Abstract
The unsaturated zone is a globally important, dynamic water store, which affects water resources, agriculture and pollutant transport. Despite this, the magnitude of unsaturated zone water storage remains highly uncertain. This work provides the first global estimates of the magnitude of this store (1.0 x105 km3) in comparison to recent estimates of global modern groundwater (3.5x105 km3), before presenting a roadmap for improved representation of the unsaturated zone in global hydrological models.
Résumé
La zone non saturée est un réservoir d'eau dynamique d'importance mondiale, qui affecte les ressources en eau, l'agriculture et le transport des polluants. Malgré cela, l'ampleur du stockage de l'eau dans la zone non saturée reste très incertaine. Ce travail fournit les premières estimations mondiales de l'ampleur de ce stockage (1.0 x105 km3) en comparaison avec les estimations récentes des eaux souterraines modernes mondiales (3.5x105 km3), avant de présenter une feuille de route pour une meilleure représentation de la zone non saturée dans les modèles hydrologiques à l’échelle globale.
Resumen
La zona no saturada es un importante almacenamiento dinámico de agua a nivel mundial, que afecta a los recursos hídricos, la agricultura y el transporte de contaminantes. A pesar de ello, la magnitud del almacenamiento de agua en la zona no saturada sigue siendo muy incierta. Este trabajo proporciona las primeras estimaciones globales de la magnitud de este almacenamiento (1.0 x105 km3) en comparación con las estimaciones recientes de las aguas subterráneas modernas globales (3.5x105 km3), antes de presentar una guía para mejorar la representación de la zona no saturada en los modelos hidrológicos globales.
摘要
非饱和区是全球重要的动态储水地, 影响着水资源、农业和污染物传输。 不仅如此, 非饱和区水储量仍然具有高度不确定性。与最近对全球现代地下水 (3.5 x105 km3) 的估计相比, 这项工作先提出了改进全球水文模型中非饱和区概化的路线图, 再首次提供了对非饱和区储量 (1.0 x105 km3) 的全球估计。
Resumo
A zona não-saturada é um reservatório dinâmico de água, globalmente importante, que afeta os recursos hídricos, a agricultura e o transporte de poluentes. Apesar disso, a magnitude do armazenamento de água na zona não-saturada permanece muito incerto. Este trabalho fornece a primeira estimativa global da magnitude deste reservatório (1.0x105 km3) em comparação com estimativas globais recentes da água subterrânea moderna (3.5x105 km3), antes de apresentar um roteiro para uma melhor representação da zona não-saturada em modelos hidrológicos globais.
Similar content being viewed by others
Avoid common mistakes on your manuscript.
Introduction
Water stored in the unsaturated zone has been shown to influence pollutant transport, storage and fate (Koroša et al. 2020), surface vegetation (Querejeta et al. 2007) and biogeochemical cycling (Kim et al. 2017). The magnitude of water storage in the unsaturated zone has been highlighted to be large and of regional importance. For example, Zhu et al. (2019) estimate a volume of water up to 3.1 x 1012 m3 stored in the unsaturated zone of the Chinese Loess Plateau, which accounts for 42.1% of the water resources in this region. However, understanding the magnitude of water storage in the unsaturated zone at the global scale is currently very limited. Here, an overview is provided of the current state of the science related to unsaturated zone water storage at the global scale. A first estimate of the possible magnitude of the store is made and a roadmap to improve the representation of the unsaturated zone in global hydrological models (GHMs) is presented.
Monitoring and modelling unsaturated zone water storage at the global scale: a brief synopsis of the state of the science
Current understanding of water storage in the unsaturated zone at the global-scale is primarily based on soil moisture estimates from in situ measurements (Vereecken et al. 2008) and remote sensing (Vereecken et al. 2016; Peng et al. 2017). The development of a global-scale soil moisture monitoring network (Dorigo et al. 2011) has allowed large-scale evaluation of soil moisture data for incorporation into GHMs. However, a significant proportion of the globe has a depth to water table of over 10 m, with a modelled maximum depth to water of 236 m (Fan et al. 2013). In-situ and remotely sensed soil moisture data can only penetrate c. 1 m depth (Ahlmer et al. 2018), and consequently this leaves a large proportion of storage in unsaturated bedrock relatively unaccounted for (Dawson et al. 2020).
Very few studies have characterised moisture and storage in the deep unsaturated zone (taken here to be beneath the zero-flux plane, the depth below which water movement is downward) (Ireson et al. 2006; Zhu et al. 2019). Studies of the deeper unsaturated zone are principally carried out with the aim of quantifying recharge to the saturated zone (Wang et al. 2009; Mattern and Vanclooster 2010; Min et al. 2015; Xiang et al. 2019). Such studies have used tracer experiments (Li et al. 2017) and solute profiling (Huang et al. 2016) alongside other conventional recharge estimation techniques (Healy and Scanlon (2010)). The rate of recharge to groundwater has been estimated in global modelling by the rate of drainage through the soil zone (Lawrence et al. 2011; Min et al. 2015) with little or no consideration of the underlying bedrock geology, or depth to water table (Schlemmer et al. 2018). Whilst remote sensing and earth observation data (e.g. GRACE) have been used extensively to identify changes in groundwater storage at the global scale (e.g. Li et al. 2019), such data cannot differentiate between water storage in the saturated and unsaturated zone without additional secondary data (e.g estimation of saturated zone water storage from in-situ measurements, see for example Brookfield et al. (2018)).
Where in situ measurements are not practical, pedo-transfer functions have been used to derive hydraulic properties of the unsaturated zone from surface soil textural classes (Tóth et al. 2015). However, these are often confined to the top few metres of the unsaturated zone (Riebe et al. 2017). Pedo-transfer functions based on soil textural classes cannot be extrapolated accurately through the unsaturated zone, due to differences in the hydraulic properties and flow mechanisms between the bedrock and overlying soils (Katsura et al. 2006). Rempe and Dietrich (2018) differentiate between soil water and water stored in fractured bedrock, describing water storage in the bedrock as ‘rock moisture’; they further highlight its importance and lack of definition and inclusion in GHMs. In the context of the global water budget, no estimate of the magnitude of global unsaturated zone water storage has been made to date.
A first estimate of the magnitude of unsaturated zone water storage at the global scale
Using a global map of depth to water table (Fan et al. 2013) and a global porosity dataset (Gleeson et al. 2014) at a 0.25-degree global resolution, the potential magnitude of water storage in the unsaturated zone globally is calculated. Assuming a global average water saturation of 25% +/- 10%, total global water storage in the unsaturated zone is estimated to be 1.0 x105km3 +/- 4.0 x 104km3. A best estimate of the modern groundwater volume in the saturated zone is 3.5x105km3 (2.4x105km3 - 3.8x106km3 accounting for uncertainty in model permeability, Gleeson et al. (2016)), with the majority of this volume thought to be stored in the upper 150 m below the ground surface (Gleeson et al. 2016). This is the same order of magnitude as the first unsaturated zone water estimate above.
A roadmap to improving representation of the unsaturated zone in GHMs
Significant uncertainties in water saturation at depth means that the estimate of global unsaturated zone water storage made above can only be used for the purpose of highlighting the possible magnitude of the store. Given the potentially significant water storage volume and its implications for water supply to surface vegetation (Beyer et al. 2016), groundwater protection (Saâdi et al. 2018) and aquifer recharge (Mattern and Vanclooster 2010), improved representation of unsaturated zone water storage in GHMs is essential.
Figure 1 outlines a roadmap to achieve this. A significant challenge at present is the lack of standardised terminology across different research communities (soil scientists, hydrogeologists, global hydrological modellers, scientists working with remote sensing earth observation datasets, e.g. GRACE) related to the unsaturated zone, particularly the distinction between shallow soil moisture and deeper unsaturated water storage. Consequently, a concerted effort to build consensus across these research communities to agree common terminology and meaning (e.g. providing definitions of the “deep” unsaturated zone) as well as data and reporting standards for future work is required. This initial task in itself is a long term investment and could be achieved through a new commission of the International Association of Hydrogeologists (IAH) and a number of community workshops. Common data and reporting standards should build on existing work to develop data models for the geosciences (e.g the Open Geospatial Commission Geoscience Markup Language). Such agreed terminology and standards should be published for reference for future research. Subsequently, a collation and synthesis of data from existing peer-reviewed and grey literature should be undertaken. Although a considerable task, this could consist of extraction of data related to the unsaturated zone (e.g. unsaturated zone water content) and how this varies with depth, time and hydrogeological setting. Data mining techniques could be exploited to achieve this, and the resulting data should be made publicly available (including through dissemination through data-focussed journals) and evaluated to develop a conceptualisation of unsaturated zone water movement at the global scale as a function of the variables above. Both the collated data and conceptualisation would then be used to re-evaluate structure and parameterisation of unsaturated zone processes in GHMs. The differences in scale, measurement and modelling approaches between field studies and regional to global observational datasets and models makes this challenging, and so the development and use of upscaling techniques should be central to this task. Where the data and conceptualisation show GHMs to have structural and parametric deficiencies (e.g. direct connection between the soil zone and saturated zone), further work could be undertaken to refine these GHMs. It is anticipated that such refined GHMs could (1) lead to improved predictions of the impacts of global change on the hydrological cycle and (2) help identify regional scale evidence gaps where further observational studies would be of benefit to better constrain the significance of unsaturated zone water storage.
The roadmap outlined here is intended to stimulate discussion amongst the hydrogeological research community as to how to engage with the soil science and global scale hydrological modelling communities to improve representation of the unsaturated zone in GHMs. The broad activities required are outlined above and in Fig. 1. However, this essay avoids being overly prescriptive on the required tasks, as such efforts need to be driven by the wider hydrogeological research community from the bottom-up. Whilst developed at the global scale, many of the principles (agreement of terminology, data synthesis and evaluation, existing model evaluation and refinement) of Fig. 1 also apply at the scale of large river basins, and could be adapted here accordingly.
References
Ahlmer AK, Cavalli M, Hansson KK, A J, Crema S, Kalantari Z (2018) Soil moisture remote-sensing applications for identification of flood-prone areas along transport infrastructure. Environ Earth Sci 77:533. https://doi.org/10.1007/s12665-018-7704-z
Beyer M, Koeniger P, Gaj M, Hamutoko JT, Wanke H, Himmelsbach T (2016) A deuterium-based labeling technique for the investigation of rooting depths, water uptake dynamics and unsaturated zone water transport in semiarid environments. J Hydrol 533:627–643
Brookfield AE, Hill MC, Rodell M, Loomis BD, Stotler RL, Porter ME, Bohling GC (2018) In situ and GRACE-based groundwater observations: similarities, discrepancies, and evaluation in the High Plains aquifer in Kansas. Water Resour Res 54:8034–8044
Dawson TE, Hahm WJ, Crutchfield-Peters K (2020) Digging deeper: what the critical zone perspective adds to the study of plant ecophysiology. New Phytologist
Dorigo WA, Wagner W, Hohensinn R, Hahn S, Paulik C, Xaver A, Gruber A, Drusch M, Mecklenburg S, van Oevelen P, Robock A (2011) International Soil Moisture Network: a data hosting facility for global in situ soil moisture measurements
Fan Y, Li H, Miguez-Macho G (2013) Global patterns of groundwater table depth. Science 339(6122):940–943
Gleeson T, Moosdorf N, Hartmann J, Van Beek LPH (2014) A glimpse beneath earth's surface: GLobal HYdrogeology MaPS (GLHYMPS) of permeability and porosity. Geophys Res Lett 41(11):3891–3898
Gleeson T, Befus KM, Jasechko S, Luijendijk E, Cardenas MB (2016) The global volume and distribution of modern groundwater. Nat Geosci 9(2):161–167
Healy R, Scanlon B (2010) Estimating Groundwater Recharge. Cambridge University Press, Cambridge. https://doi.org/10.1017/CBO9780511780745
Huang T, Yang S, Liu J, Li Z (2016) How much information can soil solute profiles reveal about groundwater recharge? Geosci J 20(4):495–502
Ireson AM, Wheater HS, Butler AP, Mathias SA, Finch J, Cooper JD (2006) Hydrological processes in the Chalk unsaturated zone–insights from an intensive field monitoring programme. J Hydrol 330(1-2):29–43
Katsura SY, Kosugi KI, Yamamoto N, Mizuyama T (2006) Saturated and unsaturated hydraulic conductivities and water retention characteristics of weathered granitic bedrock. Vadose Zone J 5(1):35–47
Kim H, Dietrich WE, Thurnhoffer BM, Bishop JK, Fung IY (2017) Controls on solute concentration-discharge relationships revealed by simultaneous hydrochemistry observations of hillslope runoff and stream flow: The importance of critical zone structure. Water Resour Res 53(2):1424–1443
Koroša A, Brenčič M, Mali N (2020) Estimating the transport parameters of propyphenazone, caffeine and carbamazepine by means of a tracer experiment in a coarse-gravel unsaturated zone. Water Res:115680
Lawrence DM, Oleson KW, Flanner MG, Thornton PE, Swenson SC, Lawrence PJ, Zeng X, Yang ZL, Levis S, Sakaguchi K, Bonan GB (2011) Parameterization improvements and functional and structural advances in version 4 of the Community Land Model. J Adv Model Earth Syst, 3(1)
Li Z, Chen X, Liu W, Si B (2017) Determination of groundwater recharge mechanism in the deep loessial unsaturated zone by environmental tracers. Sci Total Environ 586:827–835
Li B, Rodell M, Kumar S, Beaudoing HK, Getirana A, Zaitchik BF, de Goncalves LG, Cossetin C, Bhanja S, Mukherjee A, Tian S (2019) Global GRACE data assimilation for groundwater and drought monitoring: advances and challenges. Water Resour Res 55(9):7564–7586
Mattern S, Vanclooster M (2010) Estimating travel time of recharge water through a deep vadose zone using a transfer function model. Environ Fluid Mech 10(1-2):121–135
Min L, Shen Y, Pei H (2015) Estimating groundwater recharge using deep vadose zone data under typical irrigated cropland in the piedmont region of the North China Plain. J Hydrol 527:305–315
Peng J, Loew A, Merlin O, Verhoest NE (2017) A review of spatial downscaling of satellite remotely sensed soil moisture. Rev Geophys 55(2):341–366
Querejeta JI, Estrada-Medina H, Allen MF, Jiménez-Osornio JJ (2007) Water source partitioning among trees growing on shallow karst soils in a seasonally dry tropical climate. Oecologia 152(1):26–36
Rempe DM, Dietrich WE (2018) Direct observations of rock moisture, a hidden component of the hydrologic cycle. Proc Natl Acad Sci 115(11):2664–2669
Riebe CS, Hahm WJ, Brantley SL (2017) Controls on deep critical zone architecture: A historical review and four testable hypotheses. Earth Surf Process Landf 42(1):128–156
Saâdi M, Zghibi A, Kanzari S (2018) Modeling interactions between saturated and un-saturated zones by Hydrus-1D in semi-arid regions (plain of Kairouan, Central Tunisia). Environ Monit Assess 190(3):170
Schlemmer L, Schär C, Lüthi D, Strebel L (2018) A groundwater and runoff formulation for weather and climate models. J Adv Model Earth Syst 10(8):1809–1832
Tóth B, Weynants M, Nemes A, Makó A, Bilas G, Tóth G (2015) New generation of hydraulic pedotransfer functions for Europe. Eur J Soil Sci 66(1):226–238
Vereecken H, Huisman JA, Bogena H, Vanderborght J, Vrugt JA, Hopmans JW (2008) On the value of soil moisture measurements in vadose zone hydrology: A review. Water Resour Res, 44(4)
Vereecken H, Schnepf A, Hopmans JW, Javaux M, Or D, Roose T, Vanderborght J, Young MH, Amelung W, Aitkenhead M, Allison SD (2016) Modeling soil processes: Review, key challenges, and new perspectives. Vadose Zone J, 15(5)
Wang T, Zlotnik VA, Šimunek J, Schaap MG (2009) Using pedotransfer functions in vadose zone models for estimating groundwater recharge in semiarid regions. Water Resour Res, 45(4)
Xiang W, Si BC, Biswas A, Li Z (2019) Quantifying dual recharge mechanisms in deep unsaturated zone of Chinese Loess Plateau using stable isotopes. Geoderma 337:773–781
Zhu Y, Jia X, Qiao J, Binley A, Horton R, Hu W, Wang Y, Shao MA (2019) Capacity and Distribution of Water Stored in the Vadose Zone of the Chinese Loess Plateau. Vadose Zone J, 18(1)
Acknowledgments
SYB, MJA, DCG and JPB publish with permission of the Director, British Geological Survey. This research was funded by the British Geological Survey’s Environmental Change, Adaptation and Resilience Challenge National Capability programme (UK Research and Innovation).
Author information
Authors and Affiliations
Corresponding author
Ethics declarations
Conflict of interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.
Additional information
Publisher’s note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Rights and permissions
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
About this article
Cite this article
Bunting, S.Y., Ascott, M.J., Gooddy, D.C. et al. Towards improved global estimates and model representations of water storage in the unsaturated zone. Hydrogeol J 30, 1933–1936 (2022). https://doi.org/10.1007/s10040-022-02520-6
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1007/s10040-022-02520-6