Out of all our natural resources, water underpins sustainable development by far the most, and is thus critical in achieving the Sustainable Development Goals (SDG) defined by the United Nations in 2015. While water is the focus of only 1 of the 17 SDG, guaranteeing safe and sustainable drinking water resources to future societies is elementary for achieving every single SDG. Groundwater (GW) – the most elusive component of the water cycle – comprises more than 30% of the word’s freshwater resources. While often overlooked or dramatically over-simplified in hydrological analyses, a robust assessment of the temporal and spatial distribution of GW quantity, quality and renewal rates was identified as a key factor in designing resilient and sustainable water resources management plans for future societies. One of the hydrologic systems most in need of improved methods for the development of integrated water management plans are tectonically active, volcanic watersheds, particularly volcanic island states, as volcanic systems represent an important and highly valuable source of clean water, owing to their typically large subsurface permeability & porosity (i.e., outstanding filtering capacity) and good water quality. The water cycle of the volcanic island nation Japan (JP) with its over 100 active volcanos, for example, is deeply linked to the volcanic activity of the Pacific Ring of Fire. The tendency of volcanic systems to form complex networks of faults, fissures, and clinkers makes Japan’s watersheds some of the most complex on Earth, with groundwater-surface water (GW-SW) interactions being even more dynamic than elsewhere and GW flow paths extremely difficult to identify and characterize. To design resilient and sustainable water resources management plans of volcanic watersheds for future societies, a robust conceptual understanding and accurate quantification of GW recharge rates, flow paths, and SW-GW interactions are indispensable. However, most of the widely employed hydrological tracer and modelling methods are unsuitable for a robust assessment of GW circulation, renewal rates, and GW-SW interactions in volcanic systems, and conceptual gaps in understanding volcanic GW circulation exist. Considering the accelerated anthropogenic and climate-induced changes to our environment, international crises and increasing water use conflicts, an interdisciplinary understanding of volcanic GW systems is more important than ever before. To overcome these impediments and provide guidance for the design of resilient sustainable integrated water resources management plans for future societies, new hydrological tracer and modelling methods that can be used to detect and quantify deep GW circulation in tectonically active, volcanic watersheds must be developed. In this research project, we thus develop a framework for the integrated assessment of water resources in volcanic regions, building on technologies that only recently became accessible for watershed-scale hydrological studies: Integrated surface-subsurface hydrological modelling, high-resolution 3-D models of complex geological systems, continuous on-site monitoring of dissolved gases and microbes in water, and high resolution local/regional projections of climate change until the end of the 21^st century. The watershed of Mt. Fuji, JP, where deep GW was found to flow up along the tectonically most active fault system of JP and enrich shallow GW and springs based on the aforementioned tracers, will serve as experimental site. Continuous monitoring stations of deep and shallow GW and spring sites will be setup, combining novel gas-equilibrium membrane-inlet portable mass spectrometry and online flow cytometry technologies. The continuous tracer analyses will be complemented with a large body of additional tracer analyses and hydraulic observations, serve to detect the influence of seismicity on deep GW circulation, and be used for calibration of an integrated SW-GW flow model of Fuji watershed. Climate change and water use projections will then be used for predictive modelling with the calibrated model, and based thereon, resilient and sustainable water resources management strategies designed.