Impact of climate change on freshwater ecosystems

17 Pages·2010·1.63 MB·English

Hydrol. EarthSyst. Sci.,14,783–799,2010
Hydrology and
www.hydrol-earth-syst-sci.net/14/783/2010/
Earth System
doi:10.5194/hess-14-783-2010
©Author(s)2010. CCAttribution3.0License. Sciences
Impact of climate change on freshwater ecosystems: a global-scale
analysis of ecologically relevant river flow alterations
P.Do¨llandJ.Zhang
InstituteofPhysicalGeography,GoetheUniversityFrankfurt,FrankfurtamMain,Germany
Received: 8February2010–PublishedinHydrol. EarthSyst. Sci.Discuss.: 15February2010
Revised: 4May2010–Accepted: 10May2010–Published: 21May2010
Abstract. River flow regimes, including long-term average withdrawalshadcausedcomparableshiftsonlessthan5%of
flows, seasonality, low flows, high flows and other types of thelandareaonly. Long-termaverageannualriverdischarge
flowvariability,playanimportantroleforfreshwaterecosys- is predicted to significantly increase on one half of the land
tems. Thus, climate change affects freshwater ecosystems area,andtosignificantlydecreaseononequarter. Damsand
not only by increased temperatures but also by altered river withdrawalshadledtosignificantdecreasesononesixthof
flow regimes. However, with one exception, transferable thelandarea,andnowheretoincreases.
quantitative relations between flow alterations and ecologi- Thus, by the 2050s, climate change may have impacted
calresponseshavenotyetbeenderived. Whiledischargede- ecologicallyrelevantriverflowcharacteristicsmorestrongly
creasesaregenerallyconsideredtobedetrimentalforecosys- thandamsandwaterwithdrawalshaveuptonow. Theonly
tems, the effect of future discharge increases is unclear. As exception refers to the decrease of the statistical low flow
afirststeptowardsaglobal-scaleanalysisofclimatechange Q , with significant decreases both by past water with-
90
impactsonfreshwaterecosystems,wequantifiedtheimpact drawalsandfutureclimatechangeononequarteroftheland
ofclimatechangeonfiveecologicallyrelevantriverflowin- area.However,damimpactsarelikelyunderestimatedbyour
dicators, using the global water model WaterGAP 2.1g to study. Considering long-term average river discharge, only
simulate monthly time series of river discharge with a spa- a few regions, including Spain, Italy, Iraq, Southern India,
tialresolutionof0.5degrees. Fourclimatechangescenarios WesternChina,theAustralianMurrayDarlingBasinandthe
basedontwoglobalclimatemodelsandtwogreenhousegas High Plains Aquifer in the USA, all of them with extensive
emissionsscenarioswereevaluated. irrigation,areexpectedtobelessaffectedbyclimatechange
We compared the impact of climate change by the 2050s thanbypastanthropogenicflowalterations. Insomeofthese
totheimpactofwaterwithdrawalsanddamsonnaturalflow regions,climatechangewillexacerbatethedischargereduc-
regimes that had occurred by 2002. Climate change was tions, whileinothersclimatechangeprovidesopportunities
computedtoalterseasonalflowregimessignificantly(i.e.by for reducing past reductions. Emissions scenario B2 leads
more than 10%) on 90% of the global land area (excluding to only slightly reduced alterations of river flow regimes as
GreenlandandAntarctica),ascomparedtoonlyonequarter compared to scenario A2 even though emissions are much
of the land area that had suffered from significant seasonal smaller. Thedifferencesinalterationsresultingfromthetwo
flowregimealterationsduetodamsandwaterwithdrawals. applied climate models are larger than those resulting from
Due to climate change, the timing of the maximum mean the two emissions scenarios. Based on general knowledge
monthlyriverdischargewillbeshiftedbyatleastonemonth about ecosystem responses to flow alterations and data re-
on one third on the global land area, more often towards latedtoflowalterationsbydamsandwaterwithdrawals,we
earliermonths (mainlydue toearliersnowmelt). Damsand expectthatthecomputedclimatechangeinducedriverflow
alterationswillimpactfreshwaterecosystemsmorestrongly
thanpastanthropogenicalterations.
Correspondenceto: P.Do¨ll
([email protected])
PublishedbyCopernicusPublicationsonbehalfoftheEuropeanGeosciencesUnion.
784 P.Do¨llandJ.Zhang: Impactofclimatechangeonfreshwaterecosystems
1 Introduction ecosystems include overexploitation, water pollution, habi-
tatdestructionordegradation(e.g.relatedtochangesinflu-
Climatechangeisimpactingfreshwaterecosystemsnotonly vial morphology) and invasion by exotic species (Dudgeon
by changing temperatures but also by changing water flow etal.,2006). Theircombinedeffectshaveledtodeclinesin
regimes. The term “flow regime” refers to the pattern of biodiversitythatarebyfargreaterthanthoseinthemostaf-
flowvariability. Flowregimesaredescribedbycharacteris- fectedterrestrialecosystems(Dudgeonetal.,2006).Accord-
ticslikelong-termannualandmonthlymeans,statisticallow ingtotheMillenniumEcosystemAssessment(2005),popu-
andhighflows,dailytointerannualvariability,andthetiming lations of freshwater species (included in the Living Planet
of flows. Many studies have shown that flow regimes play Index)declined,between1970and2000,by50%,compared
a major role in determining the biotic composition, struc- to30%formarineandforterrestrialspecies.Itisnotpossible
ture,functionanddiversitywithinriverecosystemsandthat to attribute the decline of freshwater ecosystems to the dif-
riverflowalterations(andtheresultingwaterstoragechanges ferentstresses, whichvarystronglywithgeographicregion.
e.g. in wetlands) may have a strong impact on freshwater Do¨lletal.(2009)foundthatriverflowregimealterationsare
ecosystems (Poff and Ward, 1989; Arthington and Pusey, strongest in semi-arid regions with extensive irrigation but
1993; MatthewsandMarsh-Matthews,2003; PoffandZim- alsodownstreamoflargedams. Intheseregions, riverflow
merman, 2010). Additionally, river flow alterations influ- alterationscanbeexpectedtobeamajorcauseofbiodiver-
ence other abiotic characteristics of freshwater ecosystems sitydecline.
that affect the well-being of organisms, in particular water Investigating human impacts on river flow regimes in the
quality, sediment transport and water temperature. There- 20th century and approaches for managing river basins in a
fore, climateimpactassessmentsshouldincludeananalysis sustainablemanner,theimportanceofflowregimesforriver
offreshwaterecosystemchangesthatmaybecausedbycli- ecosystems has been well documented (e.g. Richter et al.,
matechangeinducedriverflowalterations. 2003). Reno¨fa¨ltet al.(2010)reviewed literature onecosys-
Most analyses of climate change impacts on freshwater tem responses to flow alterations caused by hydropower
ecosystems focused on the impact of temperature changes generation, distinguishing flow magnitude, frequency, tim-
(Millenium Ecosystem Assessment, 2005; Fischlin et al., ing, duration and rate of change as well as changed water-
2007). Some considered changes in mean precipitation, as landscapeinteractions. AccordingtoDudgeonetal.(2006),
a surrogate variable for river flows (Buisson et al., 2008; freshwaterbiodiversityisrelatedtohighfloweventsthatin-
Lasalle and Rochard, 2009). Few analyses considered the fluencetheriverchannelshapeandallowaccesstootherwise
effects of changing water flows. Kundczewicz et al. (2007) disconnectedfloodplainhabitats,andtolowfloweventsthat
reviewed some publications that deal with the impacts of limit overall habitat availability and quality. Many charac-
climate change induced river flow alterations on freshwater teristics of the flow regime, in particular seasonality, inter-
ecosystems, discussing decreased habitat availability due to annual variability and timing of particular flow events, af-
decreased ice-jam flooding, impacts of lower water column fectlife-historypatternslikespawningandrecruiting(Dud-
depthonspawningofsalmon,andimpactsofreducedrunoff geonetal.,2006). Invasionsbyintroducedorexoticspecies
onbreedinggroundsforwaterbirds. Inareviewontheim- at the expense of native biota are more likely to succeed
pact of climate and land use change on Alpine brown trout, in an altered flow regime (if the former happen to be more
Scheurer et al. (2009) concluded that climate change in- adapted to the altered flow regime; Dudgeon et al., 2006).
ducedincreasesofriverdischargeandsedimentloadsinwin- To characterize human alteration of freshwater ecosystems
ter and early spring could be especially harmful for brown due to changed river flow regime in a satisfactory manner,
troutreproductionanddevelopmentofyounglifestages. Er- a large number of data that allow relating freshwater bio-
win(2009)reflectedonthechallengestowetlandconserva- diversity to specific flow regimes and regime alterations is
tion and restoration under climate change, pointing out the required. However, knowledge of freshwater species is still
need to reduce non-climate stressors, including monitoring, verypoor,inparticularindevelopingcountries,anditisnot
in particular of invasive species that are favored by climate likely to significantly improve in the near future (Revenga
change. Hewarnedthatanumberofwetlandswilldisappear et al., 2005). Based on an extensive literature review, Poff
duetoclimatechange,especiallythedrier-endwetlands.As- andZimmerman(2010)concludedthatexistingliteratureon
sessingtheimpactsofclimatechangeonwaterbirdsthatde- ecological responses does not allow deriving general quan-
pend on inland freshwater systems, Finlayson et al. (2006) titative relationships between flow alteration and ecological
alsoindicatedsemi-aridandaridregionsasmajorvulnerable response. However,theliteraturedoessupporttheinference
regions. that the risk of ecological change increases with increasing
Climate change puts additional stress on freshwater magnitudeofflowalteration(PoffandZimmermann,2010).
ecosystems that have already been heavily stressed by hu- To assess ecologically relevant changes of river flow
manactionsunrelatedtoclimatechange. Waterwithdrawals regimes, the Indicators of Hydrologic Alteration (IHA) ap-
and man-made reservoirs have significantly modified river proach of Richter et al. (1996) has been widely adopted
flowregimes(Do¨lletal.,2009).Otherstressesonfreshwater or adapted (e.g. by Black et al., 2005). In this method,
Hydrol. EarthSyst. Sci.,14,783–799,2010 www.hydrol-earth-syst-sci.net/14/783/2010/
P.Do¨llandJ.Zhang: Impactofclimatechangeonfreshwaterecosystems 785
two sets of flow time series representing natural and an- and dams were not taken into account. To illustrate possi-
thropogenicallyalteredconditionsatthesamesitearecom- ble ecological consequences of alterations of river flow al-
paredusing32indicatorsofflowmagnitude,frequency,du- terations,weappliedtheempiricallydeterminedrelationship
ration,timingandrateofchange. Largedifferencesbetween between long-term average discharge at the mouth of river
the indicators under natural and altered flow conditions are basinsandnumberofendemicfishspeciesofXenopouloset
likely to indicate that the biotic components of the aquatic al.(2005),whichwasalreadyusedbyXenopoulosetal. for
ecosystem have been altered too, with a reduction in en- translating reduction of long-term average discharge due to
demic species and possibly a decline of biodiversity. Us- climatechangeintoreductionofthenumberoffishspecies.
ingasubsetofIHA,Gibsonetal.(2005)evaluatedchanges InSect.2,wedescribethemethodstocomputeriverflows
oftheflowregimesindrainagebasinsinFloridaandWash- fortheclimaticconditionsof1961–1990and2041–2070(the
ingtonthatmayhaveoccurredby2080–2095duetoclimate 2050s)withandwithouttheimpactofwaterwithdrawalsand
change,anddiscussedtheirecologicalsignificance. Statisti- dams,andtocomputethechangeoffishspeciesnumbers. In
callydownscaledtemperaturesandprecipitationsofonecli- addition, we present the selected ecologically relevant indi-
matemodelwereusedasinputtothetwohydrologicalmod- cators of river flow alteration. In Sects. 3 and 4, results are
els. InFlorida,maximumflowswerecomputedtodecrease, shownanddiscussed. Inthelastsection,wesummarizethe
reducing floodplain-river connectivity. In Washington, cli- studyresultsanddrawconclusions.
mate change may lead to a shift of the seasonal flow max-
imum from May to January, strongly decreased minimum
flows,somewhatincreasedmaximumflows,andamuchpro- 2 Methods
longedlowflowperiodfromMaytoSeptember. Prolonged
lowflowswereconsideredtoexacerbatehigherfuturesum- 2.1 ComputationofriverdischargewithWaterGAP
mer temperatures, with negative effects on cold-water fish,
and to degrade the habitat. The computed changes in in- Withaspatialresolutionof0.5◦ by0.5◦ (55kmby55kmat
terannual variability were not considered to be reliable due the equator), the global water resources and use model Wa-
to the low spatial resolution of the climate model (Gibson terGAP simulates water flows and storages (hydrology) as
et al., 2005). The Tennant Method (Tennant, 1976), which well as human water use for all land areas of the globe ex-
has been applied for a reservoir outflow management in a cludingAntarctica(Alcamoetal.,2003). Wateruse,i.e.wa-
large number of river, cannot be used for assessing the im- ter withdrawals and consumptive water use, is estimated by
pact of altered flow regimes on freshwater ecosystems as it separate models for the sectors irrigation, livestock, house-
onlyreferstoinstantaneousflowsbutnottotheecologically holdsandindustry.TheWaterGAPGlobalHydrologyModel
relevanttemporalsequencesofflows,i.e.riverflowregimes. WGHMcomputesgroundwaterrecharge,totalrunoffgener-
Obviously, a quantitative global-scale assessment of the ation as well as river discharge, taking into account the im-
impact of climate change on freshwater ecosystems related pact of human water use and man-made reservoirs on river
to river flow alterations cannot be done yet, due to missing discharge (Do¨ll et al., 2003; Do¨ll and Fiedler, 2008). For
quantitative estimates of ecosystem responses to alterations each grid cell, the vertical water balance is computed, and
of the river flow regime. However, it is possible to do the theresultingrunoffisroutedlaterallywithinthecellthrough
firststepofsuchanassessment,i.e.toperformaglobal-scale a groundwater store and various surface water stores (if ex-
analysis of the alterations of the river flow regime that may istent). Theeffectoflakes,reservoirsandwetlandsonwater
becausedbyclimatechange. balance and flow dynamics is modeled by first routing the
The objective of this study was to evaluate, for the first runoff generated within the grid cell through so-called “lo-
time, the impact of climate change on ecologically relevant cal”lakes, reservoirsandwetlands. Theresultingdischarge
flowcharacteristicsattheglobalscale. Toputtheseclimate volumeisaddedtothedischargefromtheupstreamgridcell
changeimpactsintoperspective,futureriverflowalterations and routed through so-called “global” lakes, reservoirs and
were compared to alterations of natural river flow that had wetlands, and through the river storage compartment. The
already occurred by 2002 due to human water withdrawals differencebetweenprecipitationandpotentialevapotranspi-
and dams (Do¨ll et al., 2009). We used the most recent ver- ration is added to each surface water type within the grid
sion2.1goftheglobalhydrologyandwaterusemodelWa- cell, thus taking into account the effect of the surface water
terGAP (Alcamo et al., 2003; Hunger and Do¨ll, 2008; Do¨ll balance on cell runoff. WGHM is tuned in a basin-specific
et al., 2009) which takes into account the impact of reser- manneragainstlong-termaveragedischargeat1235gauging
voirsandwaterwithdrawalsonriverdischarge. Twodiffer- stations(HungerandDo¨ll,2008).
entemissionsscenariosasinterpretedbytwostate-of-the-art ImportantWGHMinputsaretimeseriesofmonthlyvalues
globalclimatemodelswereappliedtoestimateriverflowal- of climate variables as well as information on soil and land
terations that may have occurred by the 2050s as compared cover. Monthlyclimatedataaredownscaledtodailydata,in
to the time period 1961–1990 when anthropogenic climate the case of precipitation using the available number of wet
changewasstillsmall. Futurechangesofwaterwithdrawals dayspermonth. Monthlyclimatedata,exceptprecipitation,
www.hydrol-earth-syst-sci.net/14/783/2010/ Hydrol. EarthSyst. Sci.,14,783–799,2010
786 P.Do¨llandJ.Zhang: Impactofclimatechangeonfreshwaterecosystems
areprovidedbytheCRUTS2.1dataset(MitchellandJones, 2.1.2 Modelingtheimpactofclimatechangeonriver
2005). Asprecipitationinput,0.5◦ griddedmonthlytimese- discharge
riesoftheGPCCFullDataProductVersion3(Fuchsetal.,
2007)wereused,togetherwiththenumberofwetdaysfrom Weconsideredfourdifferentclimatechangescenarios,com-
theCRUTS2.1dataset. paring the flow regime resulting from the climate during
the time period 1961–1990 to the flow regime during the
2.1.1 Modelingtheimpactofwaterwithdrawalsand
time period 2041–2070 (the 2050s). The two IPCC green-
damsonriverdischarge
house gas emissions scenarios A2 and B2 (Nakicenovic
and Swart, 2000) were translated into climate change sce-
In WGHM, the effect of human water withdrawals is simu-
narios by two state-of-the-art global climate models, the
lated by subtracting total consumptive water use from river
ECHAM4/OPYC3 model (Ro¨ckner et al., 1996, hereafter
discharge and from water stored in lakes and reservoirs, if
referred to as ECHAM4) and the HadCM3 model (Gor-
there are any in the grid cell. Consumptive use is the frac-
don et al., 2000). In the A2 scenario, emissions increase
tionofthewithdrawnwaterthatdoesnotreturntotheriver
from 11GtC/yr (CO -equivalent) in 1990 to 25GtC/yr in
butisevapotranspirated,thuscausingflowreductions. Con- 2
the2050s,butonlyto16GtC/yrinthecaseofscenarioB2.
sumptive use of a cell is supplied from the cell itself, or
Duetolargeclimatemodeluncertainties,thesameemissions
from the neighboring cell with the highest long-term aver-
scenariosaretranslatedtoratherdifferentclimatescenarios,
ageriverdischargeifnotenoughwaterisavailableinthecell
inparticularwithrespecttoprecipitation.
itself. Domestic, industrial and livestock water use in 2002
was modeled by the respective WaterGAP water use mod- Thechangesinaveragesofmonthlyprecipitationandtem-
ules. Irrigation water use was computed according to Do¨ll perature values between the periods 1961–1990 and 2041–
and Siebert (2002) using, as input, 1) version 4.0.1 of the 2070ascomputedbytheclimatemodelswereusedtoscale
GlobalMapofIrrigatedAreasGMIA(Siebertetal., 2005), the grid cell values of observed monthly precipitation and
2) estimates of actually irrigated area per country in 2002 temperature between 1961 and 1990 that drive WGHM in
and 3) the climate data time series 1961–1990, to take into the control run. In a first step, the climate model data were
account the effect of climate variability on irrigation water linearly interpolated from their original resolutions to the
use. Whiledomestic,industrialandlivestockwateruseisas- WGHM resolution of 0.5◦. Then, in the case of tempera-
sumed to be constant within each year, irrigation water use ture,observedvalueswerescaledbyaddingtothemthedif-
variesfrommonthtomonth. Globalconsumptivewateruse ference of the climate model values of future (2041–2070)
hasmorethandoubledbetween1951and2002andreached andpresent-day(1961–1990)temperature. The30-yearper-
1300–1400km3/yr around 2000 (as compared to renewable turbedprecipitationtimeserieswasproducedbymultiplying
water resources of approximately 40000km3/yr), with irri- observedvalueswithfutureclimatemodelprecipitationasa
gation accounting for more than 90% of total consumptive ratioofthepresent-dayprecipitation. Ifpresent-daymonthly
wateruse(Do¨lletal.,2009). Consumptivewateruseispar- precipitation was less than 1 mm, precipitation was scaled
ticularlyhighinIndia,Pakistan,partsofChinaandtheUSA additively, like temperature. The impact of changed inter-
andintheMediterraneanregion,mainlyduetothelargeirri- annual variability and the predicted increased variability of
gationareasthere(Do¨lletal.,2009). daily precipitation could not be taken into account in this
The impact of dams on river discharge is computed in study.
this study by using a reservoirs and regulated lakes data set
that includes 6553 reservoirs and 52 regulated lakes world- 2.1.3 Specificationofmodelruns
wide. Thesurfaceareaofthereservoirsis291000km2,the
storage capacity 5900km3 (Do¨ll et al., 2009). The data set
Six 30-year time series of gridded monthly river discharge
wasobtainedbycombiningmainlythereservoirsincludedin
were computed by WHGM, which were then used to quan-
the Global Lakes and Wetlands Database (Lehner and Do¨ll,
tify the indicators of river flow regime alterations described
2004)withapreliminary(July2008)versionoftheGRanD
inSect.2.2.Inouranalysis,ANTconditionsrefertotheflow
database(Lehneretal.,2008,2010)The1022largestreser-
regime as impacted by human water withdrawals as well as
voirsandalltheregulatedlakesaremodeledusingaspecific
byreservoirsandregulatedlakesfortheclimateoftheyears
algorithm for reservoirs, distinguishing two types of reser-
1961–1990, with withdrawals and dams in 2002. This sim-
voirs, irrigation and non-irrigation (Do¨ll et al., 2009). The
ulation is the standard WGHM simulation for which tuning
otherreservoirsaremodeledlikenaturallakes. Themajority
against long-term annual observed discharge has been per-
ofreservoirsareinNorthAmericaandAsia.
formed. NAT refers to the naturalized regime as computed
byamodelruninwhichtherearenowaterwithdrawalsand
no reservoirs, and in which regulated lakes are treated like
naturallakes. CC-ANTreferstoanyofthefourANTmodel
runs with climate of the years 2041–2070 which differ by
Hydrol. EarthSyst. Sci.,14,783–799,2010 www.hydrol-earth-syst-sci.net/14/783/2010/
P.Do¨llandJ.Zhang: Impactofclimatechangeonfreshwaterecosystems 787
Table1.Fiveecologicallyrelevantindicatorsofriverflowalterationsduetoclimatechangeorhumanwaterwithdrawalsandreservoirs.The
term“altered”eitherreferstoalterationsduetoclimatechange(CC-ANT),ascomparedtotheclimateof1961–1990(“unaltered”=ANT),
ortoalterationsduetowaterwithdrawalsanddams,for1961–1990climate(ANT),ascomparedtonaturalizedconditionswithoutanydams
andwithdrawals(“unaltered”=NAT).
indi-cator question definition specificecologicalrelevancea
ILTA How are long-term differences between long-term average annual numberofendemicfishspecies,
average river flows river discharges under altered and unaltered groundwater-dep.floodplain
affected? conditions,inpercentoflong-termaverageun- vegetation
alteredriverdischarge
ILF How are statistical difference between long-term average Q90 habitatconditions,liketemper-
lowflowsaffected? (monthly river discharge that is exceeded in 9 ature and oxygen concentra-
outof10months)underalteredandnon-altered tion, connectivity, compatibil-
conditions,inpercentofunalteredQ90 itywithlifecycleoforganisms,
wastewaterdilution
ISA Howistheseasonal differenceinseasonalamplitude(maximummi- habitatavailabilityinparticular
amplitudeaffected? nusminimumlong-termaveragemonthlyriver onfloodplains,increaseinnon-
discharge) under altered and unaltered condi- natives
tions,in%ofunalteredamplitude
ISR Howistheseasonal meanover12monthlyvaluesofabsolutediffer- habitat conditions, compatibil-
regimeaffected? encesbetweenlong-termaveragemonthlyriver itywithlifecycleoforganisms
discharges under altered and unaltered condi-
tions,in%ofunaltereddischarge
ITS Whatseasonalflow temporal shift of month with maximum river compatibilitywithlifecycleof
shifts(will)have discharge,inmonths(ifnegative,thismonthoc- organisms,e.g.disruption
occurred? cursearlierduealteration) ofspawning,assemblagestruc-
ture,foodavailabilityfordetri-
tivorousmacroinvertebrates
aPoffandZimmerman(2010),Xenopoulosetal.(2005),Gibsonetal.(2005)
theclimatescenarios. IntheCC-ANTruns,withdrawalsand 2.3 Estimationofchangesoffreshwaterfishrichness
damsremainatthe2002level. duetochangesinlong-termaverageriverdischarge
2.2 Indicatorsofriverflowregimealteration Xenopoulos et al. (2005) derived a regression equation be-
tween the number of freshwater species in river basins and
Inourglobal-scalestudyontheimpactofwaterwithdrawals the long-term average river discharges (1961–1990) at the
and dams on river flow regimes, we developed six different mouth of the basins. They considered data from 237 river
indicatorsofriverflowregimealterationthatareecologically basinslocatedbetween42◦Nand42◦S.Thenumberoffish
relevantandcanbecomputedbyWGHMinaratherreliable species mainly relates to endemic fish, with nonindigenous
manner (Do¨ll et al., 2009). Five out of the six indicators species being assumed to be less than 5%. Long-term av-
are also used here (Table 1). The indicator related to inter- erage discharge was computed with a previous version of
annualvariabilityofmonthlyflowswasnotsuitableforthis WGHM (Xenopoulos et al., 2005). Fish species numbers
study because in our method for deriving climate scenarios inriverbasinswerefoundtodecreasewithdecreasinglong-
(Sect.2.1.2),changesofinterannualvariabilityascomputed termaverageriverdischargeaccordingto
byclimatemodelarenotrepresented,onlychangesoflong-
termaveragemonthlyprecipitationandtemperature. Lognumberoffishspeciesinbasin=
0.4·logmeanannualdischargeatbasinoutlet(m3/s) (1)
+0.6242, r2=0.57
Xenopolousetal.(2005)usedEq.(1)tocomputedecreases
of fish species richness in zero-order river basins for which
www.hydrol-earth-syst-sci.net/14/783/2010/ Hydrol. EarthSyst. Sci.,14,783–799,2010
788 P.Do¨llandJ.Zhang: Impactofclimatechangeonfreshwaterecosystems
  ECHAM4 A2 HadCM3 A2

ECHAM4 B2 HadCM3 B2

Fig.1.Impactofclimatechangeonriverdischarge:changeofmonthlylowflowsQ90,ILF,between1961–1990and2041–2070.Emissions
scenariosA2andB2asimplementedbytheglobalclimatemodelsECHAM4andHadCM3, riverflowsanthropogenicallyaltereddueto
waterwithdrawalsandreservoirsoftheyear2002(CC-ANT–ANT,in%ofANT).
river discharge at the river mouth was predicted to de- inTable1)bythe2050s,bycomparingCC-ANTwithANT.
crease due to future climate change and future water with- Q is monthly river discharge that is exceeded in 9 out of
90
drawals. They stated that the consequences of increasing 10months. Inallscenarios,thespatialpatternofincreasing
river discharge for riverine biodiversity have not been rig- or decreasing Q is roughly the same, except in Australia
90
orouslytestedandthusdidnotapplyEq.(1)forriverbasins andSouthAmerica,wherestrongdiscrepanciesbetweenthe
with future discharge increases. Following Xenopoulos et ECHAM4andHadCM3scenariosexist. Q ispredictedto
90
al. (2005), we only applied Eq. (1) in case of decreasing decrease, in all scenarios, very strongly by more than 80%
discharge. Different from Xenopoulos and colleagues, we in Northeastern Brazil, the western part of Southern Africa
determinedthedecreaseofendemicfishspecies(atequilib- andattheMediterraneanrim,whileitmayincreasebymore
rium) in the upstream basin of each 0.5◦ grid cell, not only than 100% in Northeastern China. The B2 scenarios, with
forwholezero-orderbasins. lower emissions, result in somewhat less intensive changes
thantheA2scenarios. Itisobvious,however,thatthetrans-
lationofthesameemissionsscenariobytwoclimatemodel
3 Results resultsinlargerdifferencesthanifoneclimatemodelisused
to translate the two different emissions scenarios. Particu-
In this section, the computed impact of climate change on lardiscrepanciesbetweenthetwoclimatemodelsareclearly
river flow regimes is presented, and then compared to the visible in Australia, India, West Africa and South America,
impact of dams and water withdrawals on the natural flow with HadCM3 predicting a dryer future for the same emis-
regime. Inthelastpart,weshowthepossibleimpactofde- sions scenario than ECHAM4. Both climate models can be
creasedlong-termaverageriverdischargeonthenumberof regarded as equally uncertain. The difference between the
fishspecies,asquantifiedbyapplyingEq.(1). projectionsofthetwoclimatemodelsillustratesonlyafrac-
tion of the uncertainty. Selecting other climate models as
3.1 Impactofclimatechangeonfiveecologically inputtoWGHMwillprobablyleadtoastilllargerrangeof
relevantriverflowcharacteristics projectedpatternsofQ .
90
Figure 1 shows the impact of four different climate change
scenariosonstatisticalmonthlylowflowsQ (indicatorI
90 LF
Hydrol. EarthSyst. Sci.,14,783–799,2010 www.hydrol-earth-syst-sci.net/14/783/2010/
P.Do¨llandJ.Zhang: Impactofclimatechangeonfreshwaterecosystems 789
Table 2. Global characterization of river flow regime alteration due to climate change until the 2050s, as compared to historic climate
1961–1990, andduetowaterwithdrawalsandreservoirsaroundtheyear2000, ascomparedtonaturalizedconditions. Inthe“CC-ANT
comparedtoANT”columns,thechangeinriverflowregimesbetween1961–1990and2041–2070isshown,assumingwaterwithdrawals
andreservoirsrepresentativefortheyear2000.Asclimatescenarios,theA2emissionsscenarioasinterpretedbytheglobalclimatemodels
ECHAM4andHadCM3wereused. Inthe“ANTcomparedtoNAT”columns,theimpactofwaterwithdrawalsanddamsfortheclimate
1961–1990isshown. GreenlandandAntarcticaarenottakenintoaccount. Incolumnswith“+”header,thepercentoflandareainwhich
theindicatorisatleast10%(oronemonth)isshown,incolumnswith“−”header,thepercentoflandareainwhichindicatorisminus10%
orsmaller(orminusonemonth).
%oflandareawithindicatorvalue≥|10%| medianofindicatorvaluesfortheseland
(or≥|1month|,incaseofindicatorITS) areas,in%(forindicatorITSinmonths)
CC-ANTcomparedtoANT ANTcompared CC-ANTcomparedtoANT ANTcompared
ECHAM4A2 HadCM3A2 toNAT ECHAM4A2 HadCM3A2 toNAT
Indicator + − + − + − + − + − + −
ILTA 54.4 23.1 49.9 23.8 0.002 16.2 44.9 −27.6 29.2 −29.3 19.8 −27.9
ILF 45.8 24.3 39.9 27.0 4.9 26.0 40.4 −33.7 27.7 −33.6 63.2 −52.5
ISA 55.9 26.1 53.5 23.5 0.6 14.8 56.2 −30.2 40.1 −26.6 17.2 −33.5
Ia 89.8 88.5 23.8 46.3 34.0 3.3
SR
ITS 14.3 25.1 12.1 20.7 2.7 1.7 1 −1 1 −1 1 −1
aindicatorhasabsolutevalues.
FortheA2emissionsscenario,Q ispredictedtoincrease Changesoftheseasonalregime(computedastheaverage
90
significantly,i.e.bymorethan10%,on40–46%oftheland difference of the 12 mean monthly river discharges, indica-
area (excluding Antarctica and Greenland), the median in- tor I ) are more pronounced for the ECHAM4 run than
SR
crease in these areas being 28–40% (Table 2). The ranges for the HadCM3 run (Fig. 2c and Table 2). Mean monthly
reflect the different results due to the two applied climate dischargeschangebymorethan10%(averagedoverthe12
models. The median values in Table 2 always refer to the months)onabout90%ofthegloballandareaincaseofsce-
indicated area with significant changes. Q decreases by nario A2 (Table 2). In these areas, the median change is
90
morethan10%on24–27%ofthelandarea,byamedianof 34–46%, depending on the climate model. The month with
34–34%(Table2). FortheB2scenarios,Q ispredictedto themaximummonthlyriverdischarge(indicatorI )isex-
90 TS
increasesignificantlyon38–47%ofthelandarea(withame- pected to shift by at least 1 month on 33–39% of the land
dianincreaseof27–41%)andtodecreaseon21–26%ofthe area,moreoftentoearliermonths(Table2).Theearliermax-
landarea(withamediandecreaseof30–32%)(notshownin imumflowismostlyrelatedtoearliersnowmelt.Themedian
Table2). Thisindicatesthatreducedemissionshavearather value is 1 month, but there are a significant number of grid
small beneficial effect on climate change induced low flow cellswithshiftof2oreven3to6months(Fig.2d).
reductions.
Long-term average annual discharges (indicator I ) 3.2 Comparisonofclimatechangeimpactstoimpacts
LTA
show approximately the same spatial pattern as I , with ofdamsandwaterwithdrawals
LF
somewhatlargerareaswithflowincreasesandhigherpercent
increases. (Fig. 2a, emissions scenario B2). For A2, long- Compared to the impacts that dams and water withdrawals
termaverageriverdischargeispredictedtoincreasebymore hadonnaturalriverdischargesby2002,climatechangewill
than 10% on 49–54% of the land area, by on average 29– have led, by the 2050s, to much stronger alterations of the
45%(median),whileitdecreasesbymorethan10%on23– selectedecologicallyrelevantflowcharacteristics. Different
24%ofthelandarea,byonaverage24–29%. Thechangein fromclimatechange, damsandwithdrawalsnowhereledto
seasonalamplitudes(indicatorI )correlatesstronglywith any increases of long-term average discharge (Fig. 3a, and
SA
ILTA(Fig.2bandTable2).Oneofafewexceptionsoccurs,in indicator ILTA in Table 2). They resulted in significant de-
theHadCM3B2scenario, intheupstreampartoftheAma- creases of long-term average river discharge on 16% of the
zon basin where seasonal amplitude increases even though globallandarea, whileclimatechangeispredictedtocause
long-termaveragedischargedecreases(compareFig.2awith decreaseson23–24%. Itisfurtherpredictedtocausesignif-
Fig.2b). icantlyincreasedlong-termaveragedischargeonabouthalf
of the land area (Fig. 3b and Table 2). Thus, changes be-
tweenCC-ANTandNAT,i.e.theimpactsofdam,reservoirs
and climate change on naturalized long-term average river
www.hydrol-earth-syst-sci.net/14/783/2010/ Hydrol. EarthSyst. Sci.,14,783–799,2010
790 P.Do¨llandJ.Zhang: Impactofclimatechangeonfreshwaterecosystems
  ECHAM4 B2 HadCM3 B2
a  

b

c

d
Fig.2. Impactofclimatechangeonriverdischarge: percentchangeoflong-termaverageannualriverdischarge,ILTA (a),seasonalflow
amplitudes,ISA(b),seasonalflowregimes,ISR(c)andshiftofthemonthwithmaximumflow,ITS,inmonths(d),between1961–1990and
2041–2070. EmissionsscenarioB2asimplementedbytheglobalclimatemodelsECHAM4andHadCM3,riverflowsanthropogenically
alteredduetodamsandwaterwithdrawalsoftheyear2002(CC-ANT–ANT,in%ofANT).
Hydrol. EarthSyst. Sci.,14,783–799,2010 www.hydrol-earth-syst-sci.net/14/783/2010/
P.Do¨llandJ.Zhang: Impactofclimatechangeonfreshwaterecosystems 791
pogenic change by dams and water withdrawals, with one
a
exception(Table2andFig.4and;I notshowninFig.4).
  SA
TheexceptionisI (Fig.4aandb). Whileclimatechange
LF

causes more wide-spread increases of Q than dams, the
90
  areal extent with significant decreases by more than 10% is
comparable.Whileclimatechangewillcausesuchdecreases

on 21–27% (range of all four scenarios) of the global land

area,damsandwithdrawalscausedsuchadeclineofnatural
  low flows on 26% of the land area (Table 2). The average
  (median)decreaseduetodamsandwithdrawalsinthesear-
eas was 53%, significantly larger than the average decrease

duetoclimatechange(30–34%).

Figuer5indicates,inred,allareaswheretheimpactofcli-
  b mate change on long-term average river discharge (Fig. 5a)
  or Q90 (Fig. 5b) is at least twice as large as the impact of
damsandwithdrawals. Theintensiveredcolormarksthear-

easwherechangesduetodamsandreservoirshavethesame
  signasthechangesduetoclimatechange. Inblue, areasin
  whichtheimpactofdamsandreservoirsistwiceasbigasthe
impact of climate change are indicated. Considering long-

term average river discharge, only a few areas, including

Spain, Italy, Iraq, Southern India, Western China, the Aus-
  tralianMurrayDarlingBasinandtheHighPlainsAquiferin
  theUSA,allofthemwithsignificantirrigation,showadom-
inance of past alterations (Fig. 5a). Considering Q , there
90

aremanymoreareasinwhichpastriverflowalterationsdue
c
  todamsandwithdrawalsdominateoverclimatechangeim-
pacts, including most parts of the USA, the Mediterranean
andWesternandSouthAsia(Fig.5b). Lowflowsaremore
sensitive to dams and withdrawals than long-term average
riverdischarges.
3.3 Impactofclimatechangeonnumberoffishspecies
Relatingchangesoflong-termaveragedischargetochanges
infishspeciesnumberusingEq.(1),wecomputedtheimpact
ofclimatechangeonthenumberofendemicfishspecies(at
equilibrium)intheupstreambasinofeachgridcell(Fig.6b,
HadCM3 A2). In the HadCM3 A2 scenario, 15% of the
Fig.3. Comparisonoftheimpactofclimatechangetotheimpact
globallandareawouldsufferfromadecreaseoffishspecies
ofdamsandwaterwithdrawalswithrespecttolong-termaverage
in the upstream basin of more than 10%, and 0.6% of the
annualriverdischarge(ILTA). Impactofdamsandwithdrawalson
naturalized discharge (ANT – NAT, in % of NAT) (a), impact of land from a decrease of even 50%. The median decrease
climate change on anthropogenically altered discharge (CC-ANT of fish species in areas with a more than 10% decrease is
– ANT, in % of ANT) (b), combined impact of climate change, 18.3%. As the HadCM3 predicts significant discharge de-
damsandwaterwithdrawals(CC-ANT–NAT,in%ofNAT)(c). creases in Central America and the northern part of South
Dams and withdrawals in the year 2002, climate change between America(morepronouncedthantheECHAM4model,com-
1961–1990and2041–2070accordingtotheemissionsscenarioA2 pareFig.2aandb),significantdecreasesoffishspeciesrich-
asimplementedbytheglobalclimatemodelHadCM3.
ness are visible there, which, given the high number of en-
demic fish in these regions, would mean the loss of many
fish species. For example, the number of fish species in
discharge (Fig. 3c), are more pronounced than changes be- theAmazonbasin(affectedbydamsandwithdrawalsofthe
tween CC-ANT and ANT (Fig. 3b) only if climate change year2002)iscomputedtodecreasefrom561underclimate
leadstodecreasedlong-termaveragedischarges. conditions of 1961–1990 to 511 in 2041–2070. The cor-
Alteration of all flow characteristics is much stronger in responding numbers for the Orinoco are 279 and 232, and
the case of climate change than in the case of past anthro- 191 and 159 for the Tocantins (Brazil). For two zero-order
www.hydrol-earth-syst-sci.net/14/783/2010/ Hydrol. EarthSyst. Sci.,14,783–799,2010
792 P.Do¨llandJ.Zhang: Impactofclimatechangeonfreshwaterecosystems
  ANT – NAT CC-ANT – ANT
1961-1990 2041-2070
a   b

c d

e f

Fig. 4. Comparison of the impact of climate change to the impact of dams and water withdrawals. Impact of dams and withdrawals on
naturalizeddischarge(ANT–NAT,in%ofNAT),formonthlylowflowsQ90,ILF(a),seasonalflowregimes,ISR(c),andshiftofthemonth
withmaximumflow,ITS (e),ascomparedtoimpactsofclimatechangeonanthropogenicallyaltereddischarge(CC-ANT–ANT,in%of
ANT),formonthlylowflowsQ90 (b),seasonalflowregimes(d),andshiftofthemonthwithmaximumflow(f). Damsandwithdrawals
intheyear2002,climatechangebetween1961–1990and2041–2070accordingtotheemissionsscenarioA2asimplementedbytheglobal
climatemodelHadCM3.
rivers in the semi-arid Northeast of Brazil, the situation ap- theWorld: ASpecialCollectionofRiverBasinData”,http:
pears to be more dramatic. The number of fish species in //earthtrends.wri.org/maps spatial/watersheds/index.php).
the Parna´ıba river basin may decrease from 66 to 24, and Globally,thedecreaseofriverinefishspeciesrichnessdue
in the Jaguaribe river basin from 36 to 1. Please note that toclimatechangeisstrongerthanthedecreaseduetodams
these fish numbers were computed from Eq. (1) and can andwithdrawals, whichcausedadecreaseofthenumberof
onlybeconsideredtobeveryroughestimates. IntheAma- fishspeciesintheupstreambasinbyatleast10%on10%of
zon and the Orinoco, for example, the number of endemic the land area, while on 0.6%, it caused a decrease of more
(endemic plus non-endemic) fish species was estimated to than 50% (Fig. 6a). The spatial patterns of change are very
be 1800 (3000) and 88 (318), respectively (“Watersheds of different(comp.Fig.6aandb),withclimatechangepossibly
Hydrol. EarthSyst. Sci.,14,783–799,2010 www.hydrol-earth-syst-sci.net/14/783/2010/