Abstract
Current understanding of the dynamic flow paths and subsurface water storages that support streamflow in mountain catchments is inhibited by the lack of long-term hydrologic data and the frequent use of single age tracers that are not applicable to older groundwater reservoirs. To address this, the current study used both multiple metrics and tracers to characterize the transient nature of flow paths with respect to change in catchment storage at Marshall Gulch, a sub-humid headwater catchment in the Santa Catalina Mountains, Arizona, USA. The fraction of streamflow that was untraceable using stable water isotope tracers was also estimated. A Gamma-type transit time distribution (TTD) was appropriate for deep groundwater analysis, but there were errors in the TTD shape parameters arising from the short record length of 3H in deep groundwater and stream water, and inconsistent seasonal cyclicity of the precipitation 3H time series data. Overall, the mean transit time calculated from 3H data was more than two decades greater than the mean transit time based on δ18O at the same site. The fraction of young water (Fyw) in shallow groundwater was estimated from δ18O time series data using weighted wavelet transform (WWT), iteratively re-weighted least squares (IRLS), and TTD-based methods. Estimates of Fyw depended on sampling frequency, the method of estimation, bedrock geology, hydroclimate, and factors affecting streamflow generation processes. The coupled use of Fyw and discharge sensitivity indicated highly dynamic flow paths that reorganized with changes in shallow catchment storage. The utility of 3H to determining Fyw in deeper groundwater was limited by data quality. Given that Fyw, discharge sensitivity, and mean transit time all yield unique information, this work demonstrates how co-application of multiple methods can yield a more complete understanding of the transient flow paths and observable storage volumes that contribute to streamflow in mountain headwater catchments.
Highlights
Mountain headwater catchments are critical sources of water to downstream valley-fill aquifers (Viviroli et al, 2007;Viviroli et al, 2003;Kohler and Marselli, 2009;Carroll et al, 2019;Milly and Dunne, 2020;Harpold et al, 2012;Eppolito and Fonseca, 2021;Bryan, 2021)
This recommendation originates from previous work 55 that shows how mean transit time estimates based on stable water isotopes alone may be underestimated because the tails of the transit time distributions (TTDs) that correspond to longer transit times can become truncated (Stewart et al, 2010;Stewart et al, 2012;Frisbee et al, 2013;DeWalle et al, 1997), and takes into consideration that certain model performance criteria (e.g., NashSutcliffe Efficiency) are insensitive to longer transit times (Seeger and Weiler (2014)
The current study addresses the following research questions at a high-elevation, sub-humid 90 mountain site: (i) what is the appropriate TTD type and mTT for the deep groundwater system that supports streamflow? (ii) What are the fraction of young water (Fyw) and resulting Fyw-based catchment storage estimates calculated from age tracers applicable to younger and older groundwater and stable water isotope and 3H time series data, respectively? (iii) What is the discharge sensitivity of Fyw as determined by stable water isotope tracers? Following a description of the field site and data, we describe theoretical models and estimation 95 methods for 3H-based TTD and mTT estimation and Fyw and its discharge sensitivity
Summary
Mountain headwater catchments are critical sources of water to downstream valley-fill aquifers (Viviroli et al, 2007;Viviroli et al, 2003;Kohler and Marselli, 2009;Carroll et al, 2019;Milly and Dunne, 2020;Harpold et al, 2012;Eppolito and Fonseca, 2021;Bryan, 2021). Dual tracer approaches using stable water isotopes and tritium (3H) are recommended to determine the contribution of deep groundwater to streamflow in mountain sites characterized by fractured bedrock aquifers (Stewart et al, 2010;Stewart et al, 2012). This recommendation originates from previous work 55 that shows how mean transit time (mTT) estimates based on stable water isotopes alone may be underestimated because the tails of the transit time distributions (TTDs) that correspond to longer transit times can become truncated (Stewart et al, 2010;Stewart et al, 2012;Frisbee et al, 2013;DeWalle et al, 1997), and takes into consideration that certain model performance criteria (e.g., NashSutcliffe Efficiency) are insensitive to longer transit times (Seeger and Weiler (2014). The current study leverages the use of 3H as a tracer with longer-period variations as a means to more completely characterize deeper flow paths that contribute to streamflow
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