Human Appropriation of Renewable Fresh Water Author s Sandra L Postel Gretchen C Daily Paul R Ehrlich Source Science New Series Vol 271 No 5250 Feb 9 1996 pp 785 788 Published by American Association for the Advancement of Science Stable URL http www jstor org stable 2889886 Accessed 24 09 2008 10 49 Your use of the JSTOR archive indicates your acceptance of JSTOR s Terms and Conditions of Use available at http www jstor org page info about policies terms jsp JSTOR s Terms and Conditions of Use provides in part that unless you have obtained prior permission you may not download an entire issue of a journal or multiple copies of articles and you may use content in the JSTOR archive only for your personal non commercial use Please contact the publisher regarding any further use of this work Publisher contact information may be obtained at http www jstor org action showPublisher publisherCode aaas Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission JSTOR is a not for profit organization founded in 1995 to build trusted digital archives for scholarship We work with the scholarly community to preserve their work and the materials they rely upon and to build a common research platform that promotes the discovery and use of these resources For more information about JSTOR please contact support jstor org American Association for the Advancement of Science is collaborating with JSTOR to digitize preserve and extend access to Science http www jstor org The Amazon River accounts for 15 of global runoff 1 It is currently accessible however to 25 million people 12 0 4 of world population and no massive expansion of irrigation is likely that would warrant major diversiolnsfrom it We thus consider 95 of its flow inaccessible The Zaire Congo ranks second in global runoff 3 5 of the total 11 and supports 1 3 of world population 12 We judge half of its flow to be inaccessible for purposes of irrigation and in duLstrial and municipal use over the next 30 years The final subtraction is for the remote rivers of North America and Eurasia 55 of which have no dams on their main channels 13 Most of this river flow is in tundra and taiga biomes that are remote from population centers The combined average annual flow of these northern untapped rivers is 1815 km3 year and we subtract 95 of it Together the inaccessible remote flows of the Amazon Zaire Congo and northel rntier uindevelopedrivers amount to 7774 km3 per year Table 3 or 19 of total annual ruLnoff This leaves 32 900 km3 geographically accessible Our estimate is conservative because we made no subtractions for 10 300 1 Total many particularly northern rivers that have very large flows relative to the human population size and water needs of their geographic areas 14 We next adjusted for temporal inaccessibility Irrigated agriculture industry and households require that water be supplied when and where it is needed This degree of control over runoff is not easy to achieve Approximately 11 100 km3 of global runoff 27 of the total is renewable ground water and base river flow 6 As long as extraction does not exceed replenishment these souLrcescan provide a reliable renewable supply The remaining runoff 29 600 kmi3 is much harder to capture because most of it is flood water In Asia for instance 80 of runoff occurs from May to October 4 15 Capturing flood runoff generally requires the construction of dams The present storage capacity of large dams collectively totals 5500 kmi3 of which 3500 km3 is actively used in the regulation of river runoff 6 16 Adding together the base flow and the surface runoff controlled by dams gives an estimate of the total stable flow Assuming that the geographically accessible ruLnoffis l km3 year 1 K Total runoff 40 700 km3 year evapotranspiration onland 0 69 600 km3 year e 0 e Remote flow 7774 km3 year Uncaptured floodwater 20 426 km3 year Geographically and temporally accessible runoff AR 122 500 Human appropriation of ET 1 8 200 km3 year 26 km3 yea r Withdrawals Instream uses 4430 km3 year 35 2350 km3 year 1 9 Human appropriation of AR 6780 km3 year 54 Human km3 year 30 Human appropriation of total RFWSland 24 980 km3 year 23 Fig 2 Flow diagram of analysis of human appropriation of RFWSland The final box shows human appropriation of estimated accessible RFWSla ndto be 30 24 980 km3 82 1 00 km3 and hutman appropriationof total RFWSIand to be 23 24 980 km3 110 300 km3 786 SCIENCE VOL 271 n third use and iii huLmaninstream flow needs Together withdrawalsand instream uses providea measLreof hLuman appropriation of runoff and we estimatethem separatelyhere uses by far the most AR AgricuLlture worldwide We estimated agriculttLral water withdrawalsby mLltiplyingan averagewater applicationrateof 12 000 m3 ha 17 by the 1990 estimateof 240 millionhectaresof worldirrigatedarea 10 This yieldsa total water demand of 2880 km3 agricuLltural Table 4 The ratio of consumption to withdrawalsvarieswith climaticfactors the cropsgrown and irrigationefficiency and typicallyrangesbetween 50 and 80 4 We assUrme that 65 of agricultuLral water withdrawalsareconsumed for a globaltotal of 1870 km3 Industrialwater use has leveled off or declined in many wealthier countries but is growing rapidly in much of the developing world 2 Shiklomanov 4 estimatedthat ulseis 975 km3 globally includinduLstrial In ing the thermoelectricpower induLstry contrastto agricuLlture only a smallshareof waterulsedin induLstry is consumed most of it is dischargedback to the environment Table 2 Share of global runoffand population by continent Region appropriation of accessible RFWSt ama 24 980 dividedbetweenbase and flood flow in the same proportionthat total ruLnoffis we then reducedthe estimateof total baseflow by the shareof it contained in the remote rivers 2100 km3 0 27 X 7774 kim3 leading to an accessiblebase flow of 9000 km3 11 100 2100 Addition to this of the estimated3500 km3 of ruLnoff regulatedby existing reservoirsyields an estimate of presenttotal AR of 12 500 km3 year We next estimatedwhat portionof AR humanitynow uses Threecategoriesof water use are i withdrawalsor abstractions which representwaterremovedfromrivers lakes and aquifersfor human activities alsoknownas waterdemandor wateruse ii consumption which refersto withdrawals that are not availablefor a second or 9 FEBRUARY1996 Europe Asia Africa North and Central America South America Australiaand Oceania
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