Climate Change Impacts on Glacier Hydrology andRiver Discharge in the Hindu Kush–HimalayasA Synthesis of the Scientific Basis

James D. Miller1*, Walter W. Immerzeel2, and Gwyn Rees1

*Corresponding author: millj@ceh.ac.uk1 Centre for Ecology and Hydrology, Wallingford, Oxford, United Kingdom, OX108BB2Department of Physical Geography, Utrecht University, Utrecht, The Netherlands

Open access article: please credit the authors and the full source.

Rising temperatures and changing

precipitation patterns across the Hindu

Kush–Himalaya (HKH) region resulting

from climate change have an influence

on water resource availability and food

security for the downstream population.

This review seeks to objectively assess

the available evidence of the impacts of

climate change on glacier hydrology and

the wider implications upon water

resources within the Indus, Ganges, and

Brahmaputra basins. Glacier meltwater

contribution to river flows is scale

dependent and varies considerably

across the east–west climatic zones of

the HKH. For the Ganges and

Brahmaputra this contribution is

estimated to be significantly less than

for the Indus to the west, with summer

monsoon rains dominating flows from

central and easterly areas, whereas

meltwater remains a significant

contributor to downstream flow of

westerly basins, which receive most

precipitation during winter. No

corroborated trends exist in observed

discharge for any basin, and such

analyses are hindered by a lack of good-

quality long-term data. Predicted

increases in temperature will drive

increased shrinkage of glaciers, leading

to initial increases in meltwater

produced, followed by subsequent

declines with reduced glacier mass. The

impacts of such changes are predicted

to be minimal for the overall discharge

of the Ganges and Brahmaputra, where

increases in rainfall may in fact lead to

increased flows but with greater

variability. Within the Indus basin,

reduced meltwater will have significant

impacts upon available runoff; however,

increased uncertainties surrounding

precipitation and socioeconomic

changes limit any conclusive

assessment of how water availability

will be affected; moreover, seasonality

of runoff may be a more important

factor. Scientific challenges and

research recommendations are

identified for the region. This review

proposes the need for the scientific

evidence pertaining to the region’s

glacier systems to be approached

objectively in the future, such that a

robust assessment of change can be

attained.

Keywords: Glacier; climate change;

hydrology; water; Himalaya.

Reviewed by the Editors:August 2012

Accepted: September 2012

Introduction

Mountains are often referred to as the‘‘water towers’’ of the world (Messerliand Ives 1997; Viviroli andWeingartner 2004) and many suchhigh-altitude areas store much of thiswater as snow and/or ice. Heavilypopulated regions adjacent to theHimalayan mountain ranges areconsidered particularly vulnerable tothe impacts of glacier retreat (Barnettet al 2005). Under climatic warming,increased melt can cause shrinkage inthe overall glacier mass, providingshort-term annual increases inmeltwater contribution todownstream river flows. As theproportion of ice cover within acatchment decreases, the variability ofriver flows both between and withinyears will also increase, as catchment

runoff is increasingly derived fromcontemporary precipitation (Reesand Collins 2006) and less from glaciermeltwater stores.

In total, 10 major river systemsfind their sources in themountainous Hindu Kush–Himalaya(HKH) region (Figure 1); 3 of these(Ganges, Indus, and Brahmaputra)are the focus of the present review.These mountainous regions of theHimalaya, Karakoram, and HinduKush that constitute the high-altitudesources of these 3 river systems willherein be referred to as theHimalayan region. This represents adiscrete hydrological focus within thewider mountainous region referredto as the HKH, illustrated in Figure 1.

This paper provides an objectivesynthesis of the evidencesurrounding observed and modeledchanges to the runoff derived fromsnow and glacier melt, along withchanging climate. Such an assessmentinforms upon the potential impactsof climate change on the waterresources of these major river basinsand identifies the most pressingknowledge gaps and associated futureresearch priorities.

Regional characteristics

The Himalayan region ischaracterized by a diversity ofclimate that causes Himalayan glaciermass balance and river flow to varyconsiderably from west to east. Themonsoon weakens from east to west,rarely penetrating as far as theKarakoram, so that summerprecipitation declines in the samedirection. The annual precipitation

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gradient ranges from over 3000 mmin the east to less than 300 mm in themore arid west (Immerzeel et al2009). Catchments in the eastern andcentral Himalaya receive more than70% of their annual rainfall in thesummer monsoon period, whereascatchments to the west of the Sutlejbasin receive more precipitation inwinter from westerlies (Bookhagenand Burbank 2010). Glaciers, thus,experience winter accumulation andsummer ablation in the west, butpredominantly synchronous summeraccumulation and summer melt inthe east (Rees and Collins 2006).

Glaciers occupy an estimated areaof 60,000 km2 within the HKH(Bajracharya and Shrestha 2011) andapproximately 44,200 km2 within thegeographical region covered in thisreview. There has been muchdiscussion recently about the stateand fate of glaciers in high mountainAsia (HMA) (Scherler et al 2011;

Bolch et al 2012; Gardelle et al 2012;Jacob et al 2012; Kaab et al 2012).Recent reviews of all available fieldand remote sensing studies thatquantify changes in glacier extentand mass balance in the Himalayasand the Karakoram (Miller et al 2011;Bolch et al 2012) indicate thatglaciers have generally been losingmass, except in the Karakorammountain range where there areindications of mass gain. In anotherstudy (Jacob et al 2012), data from theGravity Recovery and ClimateExperiment (GRACE) satellitemission were used to assess changesin ice volume. They estimate thatduring the period 2003–2007 the icemass loss over HMAwas 4 6 20 Gt y21,roughly a factor of 10 fewer thanestimated by Bolch et al (2012). Arecent study (Kaab et al 2012) basedon Ice, Cloud and Land ElevationSatellite (ICESat) laser altimetryprovides yet another estimate of

12.8 6 3.5 Gt y21 ice mass loss, withlarge variation in the HMA betweenlocations. Determining regionalchange from such remote observationsand over short time spans has evidentlimitations and neglects the regionalvariability that can occur, such asregional anomalies found in theKarakoram (Hewitt 2011).

Changes in catchment glacio-

hydrological responses/

systems

For each of the 3 major catchmentsof the Ganges, Brahmaputra, andIndus, both the observed and model-based evidence are considered inturn to provide a systematic overviewof the variability in glacio-hydrological response. This reviewfocuses primarily upon glacier meltcontributions to river flow, as lack ofinformation on snowmelt limitssubregional assessment.

FIGURE 1 River basins formed in the high mountain HKH region, highlighting the Ganges, Indus, and Brahmaputra basins that are the focus ofthe present study. (Map by Hammad Gilani, courtesy of ICIMOD)

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Ganges Basin

Analysis of existing hydrological andglacier data with models developedfor Nepalese catchments by Alfordand Armstrong (2010) indicatesvariable glacier melt contributions tostream flow between basins, rangingfrom 2 to 20%, with an average ofaround 10%, representing 4% of themean annual estimated outflow ofNepalese rivers. Immerzeel et al(2010) similarly find that snow andglacier melt amounts to 8.7% (snow5 5.5%, glacier 5 3.3%) of theoverall discharge of the Ganges Rivernaturally generated in downstreamareas. The monsoon in the humideast generates the vast majority of theriver flow even at high elevations andtherefore total discharge is primarilyaffected by monsoon rainfall (Alfordand Armstrong 2010). Findings fromthe much smaller Din Gad catchmentin the Garwhal Himalaya (Thayyen etal 2007) indicate that interannualvariability in runoff is primarilydriven by precipitation, butmeltwater plays an importantseasonal role in sustaining flowsduring dry periods. Applying asnowmelt model to the runoff fromDokriani Glacier, it was found bySingh et al (2006) that thecontributions of rainfall and glaciermelt to runoff are 13% and 87%,respectively. Recent evidence(Andermann et al 2012) also discussesthe impact of transient groundwateron river flows from Nepal, findingthat the volume of water throughthese stores is approximately 6 timeshigher than the glacier meltcontribution.

According to Shrestha andShrestha (2004), river flow data fromlarge glacier-fed rivers in Nepal showannual discharge of the Karnali(1962–2000) and Sapta Koshi (1977–1995) rivers declining, whereasannual discharges from the Narayani(1963–1994) are increasing. Studyingclimatic and hydrological trendsacross the Kosi Basin of Nepal,Sharma et al (2000a) find an overalldecrease in total annual dischargeover the period 1947 to 1993, a

pattern that was found to be moresignificant during low-flow periods.A similar analysis of the BagmatiRiver in Nepal (Sharma and Shakya2006) finds only a decreasing trend inmonsoon flows, whereas floodfrequency and duration were shownto have increased over the observedperiod (1965–2000); the majority ofthis flow is attributed to a decrease inmonsoon precipitation and littlechange in other seasons. Gautamet al (2010), however, appliedhydroclimatic assessments of a smallsnow-fed watershed in the vicinity ofKathmandu, Nepal, finding thatannual average maximum andminimum flows are increasing,validated by gauged data.

There is considerable difficulty indetecting signals of a changingclimate on actual glacier meltwatercontribution to river dischargebecause of the complexity of physicalprocesses operating within the meltzone and access constraints limitingin situ measurements of discharge.Hydrometric data from the GarwhalHimalaya (Thayyen et al 2007)indicate decreases in glacier runoffduring the 1990s, but that changingprecipitation characteristics have amore significant effect on headwaterrunoff than a receding glacier, withcontributions ranging between 2.3and 7.5%.

Despite continued glaciershrinkage as a result of increasedtemperatures, the impacts of thisincreased melting on downstreamflows is potentially negligible (Jain2008), and discharge in higherreaches basin dependent. Sharma etal (2000b)—applying a distributedhydrological model to the TamorBasin in eastern Nepal under ahypothetical scenario of unchangedland use and precipitation pattern—showed a 5uC temperature rise couldresult in a 9% decrease in averageannual runoff. In the Langtang Kholabasin in Nepal (Fukushima et al 1991;Braun et al 1993) increased summerdischarge of between 50–100% wasshown for a 2uC temperatureincrease. A more wide-scale study by

Mizra (1997) found that there isinevitable variability betweensubbasins, but that increases in themean annual runoff are expected inall subbasins considered.

Projected increases of 5% inmean annual runoff by 2070–2100for the Ganges were found by Aroraand Boer (2001), but the approachassessed only changes to themeteorological variables, notconsidering changes to thecryosphere. Immerzeel et al (2010)projected runoff changes in 2050 forthe entire upstream area with anelevation higher than 2000 masl, andin the most likely scenario a decreasein mean runoff of 17.6% wasprojected, a combined result of adecrease in precipitation and glaciermelt. However, in small high-altitudecatchments a contrasting pattern wasprojected (Immerzeel et al 2012).Modeling of catchments along theprofile of the Ganges by Rees et al(2004) revealed that changes inproportions of glacier cover inupstream areas would have an impactupon flows in headwater catchments,with increases in mean flow duringthe first decades followed bysubsequent decreases, whiledownstream the impacts were barelynoticeable. Such conclusions arecorroborated by other studies (Jain2008).

Indus Basin

The Indus river regime is affected bya range of stratified climatic zones,from high-altitude catchments withlarge areas covered by glacierscontrolling runoff via seasonaltemperature input, to mid-altitudecatchments with summer flowdominated by preceding winterprecipitation, to foothill areaspredominantly controlled by rainfallboth in winter and in the monsoonseason (Archer 2003). Snow andglacier melt is considered theprimary driver of the hydrologicalregime (Mukhopadhyay and Dutta2010). Modeling of the Upper Indusby Immerzeel et al (2010) indicatedthat some 34% of total stream flow in

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this area is generated by snowmelt,26% from glacier melt, and suchfindings are corroborated byBookhagen and Burbank (2010)—both indicating regimes that arehugely sensitive to temperaturechanges. Using the normalized meltindex to quantify meltwaterimportance, Immerzeel et al (2010)indicated that Indus flow generatedfrom snow and glacier melt is 151%greater than flow generated indownstream areas. However,observed data can suggest variablepatterns, with Winiger et al (2005)suggesting that some 70% of annualrunoff entering the plains is derivedfrom seasonal monsoon rains in thelower parts, and Singh andBengtsson (2005) finding that therelative average annualcontribution of snow and glaciermelt on the River Chenab atAkhnoor is 49%. Observed data arethus contrary to more popularconcepts of snow and glacier meltbeing the primary driver ofdownstream river flows.

Bhutiyani et al (2008) find astatistically significant decrease inthe observed average annual andsummer monsoon discharge datafrom the River Sutlej in the westernHimalaya over the period 1922–2004.Similar trends in decreasingdischarge and sediment loads havebeen observed by Tahir et al (2011).Khattak et al (2011) find positiverunoff trends during winter andspring, with negative trends duringsummer, explained by winterwarming, which produces earliermelt and reduced rainfall duringsummer. All such assessments arecomplicated by the extensive storageand irrigation network in placeupstream of areas for which gaugeddata are available.

Impacts of declining glacier masson river discharge as a result ofclimate change will be moresubstantial in the Indus basin becauseof the high proportion of dischargederived from meltwater (Rees andCollins 2006). Assessing thesensitivity of the Sutlej River basin to

climate change, Singh and Bengtsson(2004) found that 2 main featuresexist in a warmer climate: first, thatmelt contributions from lower partsare reduced because of decreasedsnow cover and a shorter meltingseason; second, that in upperglacierized areas there is increasedmelt because of an extended ablationperiod. Seasonal stream flowindicates a reduction in all seasonsexcept spring, and overall this wasfound to result in a decrease of 4–6%in average annual runoff. Singh andBengtsson (2004) suggested thattemperature increases of 1–3uCwould reduce snow melt by 11–23%in the Sutlej basin, but also indicatedan increase in glacier melt by 16–50%.

Hydrological modeling of theupper Indus basin using thesnowmelt runoff model byImmerzeel et al (2009) found thatregional warming is affecting thedischarge in the basin because ofaccelerated melting of glaciers.Under Special Report on EmissionScenarios (SRES) A2 scenario for2071–2999, using ProvidingREgional Climates for ImpactsStudies (PRECIS) regionalsimulation output, with an assumed50% decrease in glacier area, totalglacier runoff was shown to reduceby 22%; however, this was offset byincreases in rainfall by 53%,resulting in total runoff increases of7%. Late-spring and summer flowsare expected to decreasesignificantly by around 2050 and insubsequent decades (Immerzeel et al2010). Detailed modeling of climatechange impacts in the upper Indusbasin (Akhtar et al 2008) indicatesthat discharge under an SRES A2scenario for 2071–2100 will increasefor models assuming 50–100%glacier coverage, but reducesubstantially under 0% coverage.Rees et al (2004) show cleardifferences in the modeled impactsof temperature increases in the westcompared with more easterly areas,with flows in the Indus increasing by14–90% compared to mean baseline

flows in the first decade simulated,subsequently followed by decreasingflows until the 10th decade, whenflows reach between 230% and290% of baseline flows.

Brahmaputra Basin

An average monthly hydrograph ofdaily discharge data from theBahadurabad gauging station inBangladesh, for the period 1956–1993, indicates that generallydischarge relates to rainfall, with a1-month lag time, and that there isno visible spring contribution fromglacier and snowmelt (Immerzeel2008). The same study concludesthat glacier melt is not a significantcomponent of downstreamdischarge in the Brahmaputra. Suchfindings are validated by a morerecent study indicating thatmeltwater from snow and glaciersamounts to 21% of the totaldischarge generated in downstreamareas, and that less than 19% of thismeltwater is generated from glaciers(Immerzeel et al 2010). The lack ofobserved data, however, at higherreaches in particular, limitsvalidation of the hydrologicalresponses discussed.

Comparatively less research intothe impacts of climate change on theglacier hydrology and dischargeexists for the Brahmaputra basincompared to the Indus or Ganges.Rees et al (2004) found that, in theheadwaters, there was a generaldecrease in decadal mean flows forall temperature scenarios, mainly aresult of decreasing snow cover. Thepredicted decreases accord well withthe recent findings of Immerzeel etal (2010), who mention a 19.6%decrease in flow from the upperBrahmaputra basin for the period2046–2065. A detailed analysis ofchanges to downstream river flows,using discharge-weighted ensembleinputs from 12 global climatemodels (GCMs), was conducted byGain et al (2011). For both scenarios(A1B and A2) average flow and lowflow are projected to increase insize, and peak flows are expected to

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have large increases. Despite the factthat there is considerableuncertainty in predicting changes tothe South Asian monsoon, all GCMswere found to indicate an increasein discharge in the lowerBrahmaputra River owing to aprojected increase in precipitationdownstream.

Scientific challenges

and recommendations

The studies reviewed in this paperand the science related to assessingclimate change impact on waterresources are subject to a cascade ofuncertainty that is currentlyinsufficiently quantified, andscientific focus should be onreducing these uncertainties. Allthese uncertainties together result ina rather wide bandwidth regardingthe potential impact of climatechange on the region’s waterresources. The HKH is a complexregion and a number of specificrecommendations to reduce theuncertainty in water resourcesimpact studies can be made andprioritized as follows:

N Improve the poorly defined dis-tribution of precipitation data inhigh-altitude catchments;

N Quantify the spatial variation inglacial and snowmelt;

N Monitor and derive detailed pat-terns of current and future streamflow;

N Include a range of contemporarydownscaled climate models acrossdifferent emission scenarios inhydrological impact models;

N Improve glacio-hydrologicalmodel quality;

N Develop research into permafrostmelt contributions to downstreamrunoff.

These recommendations are buta few of the scientific researchpriorities required to betterunderstand how the region’shydrology is changing and what theimpacts are on water availability,

among others. However, suchresearch must also consider theutility of such science and how tocommunicate and engage withstakeholders, especially to ensuresustainable scientific researchprograms. Viviroli et al (2011)provide more comprehensiverecommendations for researchrequired to inform policy and watermanagement in the HKH and manyother high-mountain environmentsaround the world.

Discussion and conclusions

The evidence clearly indicates thatglacier shrinkage and the relativecontribution of glacier melt tothe region’s river discharge arelower than reported in theFourth Assessment Report(Intergovernmental Panel on ClimateChange [IPCC] 2007). However, thereare clearly corroborated impacts ofcontinued glacier shrinkage on thewater resources of these variousbasins, but the impacts must beframed within an appropriate spatialand temporal reference. Theavailable evidence comes fromobserved gauge data, remote sensing,and hydrological modeling studiesand does not provide equal coveragebetween the catchments. The moststriking features resulting from thissynthesis are:

N There is corroborated evidencethat, in the short term, a reductionin glacier mass from increasedtemperatures will lead to an in-crease of meltwater into the re-ceiving river, but with prolongedglacier shrinkage there will be acorrelated reduction in contribu-tion over time.

N From the minimal evidence onriver discharge trends that exists,there does not seem to be acorroborated change in the dis-charge from the Upper Indusbasin; moreover, variable patternslinked to the monsoon rainfall arenoted in the Ganges, and there islittle observed evidence for the

Brahmaputra from which to makesuch assessments.

N Glaciers to the east feeding theGanges and Brahmaputra do notprovide a significant contributionto downstream total annual dis-charge. This is primarily becauseof the fact that monsoon rainscoincide with the warmest tem-peratures at higher altitudes, withthe contribution from glaciermeltwater at such time insignifi-cant compared to the volume ofrunoff generated by rainfall. Therelative contribution to flow does,however, increase nearer to theglacier source and during springand dry periods, while also varyingconsiderably between subbasins.

N Models suggest that glaciers to thewest provide a significant contri-bution to annual discharge in thelower reaches of the Indus, be-cause of weaker monsoon rainsand greater aridity at lower alti-tudes, whereas the limited ob-served data can indicate other-wise. River basins in the westernparts of the region are highlysensitive to climate change be-cause the basins are drier and lessinfluenced by monsoon rainfall.The relatively higher dependenceon meltwater is due to the largeglacierized fraction and persistentsnow cover in the large expansesat higher altitudes, providingmeltwater stores during thewarmer seasons that do not re-ceive the same level of summermonsoon rains as towards the east.

N Predicted—but uncertain—in-creases in rainfall as a result ofclimate change will likely drivehigher annual discharge in theriver Ganges, whereas reductionsto glacier mass will have negligibleeffects on the total annual dis-charge of the Ganges. Thus, it isunlikely that the Ganges will be-come a seasonal river and that theimpact of reduced meltwater fromglaciers on annual average down-stream flows will likely be negligi-ble, contrary to more alarmistclaims.

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N Increased glacier melt as a resultof climate change will provideshort-term increases in the con-tribution to discharge of the IndusRiver, but is likely to lead todecreases in the future, as poten-tial meltwater stores will diminish.There is, however, uncertainty inhow these decreases will be offsetby changing rainfall. The season-ality of the runoff from glacierand snowmelt may be a moreimportant factor, with evidenceindicating that a change in timingdue to increasing temperatures ordecreased winter precipitationcould have significant downstreamimpacts in spring.

N Increased snow and ice melt in theheadwaters of the Brahmaputraare expected to lead to long-termdecreasing discharge, but will beoffset by increased rainfall inlower reaches—leading to anoverall increase in the down-stream annual and mean peakdischarge.

Overall we conclude that theevidence from the Himalayan regionindicates that the HKH isundergoing changes in thecryosphere as a result of climatechange, but the patterns of change,and their impacts, are by no meansuniform across the region. Glaciermelt contribution is scale dependentand will vary considerably betweenbasins in high mountain areas of theHimalaya, and meltwatercontribution to river dischargevaries considerably across thediversity of hydroclimatic conditionsand scales considered that existacross the Himalayan headwaterregions.

Fundamentally, it is the lack ofobserved data on glacier melt,precipitation, and river discharge inupper reaches that limits rigorousanalysis of change. Uncertainty in thepredictive ability of GCMs and theresponse of glaciers to temperatureincreases further limits theconfidence that can be ascribed tomodeling of future change. Climate

models consistently project anincrease in temperature over thecoming decades, whereas forprecipitation the models show lessagreement, but the general directionof change is that precipitation willincrease consistently with anacceleration of the hydrologicalcycle.

Whether a basin will face anincrease or a decrease in wateravailability depends mostly on themagnitude and direction of thechange in precipitation and thescale being considered. These maywell be much stronger determinantsthan the presence of glaciers andrequire detailed assessment ofscenarios being generated for thefifth assessment report of theIPCC.

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