Hydrological systems are undergoing profound transformations under the combined effects of climate variability, climate change, land-use change, and intensifying anthropogenic pressures. Deciphering these evolving dynamics requires robust tracers and analytical frameworks that can capture the interplay between natural and anthropogenic drivers. Recent advances in isotopic and geochemical tools provide powerful means to trace water fluxes, identify sources and pathways, and quantify the impacts of environmental change across multiple spatial and temporal scales. They also offer breakthroughs in hydrosystem characterization and new opportunities to couple tracer data with predictive modeling. This session will highlight the contributions of stable and radiogenic isotopes, elemental geochemistry, and emerging multi-tracer approaches, and how to couple them to numerical modelling in hydrological research. Special attention will be given to methodological advances (e.g., high-frequency isotope monitoring, compound-specific isotope analysis, geochemical fingerprinting) and to integrative applications that connect hydrology, geochemistry, climate science, and traditional or local knowledge. The session will also investigate how isotopic and geochemical datasets can improve predictive modeling, identify early-warning signals of critical thresholds in hydrosystems, and guide evidence-based adaptation strategies. Case studies from diverse regions are welcome to illustrate how multi-method approaches generate actionable knowledge for sustainable water management and resilience to hydrological changes.