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

Abstract

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.

Depth distribution of soil water sourced by plants at the global scale: A new direct inference approach

“…To minimize energy use, plants are likely to extract water from soil layers with highest rooting density assuming the soil is uniformly wet (Adiku, Rose, Braddock, & Ozier‐Lafontaine, ) and at the highest available water potential (i.e., easiest to withdraw; Gardner, ). Moreover, plants can often take up relatively “new” water (Sprenger et al, ) although some recent studies showed that water transpired from trees during summer originated from rain that fell during the previous winter (Allen et al, ; Brinkmann et al, ).…”

“…Thus the role of deep root water uptake across ecosystems may have been underestimated (Pierret et al, 2016). Studies have shown that in very deep soil, some trees can develop deep roots that may access "old" waters (Sprenger et al, 2019;Zhang et al, 2017).…”

Using isotopes to incorporate tree water storage and mixing dynamics into a distributed ecohydrologic modelling framework

“…Emerging methods using stable isotopes in water ( 2 H and 18 O) have led to insights into the age distributions of soil water percolate and plant root water uptake (RWU; e.g., Sprenger et al, 2019Sprenger et al, , 2018Evaristo et al, 2019;Benettin, Queloz, Bensimon, McDonnell, & Rinaldo, 2019;Knighton, Coneelly, & Walter, 2019). Recent studies have employed stable isotopes and variations of the end member mixing analysis (EMMA) methodology to estimate the proportional contribution of different subsurface water sources to RWU.…”

Spatiotemporal variability in water sources of urban soils and trees in the semiarid, irrigated Salt Lake Valley

“…Mismatches between stem water and available water sources might reflect spatial heterogeneity of water sources in the soil (Brunel, Walker, & Kennett-Smith, 1995) as well as limitations in our current understanding of soil water isotopic composition . Instead of a single well-mixed reservoir of soil moisture (Hewlett & Hibbert, 1967), soil water may be ecohydrologically partitioned into multiple, unmixed water pools that vary in age and mobility (McCutcheon, McNamara, Kohn, & Evans, 2017;Sprenger et al, 2019;Sprenger, Leistert, Gimbel, & Weiler, 2016). This incomplete mixing and compartmentalisation of water sources has been described as the "two-water worlds" hypothesis based on observations of plants that appear to utilize soil water with low mobility instead of water that follows preferential flow pathways (Brooks et al, 2010;McDonnell, 2014).…”