Large scale hydrologic and tracer aided modelling: A review

Stable isotopes in water (oxygen-18 and deuterium) are hydrologic tracers, which have been embedded into both analytical mass balance and physically based continuous simulation models, improving hydrologic process partitioning and regional source identification, resulting in early climate change detection. Recent advances in the field of isotope hydrology have improved coordinated investment in global monitoring networks, invested in integrated modelling tools to support prediction and model evaluation, and expanded the scale of analyses possible. This review focuses on the applications of isotope tracers to large scale hydrologic modelling, its inception and evolution, and future directions for advancing this field with emphasis on the utility of isotope tracers for operational water resources prediction. The hydrologic community has begun to steer toward alternative hydrologic data due to the coinciding realities of limited data records and urgent data needs. Water resources infrastructure and management requires projections of water supply beyond the 50- to 100-year design life, with the general rule of flow frequency necessitating a 100- to 200-year assessment, however, there exist few data networks globally with records sufficient to support this kind of long-term assessment, and climate change is increasing the frequency of these extreme events. Historically, large-scale, and long time series simulation has meant less attention on process function (model fidelity) due to computational constraints, however, we argue that this may no longer be adequate under increasingly complex climate-hydrologic feedbacks, and integrated system models that incorporate human decisions, environmental health and socio-economic pressures. To expand the uptake of isotope tracers in practice, we emphasize their value for extreme quantile prediction under future projections and call for the community of practice to support data networks and accessibility, support peer review of large-domain (less data intensive) applications, and employ the FAIR (findable, accessible, interoperable, reusable) scientific principles. Though still an emerging scientific field, isotope hydrology has considerable value to the practitioner community with the proper education and training necessary to build confidence, underpinned by well developed and accessible tools for deployment.