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

• Published in: Reviews of geophysics (1985) Vol. 57; no. 3; pp. 800 - 834
• Main Authors: Sprenger, Matthias, Stumpp, Christine, Weiler, Markus, Aeschbach, Werner, Allen, Scott T., Benettin, Paolo, Dubbert, Maren, Hartmann, Andreas, Hrachowitz, Markus, Kirchner, James W., McDonnell, Jeffrey J., Orlowski, Natalie, Penna, Daniele, Pfahl, Stephan, Rinderer, Michael, Rodriguez, Nicolas, Schmidt, Maximilian, Werner, Christiane
• Format: Journal Article
• Language: English
• Published: 01.09.2019
• Subjects: critical Zone; stable isotopes; terrestrial water cycle; tracer hydrology; travel times; water ages
• ISSN: 8755-1209; 1944-9208
• DOI: 10.1029/2018RG000633

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. Plain language Summary Investigating how long it takes for a drop of rainwater until it is either evaporated back to the atmosphere, taken up by plants, or infiltrated into groundwater or discharged in streams provides new understanding of how water flows through the water cycle. Knowledge about the time water travels further helps assessing groundwater recharge, transport of contaminants, and weathering rates. Such water age studies typically focus either on water in individual compartments of the water cycle such as soils, groundwater, or stream runoff. But we argue that the interfaces between these compartments can have an influence on the water age. Here, we present methods how water ages can be estimated using tracers and hydrological models. We further discuss the “demographics of water” (water age distribution) in the critical zone that spans from the tree canopy to the bottom of the groundwater. Our review highlights how water flows and mixes between plants, soils, groundwater, and streams and how this interaction affects the water ages. This way, our work contributes toward a better understanding of vital resource water sustaining the life in the Earth's living skin. Key Points New tracer techniques now allow tracking water at high spatiotemporal resolution across the vastly varying water ages in the water cycle Exchanges of water between hydrological compartments at key interfaces influence the water age distribution more than previously assumed Variation from complete to nearly absent mixing of water at the interfaces in the critical zone affects the water ages in compartments