FINAL REPORT - MekongInfo · Mekong Mainstream, IBFM and Potential Landuse Impacts 1.1 General...

MekongRiverCommission

December 2006

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An Evaluation of Landuse and Climate Change on the Recent Historical Regime of the Mekong

INTEGRATED BASIN FLOW MANAGEMENT

P.T. Adamson

FINAL REPORT

Table of Contents

Objectives, Scope and Summary 1

PART I: Aspects of the Natural Flow Regime of the Mekong Mainstream, IBFM and Potential Landuse Impacts 5

1.1 General Overview. 5

1.2 The potential impacts of landuse change. 6

1.3 Aspectsofthelongtermdynamicsoftheproposedflowindicatorsandhydro-biological seasons. 8

1.4 Aspectsofthegeographyandvarianceoftheflowseasons. 11

PART II: Aspects of Climate Change 15

2.1 Aspects of Climate Change and the Tibetan Plateau. 15

2.2 Climate Change and the SW Monsoon in the Mekong Region 16

PART III: Summary Conclusions 23

References 25

Annex1. Metricsusedtodefinethestartandendofthefourflowseasons 27

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Objectives, Scope and Summary

Thereisnouniversalprescriptionforthemaintenanceofariver’shealth,eachhydro-ecologicalsystem has an implicit natural dynamic equilibrium that needs to be evaluated in a way that effectivelyresultsinenvironmentalflowcriteriathatemergeselfevidently.Thisrequiresamuch more perceptive understanding of the structure and function of the hydrological regime than is commonly the case. From a hydrological perspective, three factors should initially governtheselectionofenvironmentalflowindicators:

Issues of scale.InalargeinternationalriverbasinsuchastheMekong,themacro-geographicscalemeansthatsignificantlocalimpactsonflowssoonbecomespatiallyimperceptible. For example, the hydrological effects of a tributary hydropower scheme tendtobecomescreenedoutoncethemodifiedflowsenterthemainstream.Significantimpacts of this nature on the Basin scale will therefore tend to arise as a result of cumulative developments. Single interventions on the mainstream itself are a different matter, though eventhesehavetobelargescaleinordertogenerateimpactsthattranslateanysignificantdistancedownstreambeforebeingmaskedbynaturaltributaryinflows.Theimpactsoflanduse change also become undetectable as geographic scale increases, despite the fact thatregionaldeforestationhasbeensignificantfromthe1960stodate.Scaleissuesmustthereforeinfluencethetypeofindicatorthatisappropriateforenvironmentalmanagement.For example, an event based hydrological index such as the annual frequency and magnitudeoffloodhydrographswouldonlybeappropriateforsmalltributarysystems,where the scale is small enough for individual storm runoff events to be distinguished. In thelargertributariesandmainstreamthereiseffectivelyonlyoneannualfloodhydrographin response to the seasonal scale of the SW Monsoon. The onset and duration of this singleannualfloodisclearlyoneofthekeydeterminantsoftheregionalhydroecologicalenvironment.

The structure of the hydrological regime. This simple single amplitude of the annual mainstream hydrograph is complemented by a highly predicable phase, which together form thedefinitivefeatureofthehydrologicalregimeoflargetropicalmonsoonalrivers.OntheMekongthemeanannualvolumeofthisfloodhydrographrangesfrom65km3 at the China border to 350 km3intheCambodianfloodplain.Thetimingoftheonsetofthefloodseason,definedasthe(usually)singleupcrossingofthemeanannualdischargeinJune,hasastandard deviation of only two weeks. In other words it is highly predictable. Such temporal aspectsofthehydrologicalregimemustinturninfluencethestructureandfunctionofaquaticcommunitiesandthereforewouldsuggestthemselvesaskeyflowindicators,particularlywhengiventheirsmallinterannualvariance,amodestman-inducedchangewouldbeasignificantintervention.Ontheotherhand,volumetricflowindiceswouldbealmostmeaninglessinthefaceofthescaleofthefiguresinvolved.Regulationstoragecould,ontheotherhand,affecttheonsetofthefloodseason(anditsnarrowtemporalwindow)asreservoirstoragesrefill.Thenatureofthehydrologicalregimealsocontributesto the physical template of rivers, such as channel form and the geomorphological processes

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that determine it. Appropriate hydrological indices in this respect would be linked to the characteristicdurationofflowsaboveandbelowcriticalhydraulicthresholds.

The major types of resource development. Over the next 20 to 30 years, the major area of water resource development is anticipated to be hydropower, the fundamental feature of which is the seasonal regulation of the hydrology, that is a reallocation of water from the wet to the dry season via reservoir storage. Since hydropower schemes are in principlenonconsumptiveusersofwater,the‘atsite’meanannualflowremainsthesame–unlessofcoursetheschemeinvolvesinter-catchmentdiversion.Themajorimpactisthereforeanincreaseintheaveragedryseasonflows,anappropriatemeasureofwhichonthemainstreamistheminimumdischargeineachyearaveragedover90consecutivedays.Statisticalanalysesofthesedatacandefinethestructureandpatternofdryseasonhydrologyanduncoverappropriateenvironmentalflowindicators.Thesewouldincludetemporal aspects such as measures of onset and duration. Other resource developments, suchaswaterabstraction,principallyforirrigation,wouldtendtoreducedryseasonflows,though the prospects are that these will merely serve to moderate the increases due to hydropower regulation.

Changestosuchflowindicesprovidetheframeworkforassessingtheimpactsofdevelopmentand whether these are tolerable or not. However, the indices themselves have a natural variability and the need arises to incorporate this into the impact assessment process. Finally,beforedefiningtheirnaturalorbenchmarkvalues,itisnecessarytoestablishthatthehydrologicalindicesarethemselvesnotalreadyundergoingsystematicmodificationinresponse to climate induced change, resource development or landscape conversion. Any detectablehistoricalresponseoftheproposedindicestoman-inducedchangeprovidesinsightsinto how they may respond in the future and what the wider environmental and hydrological consequences could be.

This Report therefore sets out to inform the IBFM process with regard to these hydrological aspects , acknowledging the rather obvious fact that the biological and ecological contributions need to be made on the basis of a reasonable understanding of the key aspects of the mainstreamflowregime.Itislaidoutintwomajorsections:

Thefirstpart brieflyoverviewsthosebroadaspectsofthehydrologicalregimeconsideredto be relevant to the IBFM process within the Mekong context. The potential impacts of landusechangeonthehydrologicalregimearealsoexamined,specificallywithregardtothesignificantregionaldeforestationthathastakenplacesincethe1960s.Thissynopsisis considered particularly pertinent to IBFM since it underscores the issues of scale that have already been referred to as well as revealing how fragmentary types of change only appear to generate localised impacts. An awareness of the long term dynamics of the specificflowindicatorsandhydro-biologicalseasonsthathavealreadybeenintroducedto the IBFM procedures is considered to be fundamental. This historical scrutiny of the indices reveals just how persistent the timing, onset and duration of the seasons have beenoverthelast90orsoyearsaswellasthefactthatthereisvirtuallynogeographicalvariation. Also exposed is the quasi periodicity of aspects of the hydrological regime and the dynamic nature of the relationship between the hydrology and the environment that could be at risk from human intervention.

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The second part focuses upon the evidence within the observed hydrometeorological datathatclimaticallyinducedchangesarealreadytakingeffect.Noconfirmationthatthisis so was found for the hydrology studies reported in Part I, so the focus in Part II lies principally within the regional rainfall climate.

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PART I: Aspects of the Natural Flow Regime of the Mekong Mainstream, IBFM and Potential Landuse Impacts

1.1 General Overview.

Fivecomponentsofthehydrologicalregimeregulatethebiophysicalprocessesandthesocio-economic activities that operate within and depend upon a river system. These are, according to Richter et al.(1996):

themagnitudeoftheflows;

thefrequencyofflowsandwaterlevelsaboveandbelowcriticalthresholds;

thedurationandseverityoftheseaboveandbelowaverageevents;

theirtimingwithintheyearandtheirfrequencyofoccurrencefromyeartoyear,and;

their rate of change or how quickly one state changes to another, which is linked to aspects of resource reliability and exposure to the risk of extreme conditions, for example.

Separatingouttheconstituentelementsoftheflowregimeinthiswaynotonlyallowsabetterunderstanding of the physical nature and statistical structure of the process but also enables the consequences of human interventions and climate change to be considered explicitly in terms of their potential impacts on each of the components and their relationships with each other.

In the context of monitoring or forecasting potential changes to river regimes or setting environmentalflowstosustainecosystemsandlivelihoodstherehastobesomebenchmarkmeasure against which the magnitude of potential change can be measured and the tolerance ofthesystemtochangeitselfevaluated.Ideallythisbenchmarkstateisthenaturalflowregime, though the degree to which any river system anywhere remains in an entirely natural / pristine state is open to question. In a practical sense the natural or reference state refers to the hydrological regime being in some form of dynamic equilibrium with the river basin climateandlandscapeandthatecologicalfunctionsandsocio-economicactivitiesremainunimpaired. Few river system regimes in the developed world remain entirely unaffected by human disturbance, thus the emphasis lies with sound environmental management and the implementation of recovery and rehabilitation programmes to restore their natural ecological and geomorphological integrity.

The Mekong system presents a hydrological regime that can in fact be regarded as natural, which echoes the fact that it is in long term dynamic equilibrium with the regional climate and landscape. In order to establish this though, it is necessary to demonstrate that there are no monotonic(systematicandinonedirection)trendsinthemagnitudeandpatternoftheflow

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regimeandthatallhydrologicalvariabilityandfluctuationattheseasonal,annual,interannualand interdecadal time scales is the result of natural hydroclimatic variability.

ThattheflowregimeoftheMekongmainstreamremains‘undisturbed’isnot,however,aninstinctive supposition in the face of, for example, regional land use and land cover change. This has primarily been in the form of tropical deforestation and forest degradation and has, according to Giril et al.(2001),beenoccurringatanunprecedentedrateandscale,particularlyfromthe1960sonwards. Othermorerecentinfluencesincludehydropowerdevelopmentandtheexpansion of water abstractions and diversion for irrigation.

1.2 The potential impacts of landuse change

The conventional viewpoint is that deforestation results in a decrease in the natural water storage capacity of a river catchment, which in turn leads to an increase in water yield, the magnitude of which varies with the local rainfall climate, the topography and the proportion, typeanddensityoftheremovedforestcover(Newson1997).Inprincipletherefore,therearetwo potential hydrological impacts of deforestation that might be distinguished:

1. total water yield isincreasedasannualevapo-transpirationdecreases,andthe

2. seasonaldistributionofflowsismodifiedasfloodrunoffincreasesanddryseasonflowdecreases.

On a country by country basis in the Mekong region the decline in the national area prescribed asforestedfrom1960onwardsisindicatedinTable1.Theinstinctiveconclusionfromsuchfigureswouldbethattheriverregimehasalreadyundergoneconsiderablechange.Thiswouldhavesignificantimplicationsfortheselectionofthebenchmarkcriteriafordefiningenvironmentalflowrequirements.

Table 1. Forest cover over Yunnan and Indo-China from the 1960s to 2000. (after Stibig et al 2004)

CountryPeriod

1960s–1970s circa1980 circa1990 circa 2000Cambodia >70%1 >70%1 67%1 53%4

Lao PDR 60%1 - 47%1 41%5

Thailand 53%2 34%2 28%2 29%4

Viet Nam 42%1 - 28%1 30%4

Burma 58%3 - 52%4

Yunnan 55%6 33%6

Notes:1MeyerandPanzer(1990);2KlankamsornandCharuppat(1994);3Perrson(1974);4FAO(2001);5MRC2003;6 Chinese National Bureau of Statistics

Shifting cultivation is often pointed out as a leading direct cause of forest loss in tropical regions.IthasbeenpractisedintheMekongBasinaselsewheresincepre-historyandthereisa general consensus that as a traditional tropical land use practise, shifting cultivation does not destroy forests so long as fallow periods are long enough, some shade trees are left standing and the felled patches are small and have a low density. However, as population pressure increasestheseconditionscannotbesustainedandsincethelate1970ssuchactivitieshavebeen

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increasingly viewed as unsustainable. The regionally sustainable balance between relatively small upland populations and large forests has been lost in the face of resettlement policies, migration and forest land development for commercial agriculture.

Superimposed on these increased population and commercial pressures over the last three decades has been the expansion of the regional forestry sector in response to the rapid international escalation of commercial demands for tropical timber. Pressure on the forests of LaoPDR,CambodiaandBurmawasintensifiedin1989,whenThailandintroducedaloggingban within natural forests, and consequently sought increased imports from its neighbours. DemandfromChina,fuelledbythepaceofnationaleconomicexpansionfromtheearly1990s,is the latest and potentially the greatest challenge to regional tropical forest conservation.

Such rates of deforestation might be expected to produce a long term, relatively smooth and systematictrendwithregardtoaspectsoftheMekongregime.Annualflowvolumesmightbeexpectedtoincreaseanddryseasonflowswouldbeexpectedtodecrease.Figure1showsthetimeseriesofthepercentagedeviations(anomalies)aboveandbelowthelongtermmeanannualflowsatVientianeandKratieoverthe46yearsbetween1960and2005.Theplotsrevealjusthowdifficultitwouldbetoconfidentlyisolateanysystematicpatterninthedatathatcouldbeattributedtohumanactivity,whiletheappropriatestatisticaltestsfortrend(MannKendal,forexample)indicatenothingofanysignificance.

Figure1.MekongatVientianeandKratie:Percentagedeviationsofannualflowsaboveandbelowthelongtermmean.1960–2005.Thesmoothlinesarethe3yearmovingaverage

Figure 2 shows the results for the wet and dry seasons, treated separately. Again there is no evidenceofanytrend–thesplitsamemeansofthedryseasonflowsareidenticalwhilesub-sampledifferencesinmeanannualfloodpeakareclimaterelated.

It appears to be the case that forest removal has to be on a very large scale and attributed to clear-cuttingtocreateagriculturallandinorderfortheretobeunequivocalimpactsonthehydrology of large catchments with areas in excess of circa 1000 km2. In the Mekong region, althoughclear-cuttinghasbeenafactorinforestremoval,ithasnotbeenonthescalethathasbeen seen elsewhere in SE Asia, in Indonesia for example. Rather, upland forests, where they have been removed or degraded, have been replaced by fragmented landscapes consisting of remnantforestpatchesandvarioushuman-disturbedlandcovers.

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Sub-PeriodMean Annual Flood

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Vientiane Kratie Vientiane Kratie1960-1982 17400 54 300 1960-1982 1 250 2 4501983-2005 15 000 47500 1983-2005 1 220 2 450

Figure2.MekongatVientianeandKratie:Percentagedeviationsofannualmaximumflowsandmeandryseasonflowaboveandbelowtheirlongtermmeanvalues.1960–2005.Thedryseasonflowisdefinedastheannualminimum90daydischarge

Such fragmentation results from a myriad of activities, including timber extraction, shifting agriculture, permanent cultivation, forest gathering, dwelling construction, road building, andinsomecases,re-vegetation(Ziegleret al.2004).Thehydrologicalfunctionalityofsuchfragmented landscapes is only distinguishable from that of the natural forests that they have replaced at the local scale. Claims therefore that land use changes have historically had a detectableinfluenceupontheregimeoftheMekongcannotbesubstantiatedbydataanalysis.

1.3 Aspectsofthelongtermdynamicsoftheproposedflowindicatorsandhydro-biologicalseasons

Theproposedflowindicatorsandflowseasonswillonlybeeffectiveifchangestothemimposedbyhumaninterventioncanbedistinguishedfromtheirnaturalvariability.Mosthydro-climaticvariables have both long and short term quasi periodicities and oscillations superimposed on them and a characteristic variance from year to year. It turns out that this variance, measured -say-intermsoftheannualstandarddeviationoftheprocess,isquitesmall.Aswillbedemonstrated,thisisespeciallysowithregardtotheonset/endoftheflowseasons.Thisnarrow

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variancemeansthatsmallchangeswillgenerallybesignificantstatisticallyandpotentiallysignificantecologically,whichmakestheproposedindicatorsefficientdetectivemechanismsandeffectiveindicesofhydrologicalmodification.

Thetimeframeoverwhichthehydro-climaticandhydro-environmentalconditionsintheMekong region have been relatively uniform would be expected to cover the late Holocene and extend back approximately 6000 years. Figure 3 gives a clear indication of the pattern, frequencyandrangeofselectedannualflowvectorsoverthelast80to90years.

Figure3. MekongatVientianeandKratie.Timeseriesplotsofselectedannualflowvectors.Noneindicatesanyevidenceofastatisticallysignificantmonotonictrend,thoughthefrequencypatternoftheannualmaximumdischargedoesappeartoshowachange-pointintheearly1980s

These data reveal:

Nosignificantmonotonictrends,butasequenceofoscillationsandchangepoints,particularlyevidentintheannualmaxima.Theseperiodicfluctuationsareclimaterelated and are considered in detail in the climate change studies in the context ENSO periodicities.

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Evidenceofamorepluvialperiodbetweenthelate1930sandtheearly50s.Thisisrevealed more persuasively in the climate change studies where it is considered along with the available regional rainfall data.

Of the potential impacts of water resources development upon the regime of the Mekong, those concerning the dry season hydrology are generally recognised as likely to be amongst the most manifest and ecologically important. In this regard, the degree to which the mean level ofthedryseasonflowsisquasiperiodicisparticularlysignificant.Figure4confirmsthatthesedynamicoscillationsaresubstantialandmustbeanintegralaspectofthenaturalflowregimethatorganizesanddefinestheriverecosystemsanditsoverallenvironment.

Figure4. HistoricaltimeseriesofannualdryseasondischargesatVientianeandKratie,indicatingthequasi periodic oscillations in the mean level of the process. These shifts in the mean can be as muchas+/-15%andtheaverage‘runlength’is14years(theredlinesindicatetheaveragedischargeina‘run’)

The charts show a number of interesting features:

As expected, the departures above and below the long term mean are in general accordance along the mainstream. The typical length of a ‘run’ or ‘anomaly’ is 14 years andoversuchaperiodtheaveragedryseasonflowcanbesystematicallyasmuchas15to20%fromthelongtermvalue.

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Thesignificanceofanyimpactsfromhumanactivityuponthedryseasonhydrologyisnotthereforesimplyanissueofamodificationtothelongtermseasonalaverageflow.Areductionoftheyeartoyearvarianceofthedryseasonflowsandadecreaseintheamplitudeofthesemeanperiodicitieswillpresumablyhaveaninfluenceonthelongerterm ecological equilibrium of the system. Doubtlessly this change will be imperceptible atfirstbuttheconsequenceswillaccumulateastheshiftinthemeanlevelbecomesapermanentoneasopposedtothenaturalcycleoftemporaryfluctuations.

1.4 Aspectsofthegeographyandvarianceoftheflowseasons.

AnanalysisofthehistoricalonsetanddurationofthefourproposedflowseasonsatVientianeandKratieoverthelast80to90+yearsrevealstwoimportantfeatures:

The timing of the onset and the duration of the seasons is virtually identical at Vientiane andKratie(Figures5and6)despitethefactthatthehydrologyoftheformerisdominatedbythesocalledYunnancomponentoftheMekongregime(seeChapter5oftheOverviewoftheHydrologyoftheMekongBasin,MRCS,2005),whileatKratiethehydrologicalregimeislargelydictatedbyflowsenteringthemainstreamfromthelargeleft banks tributaries in Lao PDR, downstream of Vientiane. The system is therefore entirely homogenous with regard to these temporal aspects of its hydrology.

The tabulated seasonal onset values in Figure 5 disclose a very narrow range, expressed in terms of their standard deviations from year to year. The point has already been made that any small change to these seasonal onsets and durations would be not only statisticallysignificantbutmaypotentiallybeofanecologicalandenvironmentalconsequence that is quite disproportionate to the magnitude of the temporal shift. A delay oftwoweeks,oronestandarddeviation,totheonsetofthefloodseasonisananticipatedimpactoftherefillingoflargehydropowerreservoirstorages,forexample.

AsFigure7implies,theonsetanddurationofthesehydro-biologicalseasonshasbeenremarkably consistent and unchanged over the last century and almost certainly over the last 6000+ years. These seasonal timings are arguably one of the key factors in determining the hydro-bioticequilibriumofthesystemandyet,potentially,theyarethemostexposedandsusceptibleaspectsoftheflowregimetomodificationfromregulationandreservoirstorage.

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Figure5. MekongmainstreamatVientiane(1913-2005)andKratie(1924–2005).Historicalpercentagefrequencyoftheweeksduringwhichtheannualminimumdischargeandflowseasontransitions occurred, along with their sample means and standard deviations

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Figure:6MekongmainstreamatVientiane(1913-2005)andKratie(1924–2005).Historicaltimingoftheannualminimumdischargeandthetransitionsbetweentheflowseasons.Thegraphandsummarytableindicatetheprobability(P(K>k)%)thatinanygivenyeartheminimumdischargeandseasonaltransitionswillhaveoccurredbeforeweekk(k=1,52)

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Figure7. OnsetoftheseasonsintheMekongmainstreamatVientiane(1913–2005)andKratie(1924–2005)

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PART II: Aspects of Climate Change

2.1 Aspects of Climate Change and the Tibetan Plateau.

The Tibetan plateau accounts for almost a quarter of China’s area and is one that climatologists haveidentifiedasa‘tippingpoint’fortheimpactsofglobalwarming.Suchregionswill,it is argued, show a sudden, dramatic and precipitous response to climate change. It is the headwaterofriversthatflowdowntohalfofhumanity.TheYellowRiverandtheYangtsestartinnortheasternTibetandflowacrossChina,theMekongoriginatesineasternTibetasdotheIrrawady and Salween that traverse down to Burma, Laos., Thailand, Cambodia and Vietnam The Tsang Po starts near Lake Manasarovar and travels eastwards for nearly 2000 km before cutting through the Himalaya to become the Brahmaputra and empty into the Bay of Bengal. Most of the major rivers in Nepal originate in the Tibetan plateau and cut deep gorges to flowdowntotheGanga.AndthereistheIndusanditstributarieswhichalsostartnearLakeManasarovarandflowwestwardsintoPakistanandemptyintheArabianSea.

Inawarmerworld,thereflectiveiceandsnowoftheTibetanplateauwillslowlyturntobrownand grey as it melts and retreats to reveal the ground beneath. As the ground warms, melting will accelerate. Tibet will become a much warmer place.

Figure8.HistoricalcontributionofflowsfromTibetandYunnantomonthlyflowsontheMekongmainstreamatVientiane(1913–2005)andKratie(1924–2005)

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For many regional rivers such as the Mekong any sustained change in the hydrological response oftheplateauofTibetislikelytohavesignificant,ifnotdramatic,longtermimpactsontheflowregime.AsFigure8indicatesrunofffromTibetandYunnaninChinaprovidesadominantcomponentofthelowflowhydrologythroughouttheMekongsystem:

In the upper part of the Lower Mekong system, at Vientiane, the so called ‘Yunnan Component’notonlyprovidesmostofthedryseasonflowsbutinadditionmostofthefloodwaterduringthemajorityofyears.Averagecontributionsrangefromover75%duringthelowflowmonthsinAprilandMay,toover50%duringthepeakflowmonthsofJuly,AugustandSeptember.Theyeartoyearrangeofthecontributionsis,nontheless, quite wide and indicates a complex and varying nett contribution.

MuchfurtherdownthesystematKratie,thelargeleftbanktributariesinLaosprovidemostofthefloodseasonflowonthemainstreamsuchthatthe‘YunnanComponent’isreducedtoamodest15%to20%.However,itsremainsasignificantsourceofdryseasondischarge,reachingamaximumaveragecontributioninexcessof40%inApril.

TheimplicationofthiskeyaspectoftheMekongregimeisthatitisnotthefloodseasonhydrologythatispotentiallythemostvulnerabletoclimatechangeimpacts,butthelowflowregime.ThisisparticularlynoteworthywithintheIBFMcontextasitisarguablythelowflowregimeofthesystemthatismostexposedtomodificationbyresourcedevelopment,byreservoirregulation for example. This will tend to increase the dry season hydrology, whereas in the longer term it is at risk of decreasing as the reliability and contribution of snow and glacial melt waters declines. Initially, global warming may enhance these melt water contributions but such increaseswillberelativelyshortlivedastheglaciersandsnowfieldsretreattohigheraltitudes.

ThereappearstobenosignificantevidenceasyetthatanysuchchangesaremanifestingthemselvesintheobservedMekonglowflowhydrology(seeFigure3),buttheymaybeanticipatedastheTibetaniceandsnowfieldsretreat.Theevidenceisaccumulatingofacceleratingratesofglacialandsnowfieldrecession(WWF,2005).OntheQinghai-TibetanPlateau during the past 40 years or more glacial extent has shrunk by some 6600 km2 out of a total of 110 000 km2,whichissignificant.Presently,95%ofglacialsystemsareinretreat,suchthat the long term implications for the freshwater resources of much of Asia are immense. In a study of future water resources availability in the Sutlej River system, with is major sources in theHimalayanglacialsnowfields,SinghandBengtsson(2002)foundthattheimpactofclimatechange to be more prominent on seasonal rather than annual water availability. Reduction of spring and summer meltwater would have severe implications on future regional water resources at times of the year when hydropower and irrigation demand are at their peak

2.2 Climate Change and the SW Monsoon in the Mekong Region

The Tibetan Plateau also plays a major role within in the climate system of Asia and in particular upon the timing of the SW monsoon system, through both thermal and mechanical (uplift)influences.Thisinturnaffectsglobalclimateandglobalclimaticchange(WuandZhang.1998).Consequently,anychangeinthethermalregimeoftheplateauofTibetthrough

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snowmeltandglacialretreathasthepotentialtosignificantlydisruptthepatternandintensityofthe monsoon itself.

Two aspects of the monsoon are relevant in the hydrological and water resources context and therefore to the BFM process, namely its onset and duration and its intensity, in terms of the seasonalrunoffandriverflowitgenerates.AtthemuchbroaderregionalscaleKripalani,andKulkarni(2001)examinedseasonalsummermonsoon(June-September)rainfalldatafrom120eastAsianstationsfortheperiodfrom1881to1998.Aseriesofstatisticaltestsrevealedthepresenceofshort-termvariabilityinrainfallamountsondecadalandlongertimescales,thelonger‘epochs’ofwhichwerefoundtolastforaboutthreedecadesoverIndia,Indo-ChinaandChinaandapproximatelyfivedecadesoverJapan.Inspiteofyear-to-yearfluctuationsandthedecadalvariabilityintherainfallrecords,nosignificantlong-termtrendswereobservedinthedata. The authors concluded that the observational history of summer rainfall trends in east Asia doesnotsupportclaimsofintensifiedmonsoonalconditionsinthisregionasaresultofCO2-induced global warming. As for the decadal variability inherent in the record, it ‘appears to be just a part of natural climate variations’.

TheseconclusionsarebroadlysupportedbystudiesspecifictotheMekongbasin,thoughthe scale of these regionally data based analyses needs expansion and the application of morepowerfulandrefinedstatisticaltechniques.ThefirstoftheevaluationsundertakenherecentresonthetiminganddurationofthemonsoonbasedoncriteriatakenfromKhademulet al. (2006).Theannualonsetisdefinedastheweekreceivingmorethan20mmofrainfallin1 or 2 consecutive days, provided that the probability of at least 10 mm of rainfall occurring inthesubsequentweekismorethan70%.Thelattercomponentofthecriterionscreensoutisolated storm events earlier in the year that do not fully indicate the start of ‘true’ monsoonal conditions.Thedateofmonsoonwithdrawalisdefinedasthedayuptowhichatleast30mmofrainfall accumulates over a sequential seven day period, with no subsequent rainfall for at least three consecutive weeks. Criteria such as these have found wide application across the Indian subcontinent.

Figure9showstheresults,yearbyyear,atfourlocationsinLaosandThailandthathavesuitably long uninterrupted daily rainfall records. Table 2 summarises the average historical date of monsoon onset and withdrawal at each one:

Table 2. Historical (1950-date) average date of onset and withdrawal of the SW Monsoon at selected locations in the Lower Mekong Basin.

Site Monsoon onset Monsoon endDay Date Day date

Chiang Rai 129 9thMay 311 7thNovKhonKhaen 128 8th May 289 16th OctVientiane 125 5th May 284 11th OctKhorat 128 8th May 298 25th Oct

The average annual onset dates, according to the selected rainfall criterion, are remarkably consistent between the four sites.

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This is not the case with respect to the average annual date of withdrawal, which indicate aregionalvariationintheaveragetimingofalmostamonth.Themostsignificantindication is that the termination of the Monsoon towards the north in Chiang Rai is several weeks later than elsewhere.

This characteristic later termination at Chiang Rai is very evident from Figure 10. The difference with the other sites is quite consistent over the 50+ years of analysis, while there is also an indication of a higher variability in the date of the monsoon withdrawal in these more northern regions.

A clear conclusion to be drawn from all four plots is that over the last 50+ years there is no indication that the timing and duration of the SW Monsoon has undergone any change at all, as part of any climate change process.

Figure9.HistoricalonsetandwithdrawaldatesoftheSWMonsoonatselectedsitesintheLowerMekongBasin,usingtherainfallcriteriadescribedinKhademulet al.(2006)

Turning to a consideration of rainfall totals during the course of the regional Monsoon, generallyspeakingtheresultsreportedbyKripalani,andKulkarni(2001),assummarisedabove,are supported, though caution and further analysis are necessary in equal proportion.

For example, Figure 10 shows plots of the historical time series of annual rainfall totals observed at Vientiane for two periods:

1950to2005.Duringthisperioditmightbejustifiedtoconcludeadecreaseinannualrainfall,particularlyafterthelate1960s.Statisticaltestswouldconfirmthis.

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KHORAT THAILAND KHOEN KAEN THAILAND

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However,ifthelonger,thoughdiscontinuousperiodofrecordfro1923todateisconsideredtheconclusionofadecreasingtrendbecomesmuchmoredifficulttosupport.Thereisastrongsuggestionthatthewetteryearsbetweentheearly1950sandlate1960saremerelypart of the longer term quasi periodicity of the rainfall climate, supporting the wider regionalconclusionsofKripalani,andKulkarni(2001).

Figure10.AnnualrainfallatVientiane,forthecontinuousperiodofrecordfrom1950todate(above)andfortheinterruptedperiodofrecordfrom1923todate(below).Statisticaltestindicateadecreasingtrendintheshorterperiodofrecord,butlong-termmeansoverthediscontinuousrecord(indicatedbythedashedlines)areinsignificantlydifferent.

This example, serves to illustrate the caution required when apparent trends are detected in hydro meteorological time series data. Such processes almost always have multi decadal periodicities embedded within them and it is easy to confuse these with systematic trends when only a part of such long period cycles is evident in the sample used for analysis.

A second example, may on the face of it appear more indicative of a climatically induced change in rainfall regime, but once again care and circumspection are recommended. Figure 12 showsaspectsoftheannualrainfallregimeatKoratintheupperMunRiversysteminThailand(seeFigure11,below).

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Figure 11: Selected rainfall stations in NE Thailand

TheplotsrevealasubstantialchangepointintherainfallclimateatKorat,centreduponthelate1960s,when:

Annualrainfalltotalsdecreasedsignificantly,reducingthemeanannualvaluebetween1951and1970ofover1250mm,toaround1000mmfortheyearsfollowing.

Thisshiftwasassociatedwithadeclineintheannualnumberofstormdays,definedasone upon which 25mm or more rainfall occurred.

The annual maximum rainfalls over durations such as 10 days also decreased, though perhaps not as distinctly as the above.

It would be impulsive to suppose that such a result is linked in any way to climate change for several reasons:

Thedownwardshiftinannualrainfallattheendofthe1960sparallelsthedecreasewhichoccurred at the same time for the data observed at Vientiane, where the longer term data indicates that such changes are a natural component of the long term periodicity of the regional rainfall climate.

Thechangeinthenumberofstormdaysisatleastaslikely(asnot)tobepartofthissameperiodic process. Such aspects of rainfall climate as storm frequency and severity have not yet been considered as intensively as annual and season rainfall amounts in studies of long term climate variability and the detection of climate change. Their links with annual total rainfallarecertaintobesignificantandmutuallydeterministic.

Rainfall in tropical monsoonal regions has a high spatial variance, such that observed conditions at a single location are not necessarily indicative of a large area. None of the

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otherdatainvestigatedforNEThailand(seetheirlocationsonFigure11)showedsuchsignificantshifts,thoughoveralltherewasacommonpatternlinkedtoperiodiccomponentsintrinsic to the data and therefore the process. The strengths and weaknesses of this pattern are, however, quite variable, from point to point.

Figure12.Korat.NEThailand.Aspectsoftheannualrainfallclimateoverthelast50+years,showingasignificantdecreaseinannualrainfall,thenumberofstormdaysineachyearandtheannualmaximum10dayrainfallsincethelate1960s

It seems clear therefore that unravelling the natural periodic changes in rainfall climate from themoresystematicmodificationsthatmaycomeaboutasaresultofglobalwarmingwillbeextremelydifficultwhenthefocusliesentirelywiththeobservedhydro-meteorologicaldata.Claims that the impacts of climate change are already manifest are often based upon modest samples of data which contain incomplete medium and longer term natural periodicities. It is also a fact that human memory is relatively short and therefore concepts of climatic ‘normality’ are based upon very short periods of experience. Understandably therefore, natural shifts with relatively high periodic frequencies are often seen as a consequence of global warming.

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PART III: Summary Conclusions

The major conclusions of the work reported here are as follows:

Within the context of IBFM and the processes and activities associated with protecting the regime of the Mekong and its associated ecosystem, this regime as it is at present may beregardedas‘natural’,inthesensethatonthemainstreamthereisasyetnosignificantmanifestation of man induced change.

This applies both to changes in the landscape and the hydrological consequences of water resourcesdevelopment.Inparticular,thesignificantregionaldeforestationthathastakenplacesincethe1960shasnot,asisoftenclaimed,generatedanydetectablechangeintheflowregime.This,itisconcluded,isintimatelylinkedwithissuesofscaleandthefactthat the regional process of deforestation leaves a fragmented landscape which functions hydrologically in ways similar to natural forest.

ThehydrologicalflowindicatorsdevelopedwithintheIBFMprocesshaveanaturalvariability, but there is no evidence to suggest that they themselves are undergoing systematic change.

Withintheobserveddata,bothhydrologicalandmeteorological(specificallyrainfall)thereisnosignificantevidencetosupportclaimsthatclimatechangeimpactsarealreadymanifest within the region.

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References

Adamson.P.T.(2001)PerspectivesonMekongHydrologyandHydropower. International Water Power and Dam Construction, March, 16 – 21

Giri1,C.,Shrestha,S.andM.Levy(2001)AssessmentandMonitoringofLandUse/LandCover Change in Continental Southeast Asia. Paper prepared for presentation at the Open MeetingoftheGlobalEnvironmentalChange.ResearchCommunity,RiodeJaneiro,6-8October.

KhademulM,IslamMolla,RahmanS,SumiAandPBanik.(2006)EmpiricalModeDecomposition Analysis of Climate Changes with Special Reference to Rainfall Data. Discrete dynamics in Nature and Society 2006,1-17.

Klankamsorn,B.andT.Charuppat(1994)Deforestation in Thailand. Royal Forest Dept. Bankok, Thailand.

Kripalani,R.H.andKulkarni,A.(2001)Monsoonrainfallvariationsandteleconnectionsoversouth and east Asia. International Journal of Climatology 21,603-616.

Meyer,G.andK.F.Pabzer(1990)Regionalrenewablenaturalresourcesandlanduseinventoryandmointoring.ReportPN90.2098.3-03.InterimMekongCommitteeforCoordinationandInvestment.GTZ.

MRC(2003)MekongRiverCommissionannualReport2003.Vientiane,LaoPDR.

Newson,M.D.(1997)Land, Water and Development: Sustainable Management of River Basin Systems. Routledge, London.

Perrson,R.(1974)ReviewoftheWorld’sForestResourcesintheearly1970s.ResearchNotes.No17.RoyalCollegeofForestry,Stockholm.

RichterB.D.,Baumgartner,J.V.,Powell,J.andD.P.Braun.(1996)Amethodforassessinghydrological alteration within ecosystems. Conservation Biology 10,1163-117.

SinghPandLBengtsson(2002)HydrologicalSensitivityofaLargeHimalayanBasintoClimate Change. Hydrological Processes 18 (13), 2363 – 2385.

WuG.andY.Zhang(1998)TibetanForcingandTimingoftheMonsoonOnsetoverSouthAsiaand the South China Sea. Monthly Weather Review 126 (4),913–927.

WWF(2005)AnOverviewofGlaciers,GlacierRetreat,andSubsequentimpactsinNepal,IndiaandChina.HimalayanGlacierandRiverProject,WWFNepal,Kathmandu.

Ziegler,A.D.,Giambelluca,T.W.,T.Liemet al.(2004)HydrologicalconsequencesoflandscapefragmentationinMountainousnorthernVietnam:evidenceofacceleratedoverlandflowgeneration. Journal of Hydrology 287, 124–146.

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Annex1.Metricsusedtodefinethestartandendofthefourflowseasons

Season Start EndDry Season Averagedailyflowrecession

(decrease)is1%orlessover 15 consecutive days, indicativeofbaseflowconditions.

Twice the minimum daily discharge for the current dry season occurs, indicating that discharges have increased significantlyandthelowflowseason is at an end.

Transition Season 1 End of dry season StartoffloodseasonFlood Season Daily discharge exceeds mean

annualdischargeforthefirsttime.

Last date upon which daily discharge falls below mean annual discharge.

Transition Season 2 Endoffloodseason Start of dry season

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