The Demographics of Water: A Review of Water Ages in the Critical Zone

Matthias Sprenger*, Christine Stumpp, Markus Weiler, Werner Aeschbach, Scott T. Allen, Paolo Benettin, Maren Dubbert, Andreas Hartmann, Markus Hrachowitz, More Authors

*Corresponding author for this work

Research output: Contribution to journalReview articleScientificpeer-review

109 Citations (Scopus)
128 Downloads (Pure)


The time that water takes to travel through the terrestrial hydrological cycle and the critical zone is of great interest in Earth system sciences with broad implications for water quality and quantity. Most water age studies to date have focused on individual compartments (or subdisciplines) of the hydrological cycle such as the unsaturated or saturated zone, vegetation, atmosphere, or rivers. However, recent studies have shown that processes at the interfaces between the hydrological compartments (e.g., soil-atmosphere or soil-groundwater) govern the age distribution of the water fluxes between these compartments and thus can greatly affect water travel times. The broad variation from complete to nearly absent mixing of water at these interfaces affects the water ages in the compartments. This is especially the case for the highly heterogeneous critical zone between the top of the vegetation and the bottom of the groundwater storage. Here, we review a wide variety of studies about water ages in the critical zone and provide (1) an overview of new prospects and challenges in the use of hydrological tracers to study water ages, (2) a discussion of the limiting assumptions linked to our lack of process understanding and methodological transfer of water age estimations to individual disciplines or compartments, and (3) a vision for how to improve future interdisciplinary efforts to better understand the feedbacks between the atmosphere, vegetation, soil, groundwater, and surface water that control water ages in the critical zone.

Original languageEnglish
Pages (from-to)800-834
Number of pages35
JournalReviews of Geophysics
Issue number3
Publication statusPublished - 1 Jan 2019


  • critical Zone
  • stable isotopes
  • terrestrial water cycle
  • tracer hydrology
  • travel times
  • water ages


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