AUTHOR=Dwivedi Ravindra , Eastoe Christopher , Knowles John F. , McIntosh Jennifer , Meixner Thomas , Ferre Paul A. Ty , Minor Rebecca , Barron-Gafford Greg , Abramson Nathan , Stanley Michael , Chorover Jon 

TITLE=Tandem Use of Multiple Tracers and Metrics to Identify Dynamic and Slow Hydrological Flowpaths

JOURNAL=Frontiers in Water

VOLUME=4

YEAR=2022

URL=https://www.frontiersin.org/journals/water/articles/10.3389/frwa.2022.841144

DOI=10.3389/frwa.2022.841144

ISSN=2624-9375

ABSTRACT=<p>Current understanding of the dynamic and slow flow paths that support streamflow in mountain headwater catchments is inhibited by the lack of long-term hydrogeochemical data and the frequent use of short residence time age tracers. To address this, the current study combined the traditional mean transit time and the state-of-the-art fraction of young water (<italic>F</italic><sub><italic>yw</italic></sub>) metrics with stable water isotopes and tritium tracers to characterize the dynamic and slow flow paths at Marshall Gulch, a sub-humid headwater catchment in the Santa Catalina Mountains, Arizona, USA. The results show that <italic>F</italic><sub><italic>yw</italic></sub> varied significantly with period when using sinusoidal curve fitting methods (e.g., iteratively re-weighted least squares or IRLS), but not when using the transit time distribution (TTD)-based method. Therefore, <italic>F</italic><sub><italic>yw</italic></sub> estimates from TTD-based methods may be particularly useful for intercomparison of dynamic flow behavior between catchments. However, the utility of <sup>3</sup>H to determine <italic>F</italic><sub><italic>yw</italic></sub> in deeper groundwater was limited due to both data quality and inconsistent seasonal cyclicity of the precipitation <sup>3</sup>H time series data. Although a Gamma-type TTD was appropriate to characterize deep groundwater, there were large uncertainties in the estimated Gamma TTD shape parameter arising from the short record length of <sup>3</sup>H in deep groundwater. This work demonstrates how co-application of multiple metrics and tracers can yield a more complete understanding of the dynamic and slow flow paths and observable deep groundwater storage volumes that contribute to streamflow in mountain headwater catchments.</p>