The concept of water security

In global climate circles one hears the expression ‘water security’ used with ever increasing frequency, together with declarations about the urgency to increase water security in these times of unprecedented global change and future uncertainty. There is no agreement among experts on terminology, and some show little concern over its precise meaning, but it is generally conceived as the interaction between the physical stress on water resources, the risk of water-related hazards and the coping capacity in water management of the society concerned.

The most widely used definition is that of Grey and Sadoff (2007): “the availability of an acceptable quantity and quality of water for health, livelihoods, ecosystems and production, coupled with an acceptable level of water-related risks to people, environments and economies. Although some question whether narrow definitions can adequately reflect the broad scope of the water-security concept (Cook and Bakker 2012; Lankford et al. 2013).

At its heart, water security is a societal issue and, thus, a political concern. It is the balance between physical water-resource stresses (as applied by development and measured by hydrological status) and the water-management coping capacity within the area under consideration (as indicated by the economic, technical and institutional ability to manage change) (Ait-Kadi and Lincklaen-Arriens 2012). In addition to the hydrological stress caused by water abstraction, water security is also threatened by flood and drought, and by major uncontrolled pollution. Whilst economic development normally results in improvements in national water security, this has not necessarily been accompanied by conservation of aquatic ecosystems–although, the importance of ecosystem functions and services has been increasingly recognised during the last decade.

A helpful way of visualising water security is through use of the GWP-Water Security Matrix (Ait-Kadi and Lincklaen-Arriens 2012), which indexes five factors—household, economic, urban and environmental water securities plus resilience to natural water disasters, with improvements being viewed as the successful outcome of integrated water-resource management, adequate water-supply investments and appropriate water-disaster preparedness. Many countries are becoming aware that proceeding down their current development path is likely to put significant stress on water resources—and they need to respond to the challenge of improving water security through investing more in water storage, conservation and reuse, pollution abatement and flood protection. There is also evidence of a direct correlation between drought proneness and persistent poverty in Sub-Saharan Africa (Grey and Sadoff 2007), which further increases the pressure to find reliable sources of water with sufficient storage to maintain supply through drought periods.

From a water-resource perspective, the water-security concept would be more rational if applied at the scale of a specific city, drainage sub-basin or groundwater body, and if it more clearly distinguished the differing risks and impacts of drought, flood and pollution. However, since water security is mostly a political issue it tends to be viewed from a composite national perspective (Brown et al. 2013). Thus, to date, the concept has been principally applied at the national scale—and sometimes even at global or continental scale (Rockstrom et al. 2009; Vörösmarty et al. 2010)—using parameter indexation to arrive at a ‘security score’, whose primary objective is to provide an incentive for national governments to invest in water-resource management measures.

Assessments also invariably use physical water scarcity (Falkenmark 1989) to appraise the hydrological facet of water security, relying on renewable freshwater resources data (defined by measured river discharge and estimated groundwater recharge) (FAO 2005), which results in the role of groundwater storage being largely ignored (Taylor 2009; MacDonald et al. 2012).

The central objective of this article is to motivate professional hydrogeologists to articulate the role of groundwater more confidently, consistently and clearly in the growing political debate on ‘water security’ under global change. In this context, it will be necessary to avoid mixed messages regarding the huge volume of groundwater storage, contemporary rates of resource renewability and the wider impacts of uncontrolled aquifer depletion and pollution.

A groundwater vision on water security

In principle, the existence of aquifers considerably increases water security, since the large volume of groundwater stored within aquifers can be used to buffer the effect of drought on surface-water supplies (Foster et al. 2013). It is argued here that groundwater storage plays such a fundamental role in shaping water security that it should be taken prominently into account (rather than considering only annual renewable resources to indicate physical water scarcity)—particularly because most advocates of improving water security emphasise the need to invest in water storage (albeit that they tend to consider only built reservoir infrastructure).

So what are the facets of groundwater systems that contribute most to water security? Emerging research on groundwater and droughts (Calow et al. 2010; Villholth et al. 2013), along with more established approaches to assessing groundwater vulnerability to contamination (Foster et al. 2007), can be helpful here. Incorporating groundwater considerations into the evaluation of water security will require reference to the following criteria (Table 1):

Table 1 Factors influencing groundwater resource security
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  • Groundwater storage availability: an indicator of ‘buffer capacity’ to support water-supply abstraction in extended drought, and under climate variability more generally (Fig. 1), but which will be constrained by current groundwater resource status (aquifer water-level and salinisation trends) and connectivity to surface water (since in some aquifers small storage changes can have marked impacts on river baseflow, springflow and aquatic ecosystems).

  • Groundwater supply productivity: a measure of how easy it is to abstract groundwater from an aquifer, which relates to its depth and the aquifer transmissivity (MacDonald et al. 2009a) and any evidence of reducing productivity (primarily due to falling water levels).

  • Groundwater pollution protection: in terms of the interaction between aquifer pollution vulnerability pollution pressure, the effectiveness of pollution control and aquifer protection measures, and any evidence of deteriorating quality trends.

Some of these criteria are embodied in the European Union (EU) Water Framework Directive (WFD) Common Implementation Strategy (European Commission 2008; Quevauviller 2008), in which groundwater bodies requiring protection (because of their role in drinking-water supply and/or ecological habitats) have to be defined and registered, and then subject to systematic evaluation to determine present resource and quality status, the potential risk of their degradation and the required investment in measures to secure their sustainability for human use and/or for aquatic ecosystems. The EU-WFD specifically excludes advanced water treatment from consideration to manage the groundwater-quality risk because of its major implications in terms of incremental energy consumption, carbon footprint and irrelevance to aquatic ecosystem protection.

Groundwater resilience to climate change: a useful related concept?

A most important component of water security relates to the capacity of water-resource systems to withstand climate change. This could be termed ‘resilience’, which is an important attribute of any ecosystem. However, the ‘resilience concept’ is only just beginning to be applied to groundwater. In ecological science, where the concept is well established, it encompasses two aspects of a system—ability to resist long-term damage and the time taken to recover from a perturbation (Gunderson 2000). By applying this type of approach to groundwater resources in the context of climate change, two scenarios need to be considered: resilience to long-term (inter-decadal) ‘climate change’ and to shorter-term (inter-annual) ‘climate shocks’. Accepting this definition, one finds that aquifer storage volume, transmissivity and long-term recharge all have an important impact on groundwater resource resilience.

Groundwater in larger aquifer systems has long residence and response times—a reflection of their very large natural storage and a measure of their ability to buffer climate variability. While there remains considerable uncertainty over impacts of the current global warming trend on groundwater recharge (Taylor et al. 2013a), long-term response of groundwater systems to climate variability can be identified from palaeo-hydrological evidence from many large aquifer systems in currently arid parts of the world. Isotopic techniques reveal that much of the groundwater stored (and sometimes still flowing) in large sedimentary formations in semi-arid areas was recharged by late Pleistocene and early Holocene rainfall (>5,000 years BP), when the climate in the areas concerned was cooler and wetter. Moreover, accumulations of chloride and isotopic evidence in vadose zone profiles in these areas indicate that little rainfall recharge (<5 mm/a) has since taken place. Since present-day recharge is thus responsible for, at most, only a small fraction of stored groundwater in such aquifers, these resources can be considered as sensibly non-renewable (Foster and Loucks 2006), and all such groundwater can be regarded as highly resilient to on-going climate variability, be it in the form of short-term shocks or long-term changes.

In contrast, groundwater resources in low-storage aquifers are highly dependent on modern recharge and are, therefore, less resilient to long-term changes in climate than aquifers with larger storage. How these aquifers respond to short-term (inter-annual) changes in climate is, therefore, dependent on the longer-term recharge, which will replace groundwater storage lost during drought. For example, recent research in low-permeability aquifers in West Africa has demonstrated that the mean groundwater residence time of shallow groundwater (<50 m) was 20–70 years across a range of current climate zones, suggesting that the groundwater supplies concerned were well buffered against short-term climatic variations (Lapworth et al. 2013). On-going research is highlighting the importance of episodic recharge for replenishing low-storage aquifers, again suggesting that, even (under ‘normal conditions’) significant recharge events only occur sporadically (Taylor et al. 2013b).

For many stakeholders, the resilience of water supplies dependent on groundwater resources is more important than that of the actual groundwater resource itself. A sustainable approach to developing groundwater resources usually involves balancing long-term abstraction with the long-term recharge (after taking into account the needs for environmental flows) and also implementing drought-preparedness measures for all critical waterwells and springheads such that abstraction can be sustained at reduced groundwater levels (Robins et al. 2006). It is of relevance to note in this regard that research on the behaviour of groundwater sources during drought has shown that sources in higher yielding (more permeable) aquifers are generally much more reliable than those in lower yielding aquifers (MacDonald et al. 2009b). Therefore, when estimating the resilience of groundwater supplies to climate changes, aquifer transmissivity should be considered alongside aquifer storage volume and long-term recharge.

Defining the groundwater policy position

It is incumbent upon professional hydrogeologists to explain clearly the role of groundwater in the growing debate on water security under global change. When endeavouring to do this, it will be important to emphasise that:

  • The high levels of spatial aggregation usually employed when assessing water security tend to obscure the underlying groundwater resource position, because aquifer storage often exhibits large spatial variability.

  • Natural groundwater storage plays a fundamental role in water security and should be taken much more prominently into account in all water-security assessments. However, this needs to be tempered by knowledge of the current status of groundwater resource development (Fig. 1) and the side-effects (negative and positive) of depleting groundwater storage (Alley 2007; Jarvis 2013).

  • Controlled aquifer depletion where groundwater resources are mined may sometimes be a rational option, but should always be accompanied by effective monitoring of the impacts on existing users (including ecosystems) and by careful consideration of future-use implications (Foster and Loucks 2006).

Fig. 1
figure 1

Stages of groundwater resource development and the implications of storage depletion

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By understanding the aquifer characteristics relevant to water security, it should be possible to establish aquifer typologies which will respond in a similar manner to external stresses—either from humankind or climate. These typologies could then be mapped along with current groundwater status, permitting clearer communication on the groundwater dimensions of water security at the political level. For this purpose, the role of derivative groundwater maps building on standard hydrogeological mapping (e.g. MacDonald et al. 2012) and the use of large-scale long-period numerical aquifer modeling to evaluate varying demand scenarios (Sophocleous 2012; Gleeson et al. 2012) will be especially critical.