long tails (100<T<1000 d) of transit time distributions, resulting in considerable uncertainty (± 20 % for HOAL and ± 23 % for Wüstebach) in the fraction of streamflow older than 100 d. The findings demonstrate that stable water isotope measurements from streamflow outlets is insufficient to constrain preferential groundwater flow in the two study catchments and plausibly in similar catchments characterized by large passive groundwater storage. The variability in streamflow TTD estimates arising from different groundwater storage SAS function parametrizations is considerable. Reducing uncertainty in groundwater transit time estimates and preferential flow contributions to streamflow requires complementary data sources, including multiple tracers, high-frequency tracer analysis, and groundwater-level observations, to improve catchment-scale transit time modelling.

The rainfall-runoff transformation in catchments usually follows a variety of slower and faster flow paths, leading to a mixture of "younger"and "older"water in streamflow. Previous studies have investigated the time-variable distribution of water ages in streamflow (transit time distribution, TTD) using stable isotopes of water (δ 18O, δ 2H) together with transport models based on Storage Selection (SAS) functions. These functions are traditionally formulated based on soil moisture to mimic the preferential release of younger water as the system becomes wetter. In this study, we hypothesized that, in a heterogeneous catchment with a significant fast-runoff response component, precipitation intensity, in addition to soil moisture, plays a critical role in the preferential release of younger water. To test this hypothesis, we used high-resolution δ 18O data (weekly and event-based streamflow δ18O samples) in a 66 ha agricultural catchment. We tested two scenarios of the SAS function parameterization for the preferential-flow age selection: one as a function of soil moisture only and one as a function of both soil moisture and precipitation intensity. The results showed that accounting for both soil moisture and precipitation intensity to define the shape of SAS functions for preferential flow improved the tracer simulation in streamflow (increasing the Nash-Sutcliffe efficiency from 0.31 to 0.51). This also led to a higher percentage of streamflow (an increase from 2.87 % to 4.38 %) with shorter transit times (TTs younger than 7 d), with the largest differences occurring during the summer and autumn months. This was due to the fact that incorporating both soil wetness and precipitation intensity in the SAS formulation accounts for rapid flow pathways such as infiltration excess overland flow, preferential flow through macropores, and tile drain flow - allowing precipitation water to bypass much of the soil matrix and to reach the stream with minimal storage or mixing, even under dry soil conditions. We showed for the agricultural study catchment that a significant portion of event water bypasses the soil matrix through fast-flow paths, resulting in younger water reaching the stream for both low- and high-intensity precipitation. Thus, in catchments where preferential flows and overland flow are the dominant flow processes, soil-wetness-dependent and precipitation-intensity-conditional SAS functions may be required to better describe the timescale of solute transport in modelling, which has implications for stream water quality and agricultural management practices such as the timing of fertilizer application.