Tom Gleeson

Water balance of global aquifers revealed by groundwater footprint

Groundwater is a life-sustaining resource that supplies water to billions of people, plays a central part in irrigated agriculture and influences the health of many ecosystems. Most assessments of global water resources have focused on surface water, but unsustainable depletion of groundwater has recently been documented on both regional and global scales. It remains unclear how the rate of global groundwater depletion compares to the rate of natural renewal and the supply needed to support ecosystems. Here we define the groundwater footprint (the area required to sustain groundwater use and groundwater-dependent ecosystem services) and show that humans are overexploiting groundwater in many large aquifers that are critical to agriculture, especially in Asia and North America. We estimate that the size of the global groundwater footprint is currently about 3.5 times the actual area of aquifers and that about 1.7 billion people live in areas where groundwater resources and/or groundwater-dependent ecosystems are under threat. That said, 80 per cent of aquifers have a groundwater footprint that is less than their area, meaning that the net global value is driven by a few heavily overexploited aquifers. The groundwater footprint is the first tool suitable for consistently evaluating the use, renewal and ecosystem requirements of groundwater at an aquifer scale. It can be combined with the water footprint and virtual water calculations, and be used to assess the potential for increasing agricultural yields with renewable groundwaterref. The method could be modified to evaluate other resources with renewal rates that are slow and spatially heterogeneous, such as fisheries, forestry or soil.

Towards Sustainable Groundwater Use: Setting Long‐Term Goals, Backcasting, and Managing Adaptively

The sustainability of crucial earth resources, such as groundwater, is a critical issue. We consider groundwater sustainability a value-driven process of intra- and intergenerational equity that balances the environment, society, and economy. Synthesizing hydrogeological science and current sustainability concepts, we emphasize three sustainability approaches: setting multigenerational sustainability goals, backcasting, and managing adaptively. As most aquifer problems are long-term problems, we propose that multigenerational goals (50 to 100 years) for water quantity and quality that acknowledge the connections between groundwater, surface water, and ecosystems be set for many aquifers. The goals should be set by a watershed- or aquifer-based community in an inclusive and participatory manner. Policies for shorter time horizons should be developed by backcasting, and measures implemented through adaptive management to achieve the long-term goals. Two case histories illustrate the importance and complexity of a multigenerational perspective and adaptive management. These approaches could transform aquifer depletion and contamination to more sustainable groundwater use, providing groundwater for current and future generations while protecting ecological integrity and resilience.

The pronounced seasonality of global groundwater recharge

Groundwater recharged by meteoric water supports human life by providing two billion people with drinking water and by supplying 40% of cropland irrigation. While annual groundwater recharge rates are reported in many studies, fewer studies have explicitly quantified intra-annual (i.e., seasonal) differences in groundwater recharge. Understanding seasonal differences in the fraction of precipitation that recharges aquifers is important for predicting annual recharge groundwater rates under changing seasonal precipitation and evapotranspiration regimes in a warming climate, for accurately interpreting isotopic proxies in paleoclimate records, and for understanding linkages between ecosystem productivity and groundwater recharge. Here we determine seasonal differences in the groundwater recharge ratio, defined here as the ratio of groundwater recharge to precipitation, at 54 globally distributed locations on the basis of 18O/16O and 2H/1H ratios in precipitation and groundwater. Our analysis shows that arid and temperate climates have wintertime groundwater recharge ratios that are consistently higher than summertime groundwater recharge ratios, while tropical groundwater recharge ratios are at a maximum during the wet season. The isotope-based recharge ratio seasonality is consistent with monthly outputs from a global hydrological model (PCR-GLOBWB) for most, but not all locations. The pronounced seasonality in groundwater recharge ratios shown in this study signifies that, from the point of view of predicting future groundwater recharge rates, a unit change in winter (temperate and arid regions) or wet season (tropics) precipitation will result in a greater change to the annual groundwater recharge rate than the same unit change to summer or dry season precipitation.

A glimpse beneath earth's surface: GLobal HYdrogeology MaPS (GLHYMPS) of permeability and porosity

The lack of robust, spatially distributed subsurface data is the key obstacle limiting the implementation of complex and realistic groundwater dynamics into global land surface, hydrologic, and climate models. We map and analyze permeability and porosity globally and at high resolution for the first time. The new permeability and porosity maps are based on a recently completed high-resolution global lithology map that differentiates fine and coarse-grained sediments and sedimentary rocks, which is important since these have different permeabilities. The average polygon size in the new map is ~100 km2, which is a more than hundredfold increase in resolution compared to the previous map which has an average polygon size of ~14,000 km2. We also significantly improve the representation in regions of weathered tropical soils and permafrost. The spatially distributed mean global permeability ~10−15 m2 with permafrost or ~10−14 m2 without permafrost. The spatially distributed mean porosity of the globe is 14%. The maps will enable further integration of groundwater dynamics into land surface, hydrologic, and climate models.

A New Assessment Framework for Transience in Hydrogeological Systems

The importance of transience in the management of hydrogeologic systems is often uncertain. We propose a clear framework for determining the likely importance of transient behavior in groundwater systems in a management context. The framework incorporates information about aquifer hydraulics, hydrological drivers, and time scale of management. It is widely recognized that aquifers respond on different timescales to hydrological change and that hydrological drivers themselves, such as climate, are not stationary in time. We propose that in order to assess whether transient behavior is likely to be of practical importance, three factors need to be examined simultaneously: (1) aquifer response time, which can be expressed in terms of the response to a step hydrological change (τstep ) or periodic change (τcycle ); (2) temporal variation of the dominant hydrological drivers, such as dominant climatic systems in a region; (3) the management timescale and spatial scale of interest. Graphical tools have been developed to examine these factors in conjunction, and assess how important transient behavior is likely to be in response to particular hydrological drivers, and thus which drivers are most likely to induce transience in a specified management timeframe. The method is demonstrated using two case studies; a local system that responds rapidly and is managed on yearly to decadal timeframes and a regional system that exhibits highly delayed responses and was until recently being assessed as a high level nuclear waste repository site. Any practical groundwater resource problem can easily be examined using the proposed framework.

Assessing regional groundwater stress for nations using multiple data sources with the groundwater footprint

Groundwater is a critical resource for agricultural production, ecosystems, drinking water and industry, yet groundwater depletion is accelerating, especially in a number of agriculturally important regions. Assessing the stress of groundwater resources is crucial for science-based policy and management, yet water stress assessments have often neglected groundwater and used single data sources, which may underestimate the uncertainty of the assessment. We consistently analyze and interpret groundwater stress across whole nations using multiple data sources for the first time. We focus on two nations with the highest national groundwater abstraction rates in the world, the United States and India, and use the recently developed groundwater footprint and multiple datasets of groundwater recharge and withdrawal derived from hydrologic models and data synthesis. A minority of aquifers, mostly with known groundwater depletion, show groundwater stress regardless of the input dataset. The majority of aquifers are not stressed with any input data while less than a third are stressed for some input data. In both countries groundwater stress affects agriculturally important regions. In the United States, groundwater stress impacts a lower proportion of the national area and population, and is focused in regions with lower population and water well density compared to India. Importantly, the results indicate that the uncertainty is generally greater between datasets than within datasets and that much of the uncertainty is due to recharge estimates. Assessment of groundwater stress consistently across a nation and assessment of uncertainty using multiple datasets are critical for the development of a science-based rationale for policy and management, especially with regard to where and to what extent to focus limited research and management resources.

Enhanced groundwater recharge rates and altered recharge sensitivity to climate variability through subsurface heterogeneity

Significance Understanding the implications of climate changes on hydrology is crucial for water resources management. Widely used global hydrological models generally assume simple homogeneous subsurface representations to translate climate signals into hydrological variables. We study groundwater recharge in the carbonate rock regions of Europe, Northern Africa, and the Middle East, which are known to exhibit strong subsurface heterogeneity. We demonstrate that subsurface heterogeneity alters the sensitivity of recharge to climate variability and enhances recharge estimates, resulting in potentially more available water per capita, than previously estimated. Our results are opposing previous modeling studies on future groundwater availability that assumed homogeneous subsurface properties everywhere. We suggest that water management strategies in regions with heterogeneous subsurface properties need to consider these revised estimates. Our environment is heterogeneous. In hydrological sciences, the heterogeneity of subsurface properties, such as hydraulic conductivities or porosities, exerts an important control on water balance. This notably includes groundwater recharge, which is an important variable for efficient and sustainable groundwater resources management. Current large-scale hydrological models do not adequately consider this subsurface heterogeneity. Here we show that regions with strong subsurface heterogeneity have enhanced present and future recharge rates due to a different sensitivity of recharge to climate variability compared with regions with homogeneous subsurface properties. Our study domain comprises the carbonate rock regions of Europe, Northern Africa, and the Middle East, which cover ∼25% of the total land area. We compare the simulations of two large-scale hydrological models, one of them accounting for subsurface heterogeneity. Carbonate rock regions strongly exhibit “karstification,” which is known to produce particularly strong subsurface heterogeneity. Aquifers from these regions contribute up to half of the drinking water supply for some European countries. Our results suggest that water management for these regions cannot rely on most of the presently available projections of groundwater recharge because spatially variable storages and spatial concentration of recharge result in actual recharge rates that are up to four times larger for present conditions and changes up to five times larger for potential future conditions than previously estimated. These differences in recharge rates for strongly heterogeneous regions suggest a need for groundwater management strategies that are adapted to the fast transit of water from the surface to the aquifers.

Groundwater flow systems theory: research challenges beyond the specified-head top boundary condition

Groundwater flow systems theory : research challenges beyond the specified-head top boundary condition

The return of groundwater quantity: a mega-scale and interdisciplinary “future of hydrogeology”?

A series of recent papers suggest groundwater quantity may be returning to prominence in hydrogeology research. The unsustainable depletion of groundwater has been documented on both regional (Rodell et al. 2009; Tiwari et al. 2009; Famiglietti et al. 2011) and global scales (Wada et al. 2010; Konikow 2011; Wada et al. 2012) using data synthesis and the GRACE satellite data. Additionally, how groundwater resources will be impacted by global change remains important but uncertain and difficult to predict (Green et al. 2011; Taylor et al. 2013). Recent discussions on groundwater sustainability have suggested applying cutting-edge sustainability concepts such as multigenerational goal setting and adaptive management to groundwater quantity problems (Gleeson et al. 2010, 2012). At its core, groundwater quantity is a water budget question of fluxes and stores. The critical applied questions of groundwater quantity are “how much groundwater is available for sustainable use, and what is the impact of the various uses on interconnected social, economic and environmental systems” From the perspective of the authors, as young and possibly naive early-career hydrogeologists, it is suggested that this overall question is one “future of hydrogeology”, like the other futures of hydrogeology described in the Hydrogeology Journal special edition of 2005 (Voss 2005). In the following, this vast question is tentatively divided into a series of smaller questions. Undoubtedly, other researchers will raise other questions or suggest other angles or avenues of research. The purpose of this essay is to encourage this discussion. The term “return” is used to reflect the historical trends in hydrogeologic research. In Fig. 1, the evolution of hydrogeologic research has been divided into three overlapping phases, based on trends in citations and benchmark papers (Schwartz et al. 2005; Anderson 2008). Prominent research in early quantitative hydrogeology focused on questions of “capacity” or “safe yield” when studying aquifers (Meinzer 1923; Theis 1935, 1940). Over the past ∼30–40 years, the community has been largely focused on issues of groundwater contamination and quality as well as more recently on groundwater/surface-water interactions. This research remains important and can be integrated into a holistic view of groundwater and sustainability. The trajectory of hydrogeology research has been increasing in scope, interdisciplinarity and complexity (Fig. 1). It is suggested that a return to groundwater quantity research at the mega-scale, which addresses long-term issues of sustainability, equity, ecology and economics, may have a high return-oninvestment for science and society. Here, the focus is on groundwater systems at regional (≈10-km length scale) to continental (>1000 km) scales, herein called “mega-scale”. Since the 1970s, hydrogeology has often, but not exclusively, focused on site-scale (<1 km) research to examine important water resource and contamination problems and how groundwater interacts with surface water. Regional-scale groundwater systems were first modeled in the 1960s (Toth 1963; Freeze and Witherspoon 1967; Garven 1995; Person et al. 1996) but the numerical simulation of groundwater systems over entire continents has only been recently possible (Fan et al. 2007; Lemieux et al. 2008). Additionally, continental-scale remote sensing from the GRACE satellites have only recently documented realtime groundwater depletion at the mega-scale. Numerous fundamental questions of the spatio-temporal variability of groundwater fluxes and stores remain, especially at the mega-scale. These questions resemble recurring issues in groundwater science and engineering but with a new large-scale twist:

Linking groundwater use and stress to specific crops using the groundwater footprint in the Central Valley and High Plains aquifer systems, U.S.

A number of aquifers worldwide are being depleted, mainly by agricultural activities, yet groundwater stress has not been explicitly linked to specific agricultural crops. Using the newly developed concept of the groundwater footprint (the area required to sustain groundwater use and groundwater-dependent ecosystem services), we develop a methodology to derive crop-specific groundwater footprints. We illustrate this method by calculating high-resolution groundwater footprint estimates of crops in two heavily used aquifer systems: the Central Valley and High Plains, U.S. In both aquifer systems, hay and haylage, corn, and cotton have the largest groundwater footprints, which highlights that most of the groundwater stress is induced by crops meant for cattle feed. Our results are coherent with other studies in the High Plains but suggest lower groundwater stress in the Central Valley, likely due to artificial recharge from surface water diversions which were not taken into account in previous estimates. Uncertainties of recharge and irrigation application efficiency contribute the most to the total relative uncertainty of the groundwater footprint to aquifer area ratios. Our results and methodology will be useful for hydrologists, water resource managers, and policy makers concerned with which crops are causing the well-documented groundwater stress in semiarid to arid agricultural regions around the world.