Uncertainty in Climate Change Projections of Discharge for the Mekong Riverd

Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Sciences Discussions Earth System Hydrology and erent GCMs. De- ff C threshold of “dan- ◦ 3 55%, with greatest decreases in July and + 5992 5991 , and G. Kite 16% to 1 − erences in GCM projections of future precipitation. In contrast, ff , J. R. Thompson 1,2 This discussion paper is/has beenSystem under Sciences review for (HESS). the Please journal refer to Hydrology the and corresponding Earth final paper in HESS if available. UCL Department of Geography, University College London, Gower Street, London, Department of Geography, University of Otago,Bryn P.O. Box Eithin, 56, Cefn Dunedin, Bychan New Road, Zealand Pantymwyn, Flintshire, CH7 5EN, UK there is strong consistency betweenotranspiration GCMs and in a terms shift to ofin the an both upper earlier increased Mekong potential and (Lancang evap- lessis sub-basin), substantial strong the snowmelt temperature-related enough signal season. to inchange overwhelm Indeed, discharge in the discharge, precipitation-related with scenarios uncertaintyApril–June, from in and all the GCMs decreased direction leading flow to from of increased July–August. river flow from in river discharge are greater (from August, greatest increases in Mayintra-basin and changes June) in and precipitation, result evaporationresults and from snow are complex storage/melt. highly and Whilst contrasting GCM overall is dependent primarily (in driven both by di direction and magnitude), this uncertainty sensitivity, as well as from hydrologicalby model parameter running specification. pattern-scaled This GCM is(SLURP) achieved output of through the a basin. semi-distributedcific thresholds These hydrological of pattern-scaled model global climate GCM change outputsgerous” including the allow climate postulated investigation change 2 of as spe- tailed simulated analysis of using results outputs based onbut from HadCM3 non-linear seven climate response di scenarios of reveals a annualture, relatively river ranging small discharge from a to 5.4% increasing decrease global to mean 4.5% tempera- increase. Intra-annual (monthly) changes The Mekong River Basin comprisestors a that key regional include resource agriculture, inpotential fisheries Southeast impacts Asia and of for electricity climate sec- change production.quantify on freshwater uncertainty Here resources we in within explore the these the river projections basin. associated We with GCM structure and climate Abstract Hydrol. Earth Syst. Sci.www.hydrol-earth-syst-sci-discuss.net/7/5991/2010/ Discuss., 7, 5991–6024, 2010 doi:10.5194/hessd-7-5991-2010 © Author(s) 2010. CC Attribution 3.0 License. 1 WC1E 6BT, UK 2 3 Received: 6 August 2010 – Accepted:Correspondence 6 to: August D. 2010 G. – Kingston Published: ([email protected]) 23Published August by 2010 Copernicus Publications on behalf of the European Geosciences Union. Uncertainty in climate change projections of discharge for the Mekong RiverD. Basin G. Kingston 5 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ecting the poten- ff 5994 5993 Previous studies of the hydrological impacts of potential climate change on the Given the magnitude of projected climatic changes, the importance of water for The impacts of hydrological changes resulting from projected changes in climate may Mekong have generally focussed on climateclimate forcings from change individual GCMs from or the anoutput mean ensemble from the of Japanese GCMs.nario Meteorological and Agency For a GCM example, gridded fordays, Kiem hydrological the precipitation model et IPCC and to SRES al. discharge show A1b (2008)between would that sce- used increase 1979–1998 the by and mean 5.2, annual 2080–2099.hydrological 6.3 number model and of Ishidaira and 11.7%, wet et the respectively sults mean al. suggested of (2008) increases the in employed Tyndall futureincreases a Centre Mekong occurring distributed v2.03 discharges in scenario the up middle to set. of 2080 the Their with 21st re- the Century. maximum ulation, in particular in2003). the lower Furthermore, Mekong Basin themated (from as precipitation 55 generally elasticity to greater ofproportionately 90 than greater Mekong million zero, changes by meaning river in 2025, that river flow flow MRC changes has (Hapuarachchi in et been precipitation al., esti- result 2008). in pacts of climate changesuch understanding on can future water availability resourceMekong managers of (particularly River freshwater the Commission, resources. basin MRC) authority,ment the appropriate Only fully transboundary through evaluate management proposed strategies.adaptation The developments strategies need and for is climate imple- particularly change prescientriver for for the Mekong agriculture given andlarge the flood-prone reliance fish, areas, on the and the This the vulnerability situation relative of is absence likely the of to river low-lying be management exacerbated delta infrastructure. by region the projected including substantial increases in pop- and He, 2008). socio-economic development throughout theof region hydropower), (including and the thein increasing growing the (often influence Mekong, trans-boundary) there competition is for a water clear use need for improved understanding of the potential im- their influence on recent variations in flow volumes, sediment loadings and fisheries (Li already been controversial in terms of their downstream impacts with uncertainty over resources provided by theflow, river sediment load are and vulnerable waterper to quality (Costa-Cabral changes fisher et in have al., the 2008). beencertainty seasonality Indeed, declining as fish of to over catches river whether time(Hapuarachchi this et (MRC, is, al., 2003), at 2008).draulic although least interventions there in The with is part, Mekong aconstructed some is focus due on on also un- to hydropower. the being increasing Two Chinese impacted numbersfurther large section dams by dams of are of have large-scale fishers either already the hy- under2001; been river construction Li or (at and planned Manwan throughout He, and the 2008; river Dachaoschan) basin Stone, and (Kite, 2010). The two aforementioned Chinese dams have supports unique and varied ecosystems,and with diverse a fisheries. number This oferies) is endemic provides important species the because and the staple large al., Mekong diet 2008). (in for part, approximately Particularly through 300 productivewetlands, its million areas as fish- people include well the (Hapuarachchi as Mekong et the Delta Tonle and Sap its lake associated in Cambodia. However, fisheries and other Panel on Climate Changeal., (IPCC) 2008). suggest At future leastannual increases 80% precipitation in of across precipitation the Southeast (Bates GCMsprecipitation Asia, used together et events in with and increases this theevents in IPCC (Christensen magnitude the assessment et intensity show and al., of increased 2007). frequency of both extremebe wet particularly severe and for the dry source, Mekong providing River system, food, given water, transport its role and as livelihoods a (Kite, vital 2001). regional The re- Mekong also Changing availability of freshwater resourcesconsequences is of projected likely 21st to Century betial climate one for change, of sustainable critically the development a of most2010). life important Over and Southeast livelihoods (Bates Asia, et the al., most 2008; recent Todd projections et from al., the Intergovernmental 1 Introduction 5 5 25 15 20 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | C ◦ (Kiem et al., 2008). In- ff erent CMIP-3 GCMs (CCCMA ff , accessed June 2010), this study ad- gridded CRU TS 3.0 dataset (Mitchell ◦ 5 . . The wet season lasts from mid-May to 0 ff × ◦ 5996 5995 C) using one GCM, UKMO HadCM3, are also ◦ 5 erent GCMs often disagree even in the direc- . ff ). It is a major trans-boundary river, originating at over 5100 m a.s.l. in 2 ), 12th largest in length (4350 km), and 21st largest in drainage area 3 The vast majority of the basin experiences a monsoonal climate, with seasonal pre- The Mekong is initially fed by melting snow in the Tibetan Highlands, with the pre- Whilst useful and informative, these previous studies of climate change impacts days were initially obtainedand from Jones, the 2005), 0 as described in Todd et al. (2010). Monthly data for the 268 grid Annual minimum flows occur in March or April. 3 Data and methods 3.1 Data Baseline climate data for the hydrologicalmonthly model minimum of and the Mekong maximum River temperature, Basin comprising precipitation totals and number of wet October, and accounts fortotal over annual 90% precipitation of1000 ranges annual mm precipitation on from the in highs relatively manysub-basins). arid of Korat areas. 3200 plateau mm Peak in Overall, river Eastern inin Thailand flow parts September (i.e. at or the of Chi the October, Laos, and with head Mun to the of high under the flow delta season extending (Phnom from Penh) June–November. usually occurs originates from the Lancang sub-basin.upper The Mekong lower 1 Lancang, sub-basins Nam areThe Ou, dominated Nam Mekong by Ngum 1 forest and sub-basin (bothAgriculture is deciduous forms and the the evergreen). largest greatest contributorand land-use to type Mun annual in sub-basins Pakse the dischargeannual (which lower discharge) (39%). basin, together and particularly within contribute in the approximately delta. the 10% Chi ofcipitation Pakse the mean primary source of river runo dominant land cover in the uppermontane half of semi-desert. the Lancang sub-basin Although consistingBasin of snow tundra during and covers November–March only (andsubsequent approximately is melt 5% negligible has of at a thedeed, substantial other Mekong 34% impact times), of on mean snow Mekong annual storage discharge runo and at Pakse (the terminus of the Mekong 2 basin) China Sea from the manyVietnam distributaries (Fig. within 1). its delta which lies predominantly within 2 The Mekong River Basin The Mekong River475 km is the(795 000 km world’s eighththe largest Tibetan Highlands. inand discharge The in places Mekong (annual virtually subsequentlypassing unexplored discharge: flows through Lancang Burma, through Gorge Laos, the in Thailand China’s and narrow, Cambodia Yunnan steep, Province and before discharging into the South rise in global mean temperature,(Todd a et presumed threshold al., of 2010). “dangerous”global climate mean In change temperature addition, (from 0.5 theinvestigated. to hydrological 6 impacts of a progressive change in tion of changechange (Randall impact et studies al., considerble mean an 2007). climate ensemble change. ofhttp://www.met.reading.ac.uk/research/quest-gsi/ As For GCMs part this of without reason, the resortingdresses wider it to QUEST-GSI the project ensem- is (Todd important et essentialthe issue al., 2010; that Mekong of River GCM climate CGCM31, Basin uncertainty CSIRO with Mk30, by IPSL outputs driving CM4, MPIUKMO from a ECHAM5, HadGEM1). NCAR seven hydrological CCSM30, These di model UKMO GCMs HadCM3, of are driven by the policy relevant scenario of a 2 on Mekong river flowprojections have from generally a been singlethe limited GCM by use or their of by adoptiontively ensemble masking of consistent means. the future between GCMs, climate variationjections the between of Although same GCMs future GCM is through precipitation not simulated from true temperature di for can precipitation. be Indeed, rela- pro- 5 5 25 15 20 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Pro- ff C warming us- ◦ (Su, 2000), through 2 erent climatologies includ- ff 5998 5997 erent environments across the globe. These ff C using the UKMO HadCM3 GCM, and for a 2 ◦ The present study started with the same SLURP model, topographic, land cover The Mekong River Basin has previously been modelled using SLURP, for the period Future (monthly resolution) climate scenarios for temperature and precipitation were use of aKite stochastic (2001) weather study, the generator Mekong to Basin generate was only daily modelled climate as data. far as the As Pakse with gauging the studies undertaken within theand QUEST-GSI project Taylor, (e.g. 2010), Hughes a et baselineremaining al., period 1991–1998 2010; of data Kingston 1961–1990 used(initially) was for from used validation. the for monthly calibration, Thedescribed CRU with input in TS the climate Todd 3.0 et databased dataset al., were dataset. disaggregated 2010), derived to The rather daily SLURPfor than each resolution model the sub-basin, (as creates which relatively are spatial sparse thenon averages GSOD used a of to daily drive each daily station the climate time-step, model. variable Although results SLURP are operates only considered at a monthly resolution due to the simulation period without calibration“reasonably well” and at daily a number river of flow gauging was stations (Kite, shown 2001). and to soil be data, simulated model sub-basins run and for model the parameters much used longer by 1961–1998 Kite period. (2001), In but common with with the other modelling 1998; Woo et al., 2009). 1994–1998 (Kite, 2001).basins In (Fig. this 1) previousdigital based study, elevation model. on the Land the basin coverUSGS types United was 1 within km divided States each digital into sub-basin Geological landdata were 13 cover Survey derived from map sub- from (USGS) the of the the FAO GTOPO-30 original World world, Mekong Soil with SLURP soil Map parameters model (FAO,from 1990). generated consisted the using NCDC of The GSOD station-based climate dataset. data daily used meteorological The to data whole drive 1994–1998 the period was employed as the range from smallcatchments Canadian of wetland hundreds basins ofing of square studies kilometres less in with than very CanadaTurkey 1 (Apaydin di km (Armstrong et and al., Martz,to 2006) major 2008), and river South Germany basins Korea (Viney including (Kim et upper et tributaries al., al., of 2009), the 2007; Indus Park and et Yangtze al., (Jain et 2009), al., hydrological model that operatessuccessfully on employed a in daily a time range step. of di The SLURP model has been following the analyses described by Todd etferent al. modelled (2010) global to climate span futures amagnitude (e.g. range of of Indian Amazon “plausible” monsoon dieback). dif- weakening/strengthening, 3.2 The SLURP hydrological model The hydrological model usedRiver Basin to was investigate developed climatecesses using (SLURP, change v.12.7) model the impacts (Kite, Semi-distributed 1995). on This Land the is Use-based a Mekong physically Runo based semi-distributed cific thresholds of global climatewere change generated to here be for explored1.5, a (Todd 2, et prescribed 2.5, al., 3, warming 2010). 4, of 5,ing Scenarios and global six 6 mean additional GCMs: temperature of CCCMANCAR CGCM31, 0.5, CCSM30 CSIRO 1, and Mk30, IPSL UKMO CM4, HadGEM1. MPI ECHAM5, This subset of CMIP-3 GCMs was derived generated using the ClimGenborn pattern-scaling (2006) technique and developed Todd by etthe Arnell al. procedure and (2010), outlined Os- and above.et later downscaled al., ClimGen to 2000), is daily based apressed resolution on spatial as following the change scenario assumption per generator that unitfor the (e.g., a of spatial Hulme given global GCM pattern mean (Arnell of temperaturefrom and change, climate an Osborn, is change, individual 2006). relatively ex- GCM constant This to allows be the scaled pattern up- of and climate downwards change in magnitude, enabling spe- following the procedures developedet by al. Arnell (2010). (2003)local and Station-based calibration further daily of describedNational precipitation the by Climate and daily Todd Data temperature disaggregationmeteorological Centre data procedure) stations (NCDC) (the used were by basis global obtained Kite for surface (2001). from summary the of US the day (GSOD) cells which cover the river basin were stochastically disaggregated to daily resolution 5 5 25 15 20 10 25 20 10 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ), and culties 2 ffi cients). Initial minor ffi ective, with the model continu- ff ed Complex Evolution method of ffl of the whole Mekong Basin. 2 6000 5999 erent land covers (the retention constants and ff ectively at this temporal resolution. ff erent time period for which the model was being run and the ff ), Ubon (the Chi, Mun and Chi-Mun sub-basins, 122 390 km 2 rather than the total 795 000 km 2 resolution of the input climate data. Coupled with the likely high spatial variabil- ◦ erent climate data being used. This is in common with previous research that has Improvement in model performance following the adoption of a less data-intensive Following these initial calibration attempts, more substantial changes were made The calibration of the Mekong model was determined by the SLURP model structure Initial runs of the SLURP model for the 1961–1990 calibration period indicated that ff data used to construct the gridded CRU data, the representivity of gridded datasets reasons why this maywind be speed the and case. netare Firstly, radiation typically Penman-Monteith data less PET reliable in requiresand in humidity, addition a gridded to relatively datasets, temperature. limited due(e.g., number New in The of et part former al., data to 1999). points, variables Mekong measurement River The particularly Basin di second for (i.e. factor the the isa Lancang more latter series sub-catchment), specific of two where very to the variables narrow the river gorges.the upper passes 0.5 In through section places, of these the gorgesity are of substantially narrower local than climate over this complex terrain and the relatively poor coverage of station ments, including lowerobserved modelled annual discharge hydrograph, results and wereacceptability. a still considered better to match be beyond to the the bounds of shapePET of method the suggests that data quality may be an issue. There are two principal This is thought toat a be daily because time-step, autocalibrationdischarge and introduced within the by SLURP disconnect artificially can between generatingibration daily daily only routine weather temperature, from be data working precipitation prevents performed e the and autocal- to the model.the original Penman-Monteith The methodtemperature-based potential to Linacre evapotranspiration the method. less (PET) data-intensive routine Although and was more this empirical changed resulted in from substantial improve- ing to simulate monthlyobserved discharges records which at were the substantiallyscending three higher limbs gauging than of stations, those the particularlymodel of annual during autocalibration the flood the developed peak. at risingwithin The the and SLURP, Shu University de- but of application Arizona of (SCE-UA) this is method embedded failed to improve the model calibration. manual adjustment of SLURP parameters proved ine capacities of thebasins fast (evaporation, and Manning’s roughness slow and field soil capacity stores) coe and (ii) those which vary between sub- expected given the di di shown that hydrological modelsare may changed require from recalibration station-based when recordset meteorological to al., inputs gridded 2010). datasets (Mileham et al., 2008; Xu and was based onthe modifications soil to profile parameters which describing vary (i) between water di transport through from Lancang). it would be necessarya to satisfactory modify fit between existing modelled values andthe of original observed (Kite, discharge. model 2001) Modelled parameters parameter discharge values into was using too order and high to in from all gain months the whilst the high transition flow season was too gradual. The need for re-calibration was Data from three gauginglogical stations model: Chiang-Saen were (the available terminusarea for of of calibration the 228 Lancang 000 of km sub-basin, the withPakse an SLURP (the upstream entire hydro- modelled area,calibration i.e. was the undertaken terminus of sequentiallySaen the and from Ubon Mekong before 2 upstream Pakse), sub-basin). andPakse to was stations Model downstream particularly as focussed the (i.e. on combined Chiang- station) the Chi, Chiang-Saen contribute Mun and only and 10% Chi-mun of sub-basins the (i.e. Ubon mean gauging annual flow at Pakse (in comparison to 34% the many distributaries of the550 000 river’s km extensive delta. The modelled area was therefore 3.3 Calibration and validation of the SLURP hydrological model station in Laos (i.e. the terminus of the “Mekong 2” sub-basin) which is upstream of 5 5 25 15 20 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ¨ a C in ◦ ¨ erent astil ff 6002 6001 cient for Pakse varied by less than 0.05 between the ffi e coe ff cients (R2) for monthly discharge for the 1961–1990 calibration ffi e coe ff C scenario), with slightly weaker warming occurring in the May–October period ◦ The implications of the disaggregation of monthly data to a daily time-step were in- The performance of the model varies little between the calibration and validation Further manual adjustment of model parameters was undertaken following the guide- Initial results using the UDel precipitation dataset produced a marked improvement The greatest warming inthe all 2 basins occurs from November–April (e.g. 2.5 to 3.5 4 Scenario results: prescribed warming using4.1 HadCM3 Changes in climate Changes in temperature associatedature with using prescribed the warming HadCM3 ofbasins GCM global of are mean the relatively temper- Mekong, uniformchanges. with across only the the Temperatures seven Lancang increase modelled sub-basin sub- linearly experiencing with slightly di increasing global mean temperature. month. The Nash-Sutcli original run and themean mean flows were of obtained the suggestinggregation ten that procedure. the subsequent model runs. is not Very very sensitive similar to 30-year the monthly disag- periods at Pakse (Fig. 2a).model and Similarly, observations little occurs for change Chiang-Saen inwith and the the Ubon caveat (Fig. correspondence that 2b between observed and data the for c, is respectively) the only latter available gauging up station. to 1997 for the former, andvestigated 1993 by running the disaggregationtivity of procedure the ten hydrological times model to to determine the the random sensi- sequencing of rainfall events within each is likely to reflect thedescending limbs aforementioned of discrepancies the in annual hydrograph. thefavourably with The simulation previously simulated of Pakse published the discharge models compares risingdene of and and the Mahanama, Mekong (e.g., 2002; Kite,et Hapuarachchi 2001; al., et Jayawar- 2010). al., 2008; Ishidaira et al., 2008; V for Chiang-Saen “very good”. The relatively low value for Ubon (classified as “poor”) fication scheme of Henriksen et al. (2008) the R2 value for Pakse is “excellent” and that discharge was still slightly underestimatedslightly during overestimated during the the peak transition months. andparameter low It adjustment was flow not to seasons, possible further and fromflow reasonable increase seasons modelled without river flow alsoseasons. during increasing the A the high good overestimation and fitpeak of low was and flow also low during obtained seasonrising the for discharges and transition Chiang-Saen descending were (Figs. limb successfully 2b dischargesNash-Sutcli captured and were generally 3b). for too Ubon, Although highperiod as (Figs. are 2c at 0.89 (Pakse), and Pakse 0.78 3c). (Chiang-Saen) The and 0.44 (Ubon). According to the classi- some rainfall events mayBasin, be especially in erroneously the introduced narrow and to topographically or complex Lancang omitted section. lines provided from by Kite the (2008). Mekong UDel-drivenwas simulation of bought Pakse mean substantially monthly closer discharge to the observed values (Figs. 2a and 3a). However, in model performance compared tothough those this based is on initially the surprisingthan original UDel), (the CRU these CRU TS results database 3.0 follow containsreason data. previous more for findings station Al- this (Hughes data apparent et points more contradiction al., detail 2010). is in that One regional the possible precipitationrelevant for CRU than the UDel, dataset Mekong but intermittently River thatmore, Basin captures as not (i.e. a all occurs result of beyond of the this aggregating basin and regional smoothing boundary). detail these is Further- data to create a gridded product, cipitation, which can exhibitterrain. high spatial In variability light evenalternative of over precipitation the dataset, relatively the homogeneous potential University of(UDel) poor Delaware based representivity global precipitation on of dataset Legates themodel and CRU performance was precipitation Wilmott due, (1990), data, in was part, an to used the to precipitation investigate data whether used to poor drive the model. is liable to be poor in such areas. This is likely to be particularly prescient for pre- 5 5 25 15 20 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ff ff at ff in the C sce- ◦ ff as global mean ff C), and increases ◦ C scenario. Although C for November–April ◦ ◦ C scenario, but with in- ◦ 40.1% for the 6 + C scenario, Lancang: 10%; ◦ C scenario). Increases of ap- ◦ at Pakse are projected under in- C; 20% for 6 C scenario) occur for both basins ◦ C, and ◦ ff ◦ 16.0% for the 6 C but more substantial changes occur C). ◦ − ◦ C scenario whereas the largest arises in the 6004 6003 26.7%, respectively (Table 1). Large changes ◦ 55.6% for 4 + + 2.4% and + C; ◦ 15.3%) scenarios. Increasing annual runo C; 60% for 6 C for May–October. ◦ ◦ + C scenario, 40% for the 6 1%) for most sub-basins, with the exception of the three ◦ C( ≤ ◦ 11.4% and C scenario). The Lancang sub-basin has slightly more con- C scenario (60% for the 6 ◦ ◦ − C scenario is somewhat of an outlier with a 4.5% increase in ◦ 27.0% for 2 + from the 1961–1990 baseline. ff C) and July–August (up to 8% for 2 1.4%) to 6 ◦ C for the 2 ◦ + C using the HadCM3 GCM generally show small decreases in annual runo ◦ C scenario, with changes of ◦ C scenario. The 4 ◦ erent directions, with low flows generally increasing, and high flows generally de- C; 70% for 6 Further explanation for the absence of a progressive linear trend in annual runo Relatively small changes in the annual mean runo On an annual basis, the HadCM3 precipitation climate change signal (relative to ff ◦ river flow climate change(by signal holding into temperature that attributable constant to and temperature varying and precipitation, precipitation and vice versa) (Fig. 5). between snow storage andthroughout release the in basin. thebasin In upper contrast (Chiang-Saen Mekong to gauging Basin thefrom station) and overall the shows increasing Mekong PET response, a 0.5 theLancang near-linear ( sub-basin Lancang increase is sub- in drivencounter-balanced by annual by increasing runo decreasing early peak andincreases late season at season discharge Lancang discharge, (Fig. are although 4b).enhanced thought snow-melt The to earlier early be in season a the result year. of higher This temperatures is and, demonstrated in by turn, the division of the of these contrasting trends andplanation their for impacts the on absence of monthlytemperature a river increases. progressive flow linear provides trend partial in ex- annual runo can be provided by considering the role of temperature, and specifically the balance also occur in monthly discharge (Fig.months 4a). of River annual flow peak during August flow)the and decreases 2 September on (the average bydischarge 0.2 decreases and 9.8% in (respectively) mostand for months, occur the in largest June monthlynario). ( changes These involve contrasting increases trendsfor result sub-basins because in whilst all increases months infrom of temperature sub-basin the occur year, to changes sub-basin in and precipitation vary month in to direction month (both within sub-basins). The interaction 1.5 mean annual runo creases in global mean temperatureat of Q5 up and to Q95, 6 reaching smallest annual change occurs in the 6 creasing. Reductions from the5.4%, Pakse but baseline with mean little annual apparent flow link vary to between 0.2 the to magnitude of global temperature change; the 4.2 Changes in river flow Results from the scenarios0.5 of to 6 prescribed increases inPakse with global increasing global mean mean temperature temperaturein (Table from 1). temperature However, and unlike precipitation, the changes modificationsfor to river either flow the do mean not5% discharge occur at or and a the 95% linear Q5 rate ofdi and the Q95 flows time, (i.e. respectively). the discharges Furthermore, exceeded high and low flows change in in February, May, September and October.intra-annual The signal, other with sub-basins most show showing a decreases2 more from variable October–April (by up tofrom 50% May–June for (up to 17% for 2 the UDel baseline) is smallnortherly ( sub-basins, all of which showNam increases Ou: (for the 11%; 2 Nam Ngum:variable, but 5%). demonstrates In linear contrast, ratesature. the of monthly change precipitation with The signal increasing two is globalprecipitation highly mean most for temper- nearly northerly all sub-basins,(up months; to Lancang April 16% is and decrease for the Nam theproximately only 20% 2 Ou, month for show the showing 2 notable increasing decreases sistent year-round warming but a similarInter-seasonal overall and magnitude inter-basin of patterns increasing are temperatures. creased the magnitude, same such that for warming theand is 6 varies between between 7.5 6.0 and 10.5 and 7.5 (e.g. 2.0 to 2.5 5 5 25 15 20 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | erent GCMs show contrast- ff erences in scenario precipitation that are ff 6006 6005 erence in the temperature signal over the Lancang sub- ff erences between GCMs also occur in the Lancang sub-basin C warming across seven GCMs ff ◦ C, but with variation between GCMs in the monthly patterns of ◦ 5%) increases in Chi-Mun and Mekong 2 precipitation. Inter-seasonal . 1 < C prescribed warming scenarios from the seven di ◦ erences in the precipitation climate change signal between GCMs are far greater ff C scenario is therefore thought to be the result of the combination of increasing Results from running the model with scenario precipitation and baseline temperature Di Increasing river flow in the Lancang sub-basin is likely to be responsible for the non- ◦ (and vice versa) show that it is inter-GCM di with little consistency in eithersonal the mean magnitude discharge, or or direction of highclustering change, and of for GCMs low annual so flows or it (Tablecially sea- 2, is given Fig. not that 6a). possible these to ThereGCMs seven label is GCMs any (Meehl no GCM are et particular as drawn al., aever, from whilst particular 2007). substantial a outlier, di larger espe- The population(Chiang-Saen same of gauging 23 is station), CMIP-3 true resultsfrom for for April–June, all Ubon and seven (Chi-Mun decreasing GCMs flow discharge). show in increasing July How- river and August flow (Fig. 6b). (with peak increases aroundDecember–January: April CCCMA, NCAR, and HadCM3, September, HadGEM1, and MPI). decreases in5.2 June–July and Changes in river flow Projected changes in Pakse discharge show substantial disparities between GCMs northerly sub-basins in theshows HadCM3 increases scenarios of was 6% noted anddecreasing previously. 1% annual HadGEM1 for precipitation the for also the Lancang otherbasins). and basins Nam In (peaking Ou at contrast basins, 5% to respectively,tation in but the for the two the Chi Hadley three and northerly Mun Centrewith basins, GCMs, small peaking IPSL at ( shows 5% decreasing forpatterns precipi- Lancang of and change Nam range Ou, frompeak together unimodal increase (maximum in decreases September: in January–March CSIRO, and IPSL) to bimodal patterns of varying strengths entire Mekong Basin, theprecipitation CCCMA, HadCM3, (by MPI between and 3–10%)show NCAR whereas decreases GCMs show the in increasing CSIRO, precipitationing) HadGEM1 annual of precipitation and 3, is IPSLNCAR 2 consistent (CSIRO) GCMs across and GCMs. all For 1%, the sub-basins other respectively. for GCMs, the the increase Increasing CCCMA, in (decreas- annual MPI precipitation and for the of change, or the level of similarity between sub-basins. At the annual level for the than for temperature, with little consistency in the magnitude, direction or seasonality a relatively constant temperaturesub-basins; climate the change CSIRO signal and throughout MPIin February GCMs the whereas have year the a for IPSL distinctJune. GCM most peak has in a Also broad April of peak and notebasin in the temperature compared is HadCM3 rise to the from the March– di ture other increase sub-basins. in For the allbetween Lancang GCMs sub-basin January except is HadCM3 and much the Aprilthan greater tempera- for (for than other for HadCM3 basins the during the other these Lancang four sub-basins months). temperature signal is weaker The 2 ing changes in climatecreases over of the close Mekong to Basin.rising 2 temperatures. For temperature, For all example, GCMs the show CCCMA, in- HadGEM1 and NCAR GCMs show 4 Lancang discharge (from thesonal higher changes in rain:snow precipitation ratio acrossincreasing and the PET Mekong greater throughout Basin snowmelt) the against basin. the and counterbalance sea- of 5 Scenario results: 2 5.1 Changes in climate son river flow increases, indicating therain: role snow of ratio. enhanced snowmelt and/or an increasing linear response of Pakse rivertemperature flow to only increasing climate temperatures change (seen also signal: in the Fig. Pakse 5a). The peak in the combined Pakse This shows that with increasing temperature and unchanged precipitation, early sea- 5 5 25 15 20 10 20 25 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | C sce- erence ◦ erences ff ff cients for evapora- ffi C prescribed warming was ◦ cient of the river channel. ffi erence between the reference and perturbed 6008 6007 ff 10% from the calibrated value and the model re- C prescribed warming scenario only. Di 2%, with the most sensitive parameters relating to ◦ ± ± erences in GCM precipitation (Fig. 8). However, it should be noted that these find- Despite the substantial uncertainty associated with the choice of GCM, a number of Each parameter was varied by Results of the uncertainty analysis indicate that model parameterisation generally ff can also be expected to lead to increased evaporation throughout the basin. Mekong, and of more generalon relevance water to the resources. assessmentriver For of flow climate example, is change results far impacts show more2010). that consistent As the than such, the GCM some precipitation temperatureMekong confidence signal Basin signal can (see will for Kingston be increase and placed ining Taylor, the in of first the snow half finding of and thatexamined. the ice flows calendar in since The year the this due importance upper to resultbeen of enhanced is demonstrated snow melt- for consistent accumulation the across and Mekong all melt Basin GCMs (Kiem dynamics and et has al., all previously 2005). scenarios Increasing temperature availability of freshwater resources. Single-GCM evaluationsare of climate therefore change likely impacts to beclimate wholly change inadequate impacts (and studies for potentially this misleading) major as river a basin. basisadditional for important issues are raised by these results which are both specific to the 7 Discussion This paper has presented anwithin assessment the of Mekong future Basin availabilityof of as freshwater the a resources result range ofGCM of climate and uncertainty change, hydrological model in combinedof parameterisation. with this previous an Our assessment studies evaluation results of duewe are the to show comparable Mekong climate to the (e.g., those Kiem sensitivity, overwhelming et choice dependence al., on of 2008; the Ishidaira GCM et used al., for 2008) but projections of future ings are based onin the the HadCM3 reference-perturbed percent 2 anomalynario between runs baseline are and generally less HadCM3soil than 2 water capacity and the Manning’s roughness coe di imparts little uncertainty to the climate change projections relative to that generated by run with baseline climateparameter set data. with scenario Theused climate model as data was an (the then exemplar HadCM3 scenario).runs run 2 was The using then di the comparedbetween same between the reference perturbed baseline and and perturbed runs scenariothen is model situations. greater parameterisation for the If may scenario beprojections the than a (and di the vice cause baseline, versa). of further uncertainty in climate change the most sensitive parameters inlected based the on hydrological the model. resultstivity Seven from rankings parameters initial provided were manual by Kite se- modelconstants (2008). calibration and and The parameters capacities parameter investigated sensi- of were thetion, the fast field retention and capacity and slow Manning’s soil roughness. stores, and coe tation driven trend. Asimportance with of snowmelt the and HadCM3 the results, snow: these rain results ratio of demonstrate precipitation the for likely Lancang river6 flow. Uncertainty in model parameterisation In the absence of quantitativeterisation estimates from of uncertainty an associated autocalibration withan routine, model a indication parame- manual of assessment model was parameterisation made uncertainty. to provide This was undertaken by varying discharge (Fig. 7a andcharge b). is The very temperature-only consistentclimate climate between change all signal signal seven shows in both GCMs. mean increasesilar and monthly In results decreases contrast, dis- are in the found monthly for discharge. precipitation-only ing the Sim- Lancang that (Chiang-Saen) the sub-basin (Fig. April–June 7c rising and d), trend indicat- in river flow is a temperature rather than precipi- the primary cause of variation in the overall climate change signal in Mekong (Pakse) 5 5 25 15 20 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | erent ff erent PET erences in ff ff erent sections of ff C here), and secondly, the ◦ erent climate change signals, ff erent climate trends and changes ff erent GCMs emphasise the need for multi- 6010 6009 ff cult to ascribe simple attributions to downstream ffi erent hydrological processes in di erent methods of estimating historical PET from me- ff ff Despite such uncertainty, it has been demonstrated that useful information can still Whilst further uncertainty in the climate change signal for the Mekong River Basin is Results of this study demonstrate the importance of understanding the roles and The results also demonstrate that when averaged over large areas such as the ture precipitation projections. It is likely that such changes (particularly in high and even in a region highlightedconsistent by precipitation the IPCC climate 4th change Assessment signal. Report as having a relatively be obtained, for example bychanging focussing on temperature, future changes asMekong in temperature discharge River associated is Basin with consistently byjections of all simulated earlier seven to and GCMs. reducedupper rise magnitude Mekong Accordingly, Basin snowmelt-related across are this seasonal the robust flow study even peak in has in the the indicated presence pro- of substantial uncertainty in fu- pacts on the hydrology ofbeen the shown Mekong that River projections Basin. ofupon Firstly, and hydrological the most change direction importantly, in it of theprecipitation has future basin projections are variation highly in produced dependent precipitation. bymodel di evaluations The of considerable di climate change impacts. It is notable that this is still the case jective) assessment of modelbe parameter taken uncertainty. as These indicative rather findingstainty than should in definitive. therefore a Future work moreroutines). will objective aim probabilistic to manner treat model (for uncer- example, by using autocalibration 8 Conclusions A number of important findings have resulted from this study of climate change im- likely to arise from thethe findings parameterisation presented and here structure indicate of thatsociated such the with uncertainty hydrological choice is model of much used, GCM, smallerDespite climate than this, that sensitivity, it and as- should possibly observed be baseline noted data. that this paper has only conducted an initial (and sub- suggesting that this is an area for further research. methods of estimating PET can produce markedly di for river flow. Thisthe is continental-scale further Mekong River complicated Basin.in by Although the the the hydrological time role behaviour taken of ofgarding PET for the estimation is water Mekong, of key there to to both remains pass changes and baseline substantial through and disadvantages uncertainty scenario of re- PET. many Whilstteorological di the data relative have advantages beenal., 2005), widely relatively considered little (e.g., attentionmethods has Vorosmarty remain been et when given al., transferred tocent from how 1998; work representative baseline (Kingston Lu di to et et scenario al., climatology. 2009; Kingston Indeed, and re- Taylor, 2010) has shown that di trends in discharge. interaction of changes instorage of temperature precipitation and as thesonality snow of implications or precipitation, ice. of these temperature this Together driven with changes for have changing both important magnitudes implications PET and and sea- further important issues canclimate also change be highlighted: impacts firstly, onimportance the of water potential investigating resources for changing thresholds water (possiblya resources of 4 combination on of an linear intra-annualannual basis changes response. as in In it monthly is largether river basins complication flow that is that cross added climatic givein zones, by rise the such the to relative as possibility the importance thethe non-linear of basin. Mekong, of di fur- These di factors make it di Mekong or evenrespond its in sub-basins, a neither linearIshidaira way high, et to al. low, increasing (2008). temperatures. nor Thisincreased mean This is temperature thought concurs annual across to with the river be theevapotranspiration), a Mekong complicated flow findings consequence Basin by of of may seasonally (earlier contrasting variable response snowmelt changes to versus in increased precipitation. Two 5 5 25 15 20 10 25 20 10 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ¨ otter, C., and Snid- : a global perspective, ff 6012 6011 ects of reduced land cover detail on hydrological model ff This work was supported by a grant from the UK Natural and Environ- ects of IPCC SRES* emissions scenarios on river runo ff in a part of Satluj catchment, India, Hydrolog. Sci. J., 43(6), 875–884, 1998. ff Japan Meteorological Agency (JMA) AGCM, Hydrol. Process., 22, 1382–1394,sources 2008. in Yongdam Dam Basin, Korea, Stoch. Env. Res. Riskdischarge A., 21(4), and 355–373, groundwater 2007. Hydrol. in Earth a Syst. Sci., headwater 14, catchment 1297–1308, of doi:10.5194/hess-14-1297-2010, 2010. the Upper Nile Basin, Uganda, 2005. Future hydroclimatology of the Mekong River basin simulated useing the high-resolution to Mekong and Chao Phraya basins, J. Hydrol. Eng., 7,tionship 12–26, between 2002. ENSOin: and snow Regional Hydrological covered Impacts areaFranks, of S. in Climate W., the Change Wagener, – Mekong T., Hydroclimatic and Bøgh, Variability, Yellow edited E., River by: et basins, al., IAHS Publ. 296, Wallingford, UK, 255–264, cover and its hydrologicalProcess., 22, impact 1395–1405, in 2008. the Mekong River Basinruno in the 21st Century, Hydrol. vongs, A.: Landscape structure andbasin, Hydrol. use, Process., climate, 22, and 1731–1764, water 2008. movement in the MekongWorld: River Revised Legend, World Soil Resources Report 60, FAO, Rome, Italy, 1990. the Okavango River Basin7, as 5737–5768, a doi:10.5194/hessd-7-5737-2010, result 2010. of climate change, Hydrol. EarthChipanshi, Syst. A. Sci. C.: Discuss., Usingsessments: a MAGICC Climate and Scenario SCENGENwich Generator Version UK, for 2.4 52 Vulnerability Workbook, pp., and 2000. Climatic Adaptation Research As- Unit, Nor- mate Change 2007:Fourth The Assessment Physical Report Science ofSolomon, Basis. S., the Contribution Qin, Intergovernmental of D., Panel Manning, Working M., on Group et Climate al., 1 Change, Cambridge to edited University the Press, by: UK, 849–926, 2007. groundwater resources of Denmark bygroundwater use – of surface ensemble water resource model, indicators J. and Hydrol., a numerical 348, 224–240, 2008 ter, Technical Paper of theGeneva, Switzerland, Intergovernmental 2008. Panel on Climate Change, IPCC Secretariat, and Ishidaira, H.:the Investigation YHyM, of Hydrol. the Process., 22, Mekong 1246–1256, River 2008. basin hydrology for 1980–2000 using modelling, Tyndall Centre for Climate Change Research Technical Report 52, 2006. of a hydrological model, Ecol. Model., 195(3–4), 307–317, 2006. response, Hydrol. Process., 22, 2395–2409, 2008. Hydrol. Earth Syst. Sci., 7, 619–641, doi:10.5194/hess-7-619-2003, 2003. Kingston, D. G., Todd, M. C., Taylor, R. G., and Thompson, J. R.: Uncertainty in the Kim, B. S., Kim, H. S., Seoh,Kingston, B. D. 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Kite, G.: The SLURP model, in: Computer Models of Watershed Hydrology, edited by: 5 5 25 20 30 15 10 20 25 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | C ◦ 1.9 3.0 13.4 16.2 − − 0.6 − − − 1.6 0.4 9.5 4.71.6 3.2 5.4 0.4 2.6 10.4 8.9 2.00.2 26.2 26.7 3.70.7 2.8 17.8 10.2 − − − − − − − − − − − − 7.6 5.1 8.6 7.6 4.49.9 4.5 24.0 7.7 3.6 6016 6015 18.0 18.1 6.5 11.3 11.3 11.4 − − − − − − − − − − − − − C C C C C C C C C ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ 2.0 2.5 3.0 4.0 5.0 6.0 1.5 Scenario0.5 Q51.0 Mean Q95 GCMCCCMACSIRO Q5 1.8 Mean 5.7 Q95 8.0 HadCM3 HadGEM1 IPSL MPINCAR 5.9 6.3 3.0 5.9 Percent change in Pakse (Mekong 2) annual mean, Q5 and Q95 discharges for Percent change in Pakse (Mekong 2) annual mean, Q5 and Q95 discharges for 2 C increases in global mean temperature using HadCM3. ◦ Table 2. increase in global mean temperature using seven GCMs. 0.5–6 Table 1. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | (b) 1

2 Observed Simulated Observed Simulated Observed Simulated Mekong at Pakse (Mekong 2), (a) Chi-Mun sub-catchment at Ubon (note varying 6018 6017 (c) 1961 1963 1965 1967 1969 1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1961 1963 1965 1967 1969 1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1961 1963 1965 1967 1969 1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 7 6 5 4 3 2 1 0 5 0 8 6 4 2 0

45 40 35 30 25 20 15 10 16 14 12 10

Discharge (10 Discharge Discharge (10 Discharge ) s m (10 Discharge ) s m ) s m

-1 3 3 -1 3 3 -1 3 3 (b) (c) (a)

Observed and simulated monthly discharges: The Mekong River Basin and sub-basins deﬁned by Kite (2001). Note: only the sub- -axis scales). Fig. 2. Lancang sub-catchment at Chiang-Saen, y basins modelled in the current study are labelled. Fig. 1. 4

Mekong at Pakse (a)

mean monthly discharge of (b) Chi-Mun sub-catchment at Ubon (c) C increases in global mean temperature: ◦ 6020 6019 Observed Simulated Observed Simulated Observed Simulated JFMAMJJASOND JFMAMJJASOND JFMAMJJASOND 9 8 7 6 5 4 3 2 1 0 5 0

30 25 20 15 10

3.0 2.5 2.0 1.5 1.0 0.5 0.0

) s m (10 Discharge ) s m (10 Discharge ) s m (10 Discharge

-1 3 3 -1 3 3 -1 3 3 (b) (c) (a)

1.0 °C 1.0 °C 1.5 °C 2.0 °C 2.5 °C 3.0 °C 4.0 °C 5.0 °C 6.0 Baseline °C 0.5 Baseline °C 0.5 °C 1.0 °C 1.5 °C 2.0 °C 2.5 °C 3.0 °C 4.0 °C 5.0 °C 6.0 J FMAMJ J ASOND J FMAMJ J ASOND Lancang sub-catchment at Chiang-Saen, 9 8 7 6 5 4 3 2 1 0 5 0 -axis scales). 30 25 20 15 10

(b)

) s m (10 Discharge ) s m (10 Discharge

-1 3 3 -1 3 3 (b) (a)

HadCM3 climate change signal for 0.5–6 Observed and simulated mean monthly discharges (1961–1990): mean monthly discharge ofthe the Lancang Mekong sub-catchment at at Pakse Chiang-Saen. (Mekong 2), Fig. 4. (Mekong 2), (note varying Fig. 3. 6

mean monthly discharge of the 2.0 °C 6.0 °C precipitation only; and Lancang at (a) (b) JFMAMJJASOND JFMAMJJASOND 8 6 4 2 0

5 0

C increases in global mean temperature for

1.5 °C 5.0 °C 10

30 25 20 15 10 ◦

) s m (10 Discharge ) s m (10 Discharge

-1 3 3 -1 3 3 (b) (d) 6022 6021 precipitation only. temperature only, 1.0 °C 4.0 °C (d) mean monthly discharge of the Lancang sub-catchment at (a) (b) R R 0.5 °C 3.0 °C HadGEM1 IPSL MPI NCA Baseline CCCMA CSIRO HadCM3 HadGEM1 IPSL MPI NCA Baseline CCCMA CSIRO HadCM3 J FMAMJ J ASOND J FMAMJ J ASOND temperature only, JFMAMJJASOND JFMAMJJASOND (c) Baseline 2.5 °C 9 8 7 6 5 4 3 2 1 0 5 0 30 25 20 15 10

8 6 4 2 0 5 0

10 30 25 20 15 10

Discharge (10 Discharge ) s m (10 Discharge ) s m

-1 3 3 -1 3 3

m (10 Discharge ) s ) s m Discharge (10 Discharge C climate change signal across seven GCMs:

-1 3 3 3 3 (c) -1 (a) ◦

(b) (a) 2 HadCM3 climate change signal for 0.5–6

Chiang-Saen. Fig. 6. Mekong at Pakse (Mekong 2), Fig. 5. the Mekong at Pakse (Mekong 2): Chiang-Saen: 8

7 temperature only, (c)

JFMAMJJASOND JFMAMJJASOND C scenario: maximum extent of the dis- ◦ HadCM3 NCAR

8 6 4 2 0 5 0

30 25 20 15 10 10

) s m (10 Discharge ) s m (10 Discharge

-1 3 3 -1 3 3 (d) (b)

6024 6023 erence in the perturbed parameter versus reference ﬀ CSIRO MPI CCCMA IPSL precipitation only; and Lancang at Chiang-Saen: J FMAMJ J ASOND (b) 2.5 2.0 1.5 1.0 0.5 0.0

-0.5 -1.0 -1.5 -2.0 -2.5

JFMAMJJASOND JFMAMJJASOND

Baseline HadGEM1 (%) runs model perturbed

5 0 8 6 4 2 0 and reference between difference

10 30 25 20 15 10

) s m (10 Discharge Discharge (10 Discharge ) s m

-1 3 3 -1 3 3 C climate change signal across seven GCMs for the Mekong at Pakse (Mekong 2): scenario-baseline in Disparity ◦ (c) (a)

Model parameter uncertainty for HadCM3 2 2

precipitation only. temperature only, model runs. parity between the scenario-baseline di Fig. 8. (d) Fig. 7. (a)