Safeguarding water resources for society and ecosystems requires a comprehensive understanding of hydrological fluxes within the Critical Zone, Earth's living skin where the atmosphere, hydrosphere, biosphere, and lithosphere meet. For decades, tracer-aided mixing models have been used to track water flow paths through the Critical Zone, mapping the journey of water particles from atmospheric moisture to groundwater. Recent advances in novel tracer measurements and modeling methodologies offer new insights into hydrological partitioning within the Critical Zone, enabling improved quantification of water fluxes across scales ranging from microscopic to macroscopic. Advanced tracer-aided modeling approaches enable more rigorous testing of assumptions and improved quantification of uncertainties. In this review, we (a) summarize state-of-the-art tracer and modeling techniques, with an emphasis on stable water isotope tracers, (b) synthesize insights emerging from new approaches, and (c) highlight opportunities to apply these methods in interdisciplinary Critical Zone research.

The Young Hydrologic Society (YHS) is a grassroots initiative to stimulate the interaction and active participation of early career hydrologists within the hydrological sciences community and beyond. Our vision is to reform hydrologic community towards a more active and inclusive involvement of early career hydrologists. YHS mission is to connect and inform. With YHS, we want to (1) connect early career scientists (ECS) by regularly organizing scientific and professional development sessions/workshops, as well as social events at geoscience conferences e.g. EGU, AGU, and IAHS; (2) inform YHS members about successes/issues of the hydrologic community and the current and future research topics of the field. Social media, such as Twitter, blogs or Facebook have been instrumental for YHS to reach its goals. But we noticed that, even in the era of online communication, face-to-face contact remains to be the most effective way to foster connections globally and raise awareness about our network. Actively participating in workshops, seminars, and roundtable discussions at international conferences requires more resources, but the memories are longer lasting and can be even more meaningful/inspiring than what online posts provides. YHS takes pride in its strategy to diversify and empower National Initiatives (e.g. Canadian YHS) as well as actively collaborate with other ECS networks such as Student Subcommittee of AGU Hydrology Section (H3S), Young Earth System Scientists (YESS) community, and IAHS Early Career Committee. We recommend that diversifying ECS networks, engaging ECS in convening scientific sessions, in addition to expanding ECS-focused workshops/events should be an inalienable component of geoscience conferences.