Summary
This comprehensive review synthesises research on water ages throughout the critical zone, emphasising that interfacial mixing processes between hydrological compartments—rather than individual compartment dynamics alone—critically govern water travel times and contaminant transport. The authors argue that disciplinary siloes have underestimated these interfacial dynamics and advocate for improved interdisciplinary methods combining hydrological tracers with process understanding. The review identifies both methodological advances and substantial knowledge gaps constraining quantification of feedbacks between atmosphere, vegetation, soil, groundwater, and surface water.
UK applicability
The methodological framework and tracer approaches reviewed are applicable to UK hydrological research and water quality assessment, particularly for understanding contaminant and nutrient transport in the highly variable UK critical zone. The emphasis on interfacial mixing and compartment interactions is relevant to UK soil and groundwater management under variable rainfall and land use.
Key measures
Water age distributions; water residence times; tracer methodologies (isotopic and chemical); mixing dynamics at soil–atmosphere, soil–groundwater, and other hydrological compartment interfaces
Outcomes reported
The review synthesises water age research across the critical zone (vegetation canopy to groundwater) and demonstrates that mixing processes at compartment interfaces substantially govern water travel times and age distributions. The authors identify tracer methodologies, limiting assumptions in current disciplinary approaches, and recommend interdisciplinary frameworks for understanding water residence times.
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