Groundwater And Global Change: Trends, Opportunities And Challenges

9d ago
9.82 MB
44 Pages
Last View : 8d ago
Last Download : 9d ago
Upload by : Jewel Payne

:01SIDE PUBLICATIONS SERIESGroundwater and Global Change:Trends, Opportunitiesand ChallengesJac van der GunUNITED NATIONS WORLD WATER ASSESSMENT PROGRAMME

Published in 2012 by the United Nations Educational, Scientific and Cultural Organization7, Place de Fontenoy, 75352 Paris 07 SP, France UNESCO 2012All rights reservedISBN 978-92-3-001049-2The designations employed and the presentation of material throughout this publication do notimply the expression of any opinion whatsoever on the part of UNESCO concerning the legal statusof any country, territory, city or area or of its authorities, or concerning the delimitation of itsfrontiers or boundaries.The ideas and opinions expressed in this publication are those of the authors; they are not necessarilythose of UNESCO and do not commit the Organization.Photographs:Cover: africa924 / Shutterstock Rolffimages / Dreamstime Frontpage / Shutterstock.comp.i: holgs / iStockphoto p.ii: Andrew Zarivny p.2: Aivolie / Shutterstockp.4: Jorg Hackemann / Shutterstock p.17: Jac van der Gun p.31: Aivolie / Shutterstockp.33: Hilde VanstraelenOriginal concept (cover and layout design) of series: MH Design / Maro HaasLayout: Pica PublishingPrinted by: UNESCOPrinted in Franceltd,London–Paris / Roberto Rossi

Groundwater and global change:Trends, opportunities and challengesAuthor: Jac van der GunContributors: Luiz Amore (National Water Agency, Brazil), Greg Christelis (Ministry of Agriculture, Water and Forestry,Nambia), Todd Jarvis (Oregon State University, USA), Neno Kukuric (UNESCO-IGRAC) Júlio Thadeu Kettelhut(Ministry of the Environment, Brazil), Alexandros Makarigakis (UNESCO, Ethiopia), Abdullah Abdulkader Noaman(Sana’a University, Yemen), Cheryl van Kempen (UNESCO-IGRAC), Frank van Weert (UNESCO-IGRAC).Acknowledgements Alice Aureli (UNESCO-IHP), Richard Connor (UNESCO-WWAP), Michela Miletto (UNESCOWWAP), Lucilla Minelli (UNESCO-IHP), Holger Treidel (UNESCO-IHP), Olcay Unver (UNESCO-WWAP)SummaryGroundwater – containing by far the largest volume of unfrozen fresh water on Earth – is a hugely important naturalresource. However, what the general public and most decision-makers know and understand about groundwateris usually very little. Today, knowledge of groundwater around the world, its functions and its use is increasingrapidly – and views about the many ways in which groundwater systems are linked with other systems are changingaccordingly.All around the world, groundwater is a resource in transition: its exploitation started booming only during the twentieth century (‘the silent revolution’). This boom has resulted in much greater benefits from groundwater than wereever enjoyed before, but it also triggered unprecedented changes in the state of groundwater systems. On a globallevel, the key issues that need to be addressed to ensure the sustainability of groundwater resources are the depletionof stored groundwater (dropping water levels) and groundwater pollution. Climate change will affect groundwater,but because of its characteristic buffer capacity, groundwater is more resilient to the effects of climate change thanSide publicationS SerieSi

l Summarysurface water. Therefore, in areas where climate change is expected to cause water resources to become scarcer thanthey are at present, the role of groundwater in water supplies is likely to become more dominant. Their buffer capacityis one of the major strengths of groundwater systems. It allows long dry periods to be bridged (creating conditions forsurvival in semi-arid and arid regions) and generally reduces the risk of temporary water shortages. It also smoothsout variations in water quality and causes a portion of the stored water (medium-deep to deep groundwater) to berelatively insusceptible to sudden disasters, thus making this portion suitable as an emergency water source.In terms of making a contribution to securing water availability and groundwater-related environmental values, managing groundwater resources sustainably is of vital importance to society and the environment. Nevertheless, thereare situations where sustainable exploitation of groundwater is unlikely to be achieved. Such situations include,for example, cases of tapped non-renewable groundwater resources, and many of the intensely exploited renewablegroundwater systems in arid and semi-arid zones. Such cases should be identified and the population of the areasconcerned should be prepared in good time to adapt effectively to a future when these resources will be exhausted.Groundwater governance is complex and needs to be tailored to local conditions. In the case of transboundary aquifersystems, the international dimension adds complexity. International cooperation and a wide range of internationalinitiatives produce significant added value. This cooperation is instrumental in enhancing and disseminating information about groundwater, in developing and promoting approaches and tools for its proper management, and inraising global commitment for action on priority issues, such as the millennium development goals (MDGs) andsustainable development. Ensuring that groundwater is adequately incorporated into such global actions is a challenge for all groundwater professionals.iiunited nationS World Water aSSeSSment programme

Groundwater and global change: Trends, opportunities and challengesTable of ContentsSUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .i1. GROUNDWATER IN A WEB OF INTERDEPENDENCIES . . . . . . . . . . . . . . . 32. PANORAMA OF CHANGE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43. KEY ISSUES RELATED TO GROUNDWATER . . . . . . . . . . . . . . . . . . . . . . 124. CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33Side publicationS SerieSiii

Groundwater in a webof interdependenciesIn our rapidly changing world where there are manychallenges regarding water, it is necessary to pay ampleattention to groundwater and its role in securing watersupplies and in coping with water-related risk and uncertainty. However, focusing on groundwater certainly doesnot imply that groundwater systems are self-contained,or that they can be understood and managed on the basisof hydrogeological information only. On the contrary, itcannot be overemphasized that groundwater is onecomponent in the hydrological cycle – a component thatinteracts closely with other components in the cycle atvarious temporal and spatial levels. Groundwater is alsoinvolved in a number of other cycles – such as chemicalcycles (solute transport) and biochemical cycles (biosphere) – and it is affected by climate change causedby changes in the carbon cycle. In addition, groundwaterinteractions and interdependencies are not limited tophysical systems, such as surface waters, soils, ecosystems, oceans, lithosphere and atmosphere, but are alsorelated to socio-economic, legal, institutional and political systems. Hence, groundwater is entrenched in a webof interdependencies. Changes in the state of groundwater systems are taking place due to these interdependencies, and causal chains link these changes to the driversof change (root causes).Different categories of drivers are behind the processesof change in groundwater systems. Demographic driversinclude population growth, mobility and urbanization.Population growth leads to increasing demands for waterand food and to bigger loads of waste and wastewaterbeing discharged into the environment. Expandingurbanization and shifts in land-use patterns modify thesepressures. The same is true for socio-economic drivers– to a large extent, they explain people’s demands andbehaviour with respect to groundwater. Intensive groundwater exploitation may be triggered by positive expectations on the economic profitability of groundwater, andby socio-economic conditions that allow the exploitationof this resource. Higher levels of social and economicdevelopment enable societies to adapt more easilyto changing conditions (for example, by making thetransition to a less water-dependent economy if waterbecomes scarce), and to pay more attention to sustainability. Science and technological innovation are otherdrivers that have put their mark on the utilization andstate of many groundwater systems. For example, systematic aquifer exploration and improved technologiesfor drilling and pumping have contributed significantlyto generating greater benefits from groundwater. But atthe same time, the resulting intensive pumping has oftenincreased stresses on groundwater systems, on relatedecosystems and on the environment. Science – assistedby technical innovations in fields such as water use,water treatment and water-reuse – helps to define ways ofcontrolling unintended negative impacts. Policy, law andfinance form an important category of drivers of plannedchange, in the context of groundwater resources development and management. Finally, there are two categoriesof physical drivers. The first is climate variability andclimate change – particularly as they affect aquifers inarid and semi-arid regions. Minor variations in climaticconditions there can have a pronounced influence ongroundwater in three main ways: a change in the rateof groundwater renewal, a change in the availability ofalternative sources of fresh water and a change in waterdemand. Climate change is also expected to contributeto sea level rise, which will affect aquifers in low-lyingcoastal zones, where a large percentage of the world’spopulation lives. The second category of physical driversis natural and anthropogenic hazards. This is differentfrom other categories of drivers in the sense that hazards are strongly probabilistic (disasters may or may nothappen), and usually cause a sudden change rather thana trend over time.Side publicationS SerieS3

Panorama of change2.1 Increasing our knowledge of the world’sgroundwaterIn recent years, there have been significant advances inwhat we know about the world’s groundwater resources.While these advances can be observed at all levels, thefocus here is on the global and regional levels. Therehas been remarkable progress in many areas, includingthe global-level characterization of groundwater systems,their properties and their conditions. Important recentachievements include:the consolidated version of the Groundwater Resources Map of the World under the World-wide Hydrogeological Mapping and Assessment Programme (WHYMAP,2008) (Figure 1);the outcomes of global-scale hydrological modelling, such as that on worldwide groundwater rechargewith WaterGAP Global Hydrological Model (Döll andFiedler, 2008) and with PCR-GLOBWB (Wada et al.,2010);a global assessment of current groundwater use for irrigation (Siebert et al., 2010);a comprehensive monograph on the geography of the world’s groundwater (Margat, 2008).4united nationS World Water aSSeSSment programmerapidly increasing documentation and transboundary aquifers, resulting fromtools onnumerousprojects (see section 3.5).The total volume of fresh groundwater stored on earth isbelieved to be in the region of 8 million km3 to 10 million km3 (Margat, 2008), which is more than two thousand times the current annual withdrawal of surfacewater and groundwater combined. This is a huge volume,but where are these fresh-water buffers located – andwhat fraction of their stock is available for depletion?Figure 1 answers the first question by showing the geographic distribution of the world’s major groundwaterbasins (shown in blue on the map – and covering 36%of the land area of the continents). This is where themain groundwater buffers are located. Additional ones,which are less continuous and smaller, are present inareas with complex hydrogeological structures (shown ingreen on the map – and covering 18% of the total area).And further, to a lesser extent, are groundwater reservespresent in the remaining 46% of the land area of thecontinents (shown in brown on the map).The groundwater buffers allow periodic, seasonal ormulti-annual dry periods to be bridged convenientlywithout the risk of sudden unexpected water shortages.In large parts of the world, sustainable groundwaterdevelopment is possible by alternating storage depletion

Groundwater and global change: Trends, opportunities and challengesFIGURE 1Simplified version of the Groundwater Resources Map of the World (WHYMAP, 2008)120 150 90 60 0 30 30 60 60 90 120 180 150 60 St. stanbulNew YorkSeoulTianjinTokyoTehranShanghaiLos Angeles30 OsakaLahoreCairo30 DelhiKarachiDhakaCalcuttaHong KongMexico rasLagos0 0 KinshasaJakartaLimaRio de JaneiroSao Paulo30 30 SantiagoSydneyBuenos Aires60 60 0Groundwater resources300highmedium100low20very low22000300040005000 kmSurface water & Geographygroundwater recharge (mm/a)very high10000major riverin major groundwater basinslarge freshwater lakein areas withcomplex hydrogeological structurelarge saltwater lake!selected citycountry boundarycontinuous ice sheetin areas with local and shallow aquifersduring dry periods and storage recovery during wet periods. Groundwater reservoirs are rather insensitive tovariations in the length of the dry periods, and thereforeresilient to this aspect of climate variation and climatechange. In principle it is possible to ignore the sustainability criterion and exploit a large part of the storedgroundwater volumes, but in practice it is difficult to doso and often not attractive, because depletion comes ata cost, (see Section 3).Recent outcomes of the Gravity Recovery and ClimateExperiment (GRACE) mark a major step forward in assessing groundwater storage variations in some of the world’smajor aquifer systems (Famiglietti et al., 2009; Rodell etal., 2009; Tiwari et al., 2009; Muskett and Romanovsky,2009; Moiwo et al., 2009; Bonsor et al., 2010; Chenet al., 2009). The results of the experiment suggestthat satellite mapping of the Earth’s gravity field (satellite gravimetry) is a promising innovative technique thatcan be used in hydrogeological investigations in the nearfuture.1 It can be used for monitoring long-term trends,seasonal variations and change during droughts. Globalsimulation models that link the terrestrial and atmospheric1In June 2010, NASA and the German Aerospace Centre (DLR) signedan agreement to continue the current GRACE mission through 2015( 2010-195).components of the hydrological cycle are likely to becomeanother important tool for enhancing our knowledge ofgroundwater regimes, in particular for exploring how theymay respond to climate change (Döll, 2009).2.2 The silent revolutionDuring the twentieth century, groundwater abstractionacross the world increased explosively. This was drivenby population growth, technological and scientific progress, economic development and the need for foodand income. By far the largest share of the additionalvolumes of water that have been abstracted has beenallocated to irrigated agriculture. The boom in groundwater development for irrigation started in Italy, Mexico,Spain and the United States as far back as the early partof the century (Shah et al., 2007). A second wave beganin South Asia, parts of the North China Plain, parts ofthe Middle East and in northern Africa during the 1970s,and this still continues today. The cited authors perceivea third wave of increasing abstractions in many regionsof Africa, and in some countries such as Sri Lanka andVietnam. This worldwide boom in groundwater abstraction is largely the result of numerous individual decisions by farmers – decisions made without centralizedplanning or coordination. It has been called the silentSide publicationS SerieS5

Section 2 l Panorama of changeFIGURE 2Intensity of groundwater abstraction by the year 2000, as allocated to 0.5o x 0.5o grid cells by the PCR-GLOBWB model, in mm/yearNo Data0-22 - 2020 - 100100 - 300300 - 1000Source: Wada et al. 2010. 2010 American Geophysical Union. Reproduced by permission of the American Geophysical Union.revolution (Llamas and Martínez-Santos, 2005; Llamasand Martínez-Cortina; 2009).Groundwater abstraction is very unevenly distributedacross the globe. It differs not only from country tocountry, but also shows pronounced spatial variationwithin countries, as can be observed in Figure 2. Basedon recent estimates at country level (IGRAC,2010;Margat; 2008; Siebert et al., 2010, AQUASTAT, n.d.;EUROSTAT, n.d.), the world’s aggregated groundwaterabstraction as per 2010 is estimated to be approximately1,000 km3 per year, of which about 67% is used for irrigation, 22% for domestic purposes and 11% for industry(IGRAC, 2010)2. Two-thirds of this is abstracted in Asia,with India, China, Pakistan, Iran and Bangladesh as themajor consumers (see Table 1 and Table 2). The globalgroundwater abstraction rate has at least tripled over thelast 50 years and still is increasing at an annual rate ofbetween 1% and 2%.Nevertheless, in some countries where intensive groundwater development started rather early, abstraction rateshave peaked and are now stable, or even decreasing(Shah et al., 2007), as is illustrated in Figure 3. Althoughthe global estimates are not accurate, they suggest thatthe current global abstraction of groundwater representsapproximately 26% of total freshwater withdrawal globally (Table 2), and that its rate of abstraction corresponds to some 8% of the mean globally aggregated rateof groundwater recharge. Groundwater supplies almosthalf of all drinking water in the world (UNESCO-WWAP,2009), and 43% of the global consumptive use in irrigation3 (Siebert et al., 2010).The silent revolution has contributed tremendously toeconomic development and welfare in many countries,especially in rural areas. Nevertheless, it has also introduced unprecedented problems that are difficult to control in some areas (see Section 3).2.3 Changing views on groundwaterGroundwater has become an interdisciplinary subject.Professionally, it is no longer the almost exclusivedomain of hydrogeologists and engineers; it is alsoreceiving a good deal of attention from economists,sociologists, ecologists, climatologists, lawyers, institutional experts, communication specialists and others.Analysing groundwater from these different perspectivesputs it in a wider context, resulting in changing views onthis natural resource.Changing views can be observed in the first instancein relation to the functions and value of groundwater.Measuring the importance of groundwater by comparing its recharge rate, withdrawal and stored volume tothose of surface water is gradually being replaced bymore economically and/or ecologically oriented valuing32Almost all values mentioned in this paragraph are globally aggregatedor averaged, and thus cannot be used to draw conclusions aboutconditions at a local or regional level.6united nationS World Water aSSeSSment programmeSiebert et al. (2010) estimate global consumptive irrigation water useto be 1,277 km3 per year - or 48% of global agricultural water withdrawals. Their estimate for the share of groundwater in this figure is 545 km3per year, which is fairly consistent with the estimated global groundwater abstraction for irrigation, taking into account irrigation water losses.

Groundwater and global change: Trends, opportunities and challengesFIGURE 3Table 1Groundwater abstraction trends in selected countries (in km3/year, based on Margat, 2008, with modifications)300Top ten groundwater abstracting countries (as per 501960India1970USAMexicoChinaSaudi Arabia19801990Pakistan20001. India2512. China1123. USA1124. Pakistan645. Iran606. Bangladesh357. Mexico298. Saudi Arabia239. Indonesia1410. Italy142010IranRussiaFranceSource: Adapted from Margat (2 008, fig. 4.6, p. 107).Table 2Key estimates of global groundwater abstraction (reference year: 2010)continentIrrigationkm3/yrNorth AmeriCAcompared to totalWater abStractiongroundWater abStraction *Domestickm3/yrIndustrialkm3/yrTotalkm3/yr%Total waterabstraction**km3/yrShare ofgroundwater %99261814315524275721411499South AmeriCA128626318214europe (iNCludiNgruSSiAN 6367668225730421712625666212108986100383126CeNtrAl AmeriCAANd the CAribbeANASiAoCeANiAWorld* Estimated on the basis of IGRAC (2010), AQUASTAT (n.d), EUROSTAT (n.d.), Margat (2008) and Siebert et al. (2010).**Average of the 1995 and 2025 ‘business as usual scenario’ estimates presented by Alcamo et al. (2003).Side publicationS SerieS7

Section 2 l Panorama of changeapproaches that are focused on ‘added value’ producedby groundwater. For example, studies in Spain (Llamasand Garrido, 2007) and India (Shah, 2007) have shownthat when compared to surface water, groundwater produces higher economic returns per unit of water used inirrigation. The explanation is that groundwater usuallypresents considerably less water shortage risk than doessurface water, as a result of the buffer capacity of its relatively large stored volume. Consequently, groundwater’sshare in the overall socio-economic benefit derived fromabstracted water tends to be higher than its volumetricshare in total water abstraction. Although groundwaterquantity is still often assessed in terms of recharge and/or discharge rates only, it is clear that the volumes ofstored groundwater are equally relevant in the assessment of this ‘stock-and-flow’ resource. Stored groundwater enables the functions or services of a groundwatersystem to go beyond the withdrawal of water for directconsumptive and productive use (provisioning services)and to include a number of in situ services (mostly regulatory services) as well. One of these in situ services isthe reservoir function of groundwater systems, whichallows dry periods to be bridged and – at a very largetime scales – non-renewable groundwater to be availablein areas where groundwater recharge is currently negligible (Foster and Loucks, 2006). Other in situ services arethe support of ecosystems and phreatophytic agriculture,the maintenance of spring flows and base flows, the prevention of land subsidence and seawater intrusion, andthe potential for exploiting geothermal energy or storingheat. All withdrawal and in situ services contribute tothe value of a groundwater system and should be takeninto account in groundwater resources management.A second category of changing views refers to the role ofpeople. Not long ago, the diagnostic analysis and management of groundwater resources tended to be basedalmost exclusively on an analysis of the physical components (groundwater systems and related ecosystems).There is broad consensus nowadays that socio-economicaspects deserve a share of the attention as well, preferably coupled with the physical components – for example, in a socio-ecological systems approach similar tothe ecosystem approach adopted by the Convention forBiological Diversity in 1995 (Convention on BiologicalDiversity, n.d.). Groundwater resources management islikely to be successful only if stakeholders are cooperating fully. This is because the majority of groundwatermanagement measures aim to influence or change people’s behaviour. In addition, people may be better prepared to adapt to climate change and climate variabilityif they are aware of groundwater’s potential to help themdo so. Correspondingly, substantial efforts are madein many parts of the world to draft new groundwaterlegislation and related regulatory frameworks, to raiseawareness of groundwater issues among stakeholdersand to involve stakeholders in the management of theirgroundwater resources.The debate on climate change has made it clear thathydrogeologists and hydrologists have to abandon their8united nationS World Water aSSeSSment programmetraditional implicit assumption of the stochastic stationarity of natural hydrological flow rates. The assumptionthat groundwater recharge rates assessed in the pastwould provide an unbiased estimate for future conditions is no longer appropriate. This makes a differencefor actively recharged phreatic aquifers in particular– especially when they are shallow – and less of a difference for deep confined aquifers, which tend to reactmore smoothly to climatic variations because of theirlower recharge rates and higher volumes in storage. Itdoes not play a role in the case of non-renewable groundwater resources. Finally, there is a growing recognitionof groundwater’s relatively high resilience to climatechange and climate variability. This special characteristic leads to the prediction that groundwater will play animportant role in human adaptation to climate change(see Section 3.3).2.4 Conjunctive management, integratedwater resources management and beyondThe time when groundwater used to be explored andexploited as an isolated resource is long past. Althoughthe advantages of using groundwater and surface waterin combination were recognized at least as far back asthe 1950s (Todd, 1959), the notion of joint management of these resources has been embraced much morerecently. Under this paradigm, water resources are notonly used but also managed as components of a singlesystem. This generally leads to greater flexibility in wateruse, improved water security, cheaper water supply andmore efficient use of available water resources – all ofwhich together contribute to greater total benefits.An interesting feature of conjunctive management ismanaged aquifer recharge (MAR), the intentional storageof water in aquifers for subsequent recovery or environmental benefit. It makes use of a variety of techniquesand is being applied in countless small and large schemesaround the world (Dillon et al., 2009). Box 1 provides anexample of a proposed MAR application in Namibia. Inthe case presented in Box 2, the conjunctive management of groundwater and surface water does not focusprimarily on water as an extractable resource, but ratheron how to maintain environmentally optimal groundwaterlevels, which is widely practised in the Netherlands.The next step is integration across water use sectors, as advocated by integrated water resourcesmanagement (IWRM). The Global Water Partnership(GWP, n.d.) defines IWRM as the coordinated development and management of water, land and relatedresources, in order to maximize economic and socialwelfare without compromising the sustainability ofecosystems and the environment. In many countries,this cross-sectoral approach to water has replaced traditional, fragmented sectoral approaches that ignoredthe interconnection between the various water usesand services. Tendencies can be observed (for example,

Box 1: Water augmentation to Central Areas of Namibia through managed aquifer rechargeNamibia is the most arid country in sub-Saharan Africa, and it islargely dependent on groundwater. Perennial rivers are found only onthe northern and southern borders, at a considerable distance from themajor demand centres in the Central Areas of Namibia, including thecapital city, Windhoek. Dams on the ephemeral rivers provide the mainsource of water for the country’s more urbanized central regions. Inflowinto these dams is irregular and unreliable, and evaporation rates inNamibia’s arid climate are high. As a result, the assured safe yield ofthese dams is low. The region’s growing demand for water will, in thenear future, result in existing water resources not being able the meetexpected demand in a sustainable, and from groundwater in a municipal well field. When the threedams are operated on an individual basis, the 95% safe yield is only17 Mm3 per year, mainly as a result of huge evaporation losses fromthe Omatako and Swakoppoort dams. Through integrated use of thethree dams, water is transferred and stored in Von Bach dam, whichhas the lowest evaporation rate due to the dam basin characteristics.The best option for alternative water supply augmentation to theCentral Areas of Namibia was found to be the creation of a water bankthrough managed aquifer recharge of the Windhoek Fractured RockAquifer, in combination with deep boreholes, to increase the accessto a larger volume of stored reserves. This managed aquifer rechargeoption involves taking water (when a surplus is available) from thethree-dam system on which the city relies, purifying it and injectingit into the Windhoek Aquifer via the boreholes. This reduces evaporation losses at the dams. In years when the surface sources are insufficient, the stored underground water can be abstracted. Securingwater supply through managed aquifer recharge must be fast trackedas water shortages and non-availability in times of drought will havea devastating effect on the economy. Windhoek contributes approximately 50% of the N 5.26 billion manufactured goods (excludingfish processing on shore), and the closure of industry due to nonavailability of water would result in a N 2.63 billion loss per year toNamibia (based on the 2006 Gross Domestic Product).For additional water supplies to the Central Areas of Namibia, threemain development options include:This operating procedure improves the 95% safe yield from thethree-dam system to 20 Mm3 per year. It is forecast that annualwater demand will increase from the current level of 25 Mm3 toapproximately 40 Mm3 in 2021.Managed recharge of the Windhoek Aquifer (using surplus water from the Central Area dams to increase underground reserves);Karst aquifers used only for emergency supply; and a pipeline link from the Okavango River to supply the Central Areas when required (see Figure).Managed aquifer recharge is the preferred option. Over-abstractionof the Windhoek Aquifer since 1950 has created an undergroundstorage facility estimated at 2

climate change - particularly as they affect aquifers in arid and semi-arid regions. Minor variations in climatic conditions there can have a pronounced influence on groundwater in three main ways: a change in the rate of groundwater renewal, a change in the availability of alternative sources of fresh water and a change in water demand.