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

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. © 2019. The Authors.
This review is based on discussions at the workshop on Water Ages in the Hydrological Cycle, held in the Black Forest in October 2017 and funded by the Wassernetzwerk Baden-W?rttemberg. We thank the attendees of the workshop R. B?umle, T. Gebauer, D. Gooddy, P. K?niger, M. Loschko, V. Maurer, R. Purtschert, S. Seeger, and H. Wang. M. S. thanks Wassernetzwerk Baden-W?rttemberg and the Deutsche Forschungsgemeinschaft (DFG)?Project 397306994?for funding. N. R. thanks the Luxembourg National Research Fund (C14/SR/8353440/STORE-AGE) for financial support. A. H. was supported by the Emmy Noether Programme of the Deutsche Forschungsgemeinschaft DFG (HA 8113/1-1). This research manuscript by Matthias Sprenger was supported by the Ministerium f?r Wissenschaft, Forschung und Kunst Baden-W?rttemberg (Az. 33-7533.-25-11/31).