Journal of Glaciology , Vol. 49, No.165, 2003

Tropical glacier meltwatercontribution tostreamdischarge: acasestudy in theCordillera Blanca, Peru

Bryan G. MARK,1* Geoffrey O. SELTZER2 1MaxPlanck Institute for Biogeochemistry,D-07745 Jena,Germany E-mail: [email protected] 2Department of Earth Sciences,Syracuse University,Syracuse,NewYork 13244,U.S.A.

ABSTRACT.Discharge measurements, climate observationsandhydro chemical samples gatheredmonthly ( 1998/99)inthe Yanamareyand U ruashrajuglacier-fed catch- ments ofthe CordilleraBlanca, Peru, permit ananalysis of the glaciermeltwater contribu- tionto stream-flow .Theseglacier catchments feedthe R õÂoSanta,which discharges into the PacificOcean. Based on a water-balancecomputation, glacier melt contributes anesti- mated 35%of the averagedischarge from the catchments. Forcomparison, a volumetric end-member mixingmodel of oxygen isotopes shows glacier melt contributes 30^45%to the totalannual discharge .Basedon stream geochemistry,dischargefrom the Yanamarey glaciercatchment provides30% of the annualvolume discharged from the Querococha watershed,which is 510%glacierized.By analogy ,the largerR õÂoSantawatershed, also 510%glacierized,receives atleast 12%ofits annualdischarge from melting glacier ice. Tributarywatersheds tothe R õÂoSantawith larger fractions of glaciercover have less vari- ablerunoff and enhanced discharge, demonstrating that the glacierseffectively buffer stream dischargeseasonally .With continuedglacier melting, stream-flow willlikely becomemore variable,and there willbe less dry-seasonrunoff.

1.INTRODUCTION tationcauses tropicalglaciers to accumulate primarily in the wet season,when melting is alsogreatest. Inthe Cordillera Theworld ’stropicalglaciers have been retreating overthe Blanca,70^90% of annual precipitation falls between late20th century ,raisingconcerns about regional water sup- October andMarch (Kaser andGeorges, 1997).Ablation plies inalpine tropical regions (Hastenrath andKruss, 1992; rates inthe wet seasonare 33% greater than in the dryseason Francouand others, 1995b;Ribstein andothers, 1995;Kaser, (Kaser andothers, 1990;Kaser ,1995).Seasonalincreases in 1999).Theconsequence of sustained glacier retreat is the loss of meltingduring the wetseason are caused by changes in the glaciermeltwater thatprovidesa significantcon tributionto verticalhumidity gradient over the glaciersurface, which stream discharge,which is used fordrinking water ,hydro- alters the latent-heat fluxand provides more energyto melt electric power(HE P)generationand irrigation. If these ice (Wagnonand others, 1999).However,this highermelt rate glaciersdisappear ,there couldbe water -supplycrises (Barry maynot exceed the greateramount of precipitation received andSeimon, 2000 ).Forexam ple,a studyof glacier mass duringthe wetseason that contributes tostream discharge. balancehas shown tha t50%of the meanannual water dis- Consequently,arelativelygreater proportion of glaciermelt chargefrom GlaciarY anamareyof the CordilleraBlanca, Peru, maycontribute to stream dischargein the dryseason than in comes fromnet glaciermass loss dueto retreat (Hastenrath the wetseason. andAmes, 1995).Undercurrent rates ofrecession, Glaciar Thispaper presents the results ofwork to evaluate the Yanamareywill disappear completely in half a century.Simi- hydrologicalimpact of glacier meltwater inthe Cordillera larly,GlaciarChacaltaya, Bolivia, could disappear within Blancaat the scales ofboth small glacialcatchments andthe 10years,causing an estimated 30%loss ofstream discharge largerregional watershed of the R õÂoSanta.Individual pro- (Francouand others, 2000).However,it remains questionable glaciallakes are manageable settings formeasuring the ifthe results ofthese limited studies applyto discharge in dynamicsof the annualglacier-hydrological regime, whereas 2 largervalleys, such asthe R õÂoSanta,which drains 5000 km analysesof historicalrunoff data for larger glacierized tribu- alongthe western slopes ofthe CordilleraBlanca (Fig .1). tarystreams ofthe R õÂoSantaaddress the impactof glacier Anothermajor concern is that the proportionof glacier melt towater supply at the regionalscale . melt contributingto stream dischargemay vary seasonally inhigh-elevation tropical regions. Strongly seasonal precipi- 2. SETTING

Thetropical mountains of the CordilleraBlanca extend 120km between 8. 5³and 1 0³S alonga northwest^southeast * Present address: Department ofGeography and T opo- strike, dividingstream runoffbetween the Pacificand Atlantic graphicScience, U niversityof Glasgow ,GlasgowG 128QQ, Oceans (Fig.1).Thelargest percentage of glacierizedarea in Scotland. the CordilleraBlanca discharges to the PacificOcean via the 271 Downloaded from https://www.cambridge.org/core. 13 Oct 2020 at 17:07:17, subject to the Cambridge Core terms of use. Journal of Glaciology

Fig.1.Contour map of the Cordillera Blanca showing glacierized areas,the R õÂoSanta,the Glaciares Uruashraju andYanamarey catchments (withenlargements) ,and stream gauge sites (solid circles) listed in Table 1.The larger Olleros and Querococha watersheds below the glacier catchments are outlined in dotted lines.Abox with dashed outline shows the area covered by Figure 3.

RõÂoSanta.Of the rivers drainingto the Pacificcoast of Peru, coastalcity of T rujillo.The HEP plantat Huallanca ( 1800m the RõÂoSantahas the secondlargest discharge volume, but a.s.l.)delimits the upperR õÂoSantawatershed to an area of maintainsthe most regularflow ,featuringthe least vari- 4901km2 that isreferred toas the Callejonde Huaylas and is abilityin monthly runoff over the year.TheR õÂo Santa des- the focusof these studies. cends from4300 m over300 km, draining a watershedof Theglacierized area contributing to R õÂoSantadischarge 12200 km2.Fourhydroelectric plants along the lowercanyon variesamong the tributarywatersheds alongthe Callejonde of the RõÂoSantasupply power to mines andvillages along its Huaylas,although the CordilleraBlanca has undergone an banksand to the portcity Chimbote with its largesteel mill overallreduction in glacier volume throughout the 20th andanchovy fishery industries. Other channelsdivert water century.TheGlacier Inventoryof Perulists the totalarea of fromthe riverto irrigate large sugar plantations around the glaciersin the CordilleraBlanca as 723km 2,basedon aerial

Table1.Discharge data (fromEgenor SA)and glacierized area for R õÂoSantatributary watersheds in the Callejon de Huaylas, west of the Cordillera Blanca

Station Stream Period of Watershed Glacierized Glacierized Mean annual Specific Meanmonthly Max.monthly Max./mean record area1 area (1962)1 area (1997)2 discharge discharge specificdischarge specificdischarge discharge km2 % % m3 s^1 mm a^1 mm mm

Paron Paron 1953^95 49 55 52 1.89 1220 99 140 1.41 LlanganucoLlanganuco 1 953^9785 41 36 2.99 1110 91 136 1.49 Huillca Collota1 953^8355 32 29 2.75 1580 130 204 1.57 ChancosMarcara 1 953^9722 1 25 22 8.80 1260 101 173 1.72 Cedros Cedros1 953^831 14 22 18 3.54 980 81 116 1.44 Colcas Colcas 1953^85 237 19 18 6.18 820 64 116 1.82 QuillcayQuillcay 1 953^83243 18 17 7.41 960 79 143 1.80 PachacotoPachacoto 1 953^831 94 12 8 4.40 720 59 130 2.19 Olleros Olleros1 970^971 75 11 10 4.77 860 71 132 1.84 LaBalsa Santa 1954^974768 9 8 88.40 580 47 108 2.28 QuitarascaQuitaracsa 1 953^97383 8 7 10.97 900 73 140 1.91 QuerocochaQuerococha 1 956^9662 6 3 1.70 870 71 151 2.12

1Areastaken from 1:1 00000scale maps based on 1962aerial photography . 2Areastaken from 1 997Landsat imagery . 272 Downloaded from https://www.cambridge.org/core. 13 Oct 2020 at 17:07:17, subject to the Cambridge Core terms of use. Mark and Seltzer:Tropical glacial meltwatercontribution tostream discharge

5300m a.s.l.,7 5%ofwhich is coveredby glacier ice. R unoff fromthe catchment flowsto lake Querococha, and monthly dischargerecords fromthe lakeeffluent are available for the period1 956^96.The U ruashrajucatchment covers3 .4km 2 overa slightlylarger elevation range from 4600 to 5 700m, and65% of the areais coveredby glacier ice. Runoffleaving this catchment joinsa stream thatflows past a gaugeat Olleros, andmonthly discharge records areavailable for the period1 970^97.Thelarger O lleros andQuerococha water- sheds containingthe Uruashrajuand Yanamareycatchments, respectively,areoutlined in Figure 1 . Themouth of eachglacier catchment isthe outlet froma small proglaciallake that hasformed in the bedrockin front ofeachglacier during recent glacierrecession. Theglacier catchments havevery little vegetation(mosses andgrasses) andpoor soil development over the looselyconsolidated alluviumand till. Thebedrock in the regionconsists of metamorphosedsedimentary rocks(quartzite andhorn- fels),drapingoff the centralgranodioritic batholith that forms the coreof the CordilleraBlanca (Wilson andothers, 1967).Glacialmeltwater enteringthe lakesis transmitted directlyto the outlet streams withouta significantlag due tolake storage. The abbreviations Y ANandUR Uwillbe used henceforthto identify the Yanamareyand Uruashraju glaciercatchments, respectively,andalso label discharge sites atthe outlet streams fromthe proglaciallakes (Fig .1).

3.D ATAANDMETHODS

3.1.Annual hydrology of theglacier catchments

YANandUR Uwerevisited monthly from May 1 998to August1999to collect observations of temperature andpreci- Fig.2.(a)M ean monthly precipitation shown as percentage pitation,and to measure stream discharge.W eather stations of the annual total for stations in the Callejon de Huaylas installednear the glaciertermini inthe 1980scontain totaliz- plotted over the hydrological year (July^June).(b)Mean ingrain gauges and mechanically recording thermographs monthly discharge for tributary streams of the R õÂoSanta,also installedin ventilated roofed shelters witha month-long shown aspercentage of annual total, andplotted fromJulyto storagecapacity .Battery-poweredHobo temperature loggers June.The percentage of glacierized area is also listed with (Onset ComputerCorporation) installed in the coveredshel- each named tributary in the legend. ters in1998sampled temperatures at0. 5hintervals.Tempera- ture datacoverage extended from April 1 998through January2000, with gaps due to mechanical failures of the photographyfrom 1 962and 1 970,while monitoring of select thermographsand limited memorycapacity in the data glaciershas shown recession since amaximumposition at loggers.Stream dischargewas calculated using the cross- the endof the 19thcentury (Kaser andOsmaston, 2002).The sectionalaveraging technique (M osleyand McK erchar,1993) dischargeof some tributarystreams hasbeen measured, and atthe streams dischargingfrom the proglaciallakes below historicalrecords areavailable for about 40 years(T able1 ). eachglacier terminus. Velocitywas m easuredwith a mechan- Monthlyaverage discharge from these gaugedtributary icalflow meter (Model2030R ,General Oceanics Inc.). streams enteringthe Callejonde H uaylasis higherduring Abasicwater -balancemodel holds that the totalvolume October^April,reflecting closely the highlyseasonal annual ofwater discharging from a catchment is equalto the cycleof precipitationtypical of the outertropics (Fig.2).In volumeof water entering the catchment plusa changein contrast, the temperature remains relativelyconstant, and storage.In this case study,it is assumed that the primary the annualvariation of air temperature is smaller thanthe outflowconsists ofthe totaldischarge leaving the proglacial diurnalvariation (Kaser andothers, 1990).Larger-scale lake (Q ),whereasthe onlyinput is precipitation( P ) falling climatic features such asEl Ni·o havealso been shown to t overthe watershedarea. Losses fromthe system include impactthe interannualvariability of temperature, precipi- evaporationand/ orsublimation ( E)andground-water tationand stream dischargein the tropicalAndes (F rancou recharge (Gw).Thechange in storage is consideredto be a andothers, 1995a;Ribstein andothers, 1995;W agnonand loss orgain of glacierice volume( ¢g),wherea loss (nega- others, 2001). tive ¢g)ofglacier volume contributes positivelyto Q .Thus Theindividual glacier catchments selected forthis case t the finalbalance is: studydrain to tributary streams ofthe R õÂoSantaand contain Q P E Gw ¢g : 1 GlaciaresY anamareyand U ruashraju,two small alpine t ˆ ¡ ¡ ¡ … † 3 ^1 glacierslocated in the southernCordillera Blanca (Fig .1). Measurements of Qt (m s )areconverted to a depthmeas- TheYanamareycatchment covers1 .3km 2 between4600 and ure (mm) tocompare with linear measurements ofthe 273 Downloaded from https://www.cambridge.org/core. 13 Oct 2020 at 17:07:17, subject to the Cambridge Core terms of use. Journal of Glaciology

othervariables by multiplying by the number ofseconds in the respective intervalof time betweenmeasurements (approximately1 month),andnormalizing by the areaof the glacierwatershed as determined froma digitalelevation modelof the area.Likewise, P is atotaldepth falling betweenthe same measurement dates.Because the glacier watersheds consist primarilyof relatively impermeable bed- rock,loss tothe ground-watersystem isnotconsidered sig- nificant.The water balance can be simplified and rearrangedto consider the glacialmeltwater contribution tostream dischargeas the changein the storageterm ( ¢g): ¢g P E Q : 2 ˆ ¡ ¡ t … † Alackof empiricaldata in this studyprecludes explicit monthlyquantification of E.Generally, E is small com- paredto others inglacier watershed balance equations. Y et arecent studyin the tropicalBolivian Andes has shown that sublimationaccounts for about 17%ofthe totalannual mass loss froma glacier(W agnonand others, 1999).Twocalcula- tionsof the hydrologicbudget were made in this studyto estimate the magnitudeof potential change in glacier volume.First, E isassumed negligible,and set equalto zero, Fig.3.Map of the lake Querococha watershed, showing the sothat the changein storageis assumed tobe entirely com- discharge- and water-sampling points:YAN,proglacial lake posedof glaciermelt oraccumulation. A secondcalculation discharge;Q1,non-glacier stream; Q2,downstream ofYAN; wasalso made, considering that loss by E fromthe water- Q3,discharge from lake Querococha. shed addsproportionately to the glaciermelt componentof the totalannual change in storage. This effectively increases HDO II waterequilibrator) ,andreported relativeto the the absolutevalue of ¢g;more ice volumeis requiredto be ViennaStandard Mean Ocean Water (V-SMOW)standard, removedfrom the watershedto account for evaporative/ sub- withan accuracy of 0.1%for d18O and 3% for dD. limationloss while Qt and P remainthe same. Sincethe § § Atwo-componentmixing model incorporates isotope climate is similar butdrier inthe tropicalBolivian Andes, valuesto estimate the volumeof glaciermeltwater addedto 20%of the totalannual loss is considereda conservative precipitation-generateddischarge at Y AN(Clarkand F ritz, estimate ofevaporation here basedon the results fromthe 1997).Thetotal discharge at Y AN( Q )isconsidereda mix- tropicalstudy cited above.Thus, the changein storage t ture ofprecipitation ( Q )andglacier melt ( Q ): assumed here incase oneto be entirely glacier melt is in p m the secondcase equalto only 80% ofalargerpotential total Q Q Q : 3 t ˆ p ‡ m … † changein storage when E isconsidered. Thesedischarge volumes can be combinedwith the stable- 3.2.Hydrochemistry of glacial andnon-glacial isotopevalues for each mixing component to derive a streams volume-weightedisotopic mass balancesuch that ¯ Q ¯ Q ¯ Q ; 4 Duringthe monthlyvisits tothe glacialwatersheds, water t t ˆ p p ‡ m m … † 18 wassampled at the stream-discharge measuringsites below where ¯t, ¯p and ¯m refer to the d Ovalueof the proglacial eachglacier (Fig .1),aswellas atthree additionalsites below lake,precipitation and glacier melt, respectively.Thisequa- YANwithinthe Querocochawatershed (Fig .3).Twosepar- tioncan be rearrangedand solved for the volumeof glacier atewater samples weretaken at each site inplastic N algene melt, givenmeasured orestimated d18Ovalues.As described bottles forsolute andisotopic measurements. 50mL of above, both Qt and Qp weremeasured directlyeach month. samplewere filtered througha 47 mmglass-fiberfilter for Thestable-isotope values of proglaciallake water ( ¯t) were the solute analyses.25 mL samples werealso collected for alsomeasured directlyduring each month of sampling . isotopicanalysis by completely submerging and sealing the Manyfactors, such aselevation,temperature andevapora- vialsunder the watersurface. The bottle capswere then tion,contribute to alter the isotopicvalue of glacierice sub- sealedwith tape to prevent leakage and evaporation, and ject tomelting, so the isotopicvalue of the glaciermelt samples werestored inan enclosed cabinet for up to 4 months componentis expectedto vary (Stichler andSchotterer , beforebeing transported tochilled ( 4³C) laboratorystorage. 2000;He andothers, 2001).Avalueof ^1 4.5 2.3% was used § Laboratoryanalyses were completed at Syracuse U niversity forthe melted glacierice ( ¯m),basedon the averageand duringSeptember 1999. standarddeviation of five melted glacierice andfirn Direct-current plasmaspectroscopy (Beckman Spectra- samples. Without areliablemethod to collect precipitation span-V)wasused toanalyze for Ca 2+, Na+, Fe3+, Si+, Mg2+, samples forisotopic analyses, mean monthly measurements 2+ + 2^ ^ ^ Sr and K .Concentrationsof SO 4 , Cl and NO3 were fromLa Paz, Bolivia, ( 16³17 ’ S, 68³5’ W;4071ma.s.l.)were measured usingion chromatography with a DionexD X500 used. LaPaz is the nearest Andeanstation with available chromatographysystem. Carbonatealkalinity (sum of stable-isotopedata, with records coveringthe period1 995^ 2^ ^ chargedcarbonate species, CO 3 and HCO3 ) was calcu- 97(International Atomic Energy Agency (IAEA) and latedas a residualfrom the chargebalance equation WorldM eteorologicalOrganization (WM O)globalnet- (Drever,1997).Valuesof d18O and dDweremeasured witha workfor isotopes in precipitation, http:/ /www.iaea.org/ mass spectrometer (FinniganMA T252directly coupled to a programs/ri/gnip/gnipmain.htm),andit alsohas a similar 274 Downloaded from https://www.cambridge.org/core. 13 Oct 2020 at 17:07:17, subject to the Cambridge Core terms of use. Mark and Seltzer:Tropical glacial meltwatercontribution tostream discharge

Asecondmixing model estimates the proportionof glaciermelt fromYANcontributingto discharge from lake Querocochabased on the concentrationsof cations and anions,following the procedures outlinedby Hounslow (1995).Thechemical composition of amixture oftwo waters lies alonga straight linein the Piperdiagram joining the twoend-member compositions(Piper ,1944).Therelative amountof eachend member contributingto the mixture is inverselyproportional to the distancealong the linefrom the endmember .

4. RESULTS

4.1.Annual hydrology of theglacier catchments

Within the glaciercatchments, P and Qt showan offset in respective maximathat isdistinct fromthe generalpattern seen inlarger watersheds downstream.Maximum Qt pre- cedes the peaksin average P by2^4 months (Fig.4).Alack ofavailable data for the Olleros andQuerococha water- sheds forthe periodof studyprecludes asimultaneouscom- parisonwith the glaciercatchments, sothe trend ofmean monthlyvalues over the historicalperiod of observationis used tocharacterize the largerwatersheds. Meandischarge iscloselycorrelated in time tomean P inthe largerwater- sheds, asillustrated bythe time series forQuerococha and Olleros, locateddownstream of YANandUR U,respectively (Table2; Fig .5).Within the glaciercatchments, Qt does not correlatewell with P,butis equallypoorly correlated with bothtemperature and P (Table 2). Thehydrologic balance of the glacierwatersheds calcu- Fig.4.Monthly measurements of discharge ( Qt) (mm) plotted with the monthly precipitation totals ( P) (mm) as latedmonthly (Equation ( 2))yields the changein storage observed over the 1998/99hydrological year for (a)YANand term (¢g),whichprovides an estimate ofthe meanmonthly (b) URU. contributionof glacier melt tostream discharge(Fig .6). Positivevalues of ¢g represent storagegain (net accumu- lation),anddiminish Qt,whilenegative values represent annualvariability in precipitation. The mean monthly storageloss (net melt) andan enhancement of Qt. The plot valuesprovided the stable-isotopevalues of precipitation of ¢g showsthat the maximumin specific melt (mm) occurs (¯p)inthe mixingmodel, and the standarddeviation of the inOctober andN ovemberfor Glaciares Yanamareyand monthlyvalues provided a rangein the computedvalue of Uruashraju.However ,glaciermelt contributes amaximum Qm. relativepercentage of the monthly Qt duringthe dry-season

Table 2.Mean monthly discharge (Q)in mm, precipitation (P)in mm, and temperature (T)in ³Cfor different stations in the Cordillera Blanca and the glacier catchments.The number of years available to calculate each mean monthly variable is listed. Linear correlation coefficients (Pearson’sproduct moment,r)are calculated for correlations between discharge andprecipitation anddischarge and temperature for each station

Station Numberof yearsJune July Aug.Sept.Oct. Nov.Dec.Jan.F eb.Mar.Apr.May Pearson’s(r)

Querococha Q 40 281 9192648 64 90 1 131341 541 0254 Querococha P 40 127134089 88 1 181561 541 719437 0.94 Querococha T 40 7.57 .37.77.67.57 .77.67 .57.37.37 .67 .70.41

Olleros Q 27 3427 29 37 6 17894 1 091 1513795 53 Huaraz P 40 3152362 5 7801 041 1311571200.96 Hurarz T 40 1514.51 5.21 5.51 5.315.315.315.115.11515.31 5.40.52

YAN Q 1 208206 1 90265 6 14531289263 1 341 139164 YAN P 15 11341 001 481 541 742 17252244 1 476 1180.54 YAN T 12 2.41 .81 .82.5 2.9 2. 72.91.91.922.52. 60.70

URU Q 1 1289 11342 10259467 2 121961 861 761 6794 URU P 11 1013591 081 181192071 911891 3943 22 0.5 1 URU T 9 2.72.42.22.4 2.9 2. 72.42.32.31 .92. 12.10.54

275 Downloaded from https://www.cambridge.org/core. 13 Oct 2020 at 17:07:17, subject to the Cambridge Core terms of use. Journal of Glaciology

Table 3.Amount ofglacier meltwater (mm)and relative con- tribution to average annual stream discharge (%aver Q) for the glacier catchments

Catchment Area Glacier melt Contribution km2 mm % aver Q

YAN 1.3 970 (1210) 35 (44) 10 § URU 3.4 780 (975) 36 (45) 10 §

Notes:Valuesin parenthesesrepresent larger potential estimates assuming 20%annual glacier ablation is dueto evaporation and/ orsublimation. A10%rangeis approximatedassuming 1 0%error in stream-discharge measurementand 5% in precipitationmeasurement.

arelowest in the glaciercatchments (YAN,URU),butshow ahighervariability over the yearthan the other stream samples. Isotopicvariability of stream watervaries with the size ofthe watershedand with the relativeamount of glacierrunoff contribution. There is adownstreamdecrease inthe magnitudeof annual isotope variability ( ¢d18O) amongthe stream samples, withsamples fromthe proglacial lakedischarge YANshowing 9% changeover the yearand ¹ those fromsite Q3at the outletof Querococha showing only

Fig.5.Mean monthly discharge (Q)(mm)and precipitation (P)(mm)plotted for (a)the Querococha watershed,down- stream of YAN,representing the average of 40 yearsfor both variables;and (b)the Olleros watershed, downstream of URU,representing the average of 27years of discharge data and 40 years of precipitation data from nearby Huaraz.

months (June^September).Duringthis periodof little tono precipitation,glacier melt contributes upto 1 00%of Qt. Yet overthe course ofthe wet season, ¢g shifts dramatically fromstorage release tostorage accumulation. Thetotal annual volume of glacier meltwater isexpressed asadischarge,and also as apercentageof the averageannual stream discharge(T able3) .Assumingthat the loss instorage isexclusivelyby melting ,then glacierice loss comprises 35% ofthe totalannual stream dischargein YANand36% of total dischargein UR U.If 20%of the totalstorage is lost toevap- oration/sublimation(e.g .Wagnonand others, 1999),alarger totalvolume of ice loss wouldhave to be ablatedannually , equivalentto about 44% of the totaldischarge in Y AN,and 45% in URU.

4.2.Hydrochemistry of glacial andnon-glacial streams

The d18Ovaluesof the stream waters conformclosely to the meteoric water-line,and follow a pattern ofannualvariation similar tothe isotopevalues of precipitationmeasured atthe IAEAstation in La Paz(Fig .7).Themonthly isotope meas- urements showa maximumvalue in October^N ovember anda minimum valuein March^April. The precipitation d18O varies by 10% betweena maximumin October and Fig.6.Change in glacierstoragevolume ( ¢g),expressed both in ¹ minimum inApril,which is agreatermagnitude of annual mm and as apercentage of monthly stream discharge leaving the changethan the stream waters.The average isotopic values proglacial lake ( Qt),calculated for (a)Y ANand (b)U RU. 276 Downloaded from https://www.cambridge.org/core. 13 Oct 2020 at 17:07:17, subject to the Cambridge Core terms of use. Mark and Seltzer:Tropical glacial meltwatercontribution tostream discharge

Fig.8.Results of atwo-component isotopic mixing model during the months with stream sample data over the 1998/99 hydrological year,showing the discharge due to glacier melt (Qm)and total discharge from the proglacial lake ( Qt) for (a)Y ANand (b)UR U.Discharge from precipitation is shown with bars.The error bars reflect variability of isotopic values for precipitation andglacier melt.

2% annualvariation. The slightly higher variability at site ¹ Q2than Q 1reflects the glaciermelt contribution. Theisotopic mixing model predicts that the discharge frommelting glacier ice ( Qm)hasan annual variation similar tothat calculatedfrom the hydrologicalbalance (Fig.8) .Thereis maximum Qm inOctober^N ovemberas wellas maximum glacier storage during March^April. When summed overthe hydrologicyear ,the totalvolume of Qm canbe expressed asapercentageof the total Qt to esti- mate the totalannual contribution from glacier ice melt. Thisis best presented asa range,accounting for the vari- ationin monthly isotope values of precipitation.Glacier melt accountsfor 3 1%ofthe proglaciallake discharge volume at YANand45% at UR Ubasedon the isotopevalues. Fig.7.Isotopic data used: (a)local meteoric water-line Theaveraged concentrations of major anions and cations drawn through bivariate plot of allstream samples from the fromeach stream samplesite overthe hydrologicyear (T able Querococha watershed; (b)annual range of isotopic values 4)forma mixingline when plotted on a Piperdiagram (Fig. (¢d18O)as afunction of watershed area for each sample 9).Theconcentrations from YANandthe non-glacierstream site;(c)time series of d18Ofor each stream sample over the Q1sites formthe twoend points of the mixingline .The 1998/99 hydrological year;(d)mean monthly precipitation mixedmember comingfrom Querococha, Q3, falls at a dis- (bars) and d18Ovalues for LaP az (4071m)averaged over tanceinversely proportional to the concentrationof each the existing yearsof IAEA data,1995^97,with error bars end-member contribution,such that 30%is derivedfrom showing the standard deviation of monthly isotopic values. YANand70% from the non-glacierstream Q1. 277 Downloaded from https://www.cambridge.org/core. 13 Oct 2020 at 17:07:17, subject to the Cambridge Core terms of use. Journal of Glaciology

Table 4.Average major-ion composition (in meq L^1) for sites in the Querococha watershed

2+ 2+ + + ^ 2^ ^ Site Ca Mg K Na Cl SO4 HCO3

YAN5891 1412702 955n/ a Q13495 1111378 188397 Q2397 80 1 15735679 1 Q3343 56 1 2702 391 12

On the largerscale of the tributaries tothe Callejonde Huaylas,where stream dischargeis closelytied tothe annualcycle of precipitation, the magnitudeand variability ofdischargeare directly related to the percentageof glacier coverage(Fig .10).Themagnitude of specific discharge (averageannual stream dischargenormalized by watershed area)generally increases withlarger amounts of glacier coverage.Annual stream dischargeis alsoless variablein watersheds withlarger glacierized areas, as illustrated by the ratioof maximum discharge to mean discharge as a functionof the percentageof glaciercoverage.

5.DISCUSSION

5.1.Hydrological balance methods

Utilizinga simplified hydrologicbalance to estimate glacier massbalanceis justified here basedon a lackof surface-based observations,but involves many complications (e.g. F oun- tainand others, 1999).Without energy-balancemeasure- ments andrestricted tosingle precipitation gauges in each catchment, wewere unable to quantify important issues of spatiallyvariable precipitation inputs andevaporation. Also, the totalchange in storage considered for the glacierwater- sheds actuallycontains long-term storageof water coming

Fig.10.Magnitude and variation of annual stream discharge with percentage ofglacierized area in the R õÂoSantatributaries , shown by (a)coefficient of variation (%);(b) specific dis- charge (m a^1);(c) ratio of maximum monthly discharge to mean monthly discharge ( max Q=mean Q).

fromglacier ice, firnand snow .Muchwork has been devoted toinvestigating the seasonalpatterns ofmeltwater storagein Fig.9.Piper plot of mean monthly chemistry concentrations for glaciersin different extratropicalregions (e.g. Tangbornand the stream sites within the Querococha watershed.The radii of others, 1975;O « stlingand Hooke, 1 986;Hodgkins, 200 1),and the circles represent total dissolved solids in parts per million further investigationdifferentiating the impactof seasonal according to the included scale. storageshould be conducted on tropicalglaciers. 278 Downloaded from https://www.cambridge.org/core. 13 Oct 2020 at 17:07:17, subject to the Cambridge Core terms of use. Mark and Seltzer:Tropical glacial meltwatercontribution tostream discharge

Theuse ofsimplified mixingmodels based on the solute remains small relativeto the magnitudeof precipitationin chemistry ofmeltwater hasbeen well established inglacier this tropicalsetting .Thisdistinction is importantfor under- hydrologyas amethodto separate subglacial and englacial standingwhy the annualhydrographs from the streams componentsof discharge(e.g .Collins,1978).Themethod is leavingthe glaciercatchments match more closelythe notwithout problems, and the assumptionthat the end changesin monthlyprecipitation (Fig .5). members mixconservatively is especiallyproblematical withinthe glacierenvironment (Sharp and others, 1995). 5.3.Annual meltwater contribution to streamflow Rather thandeciphering members ofenglacial and sub- While the hydrologicbalance and the isotopicmixing glacialwaters routingthrough a glacier,this studyfocuses modelpredict different percentagesof annualglacier melt onthe fateof waters leavingfrom the glaciercatchment contributionto stream-flow leavingthe glacierwatersheds, andmixing downstream. The simplified mixing-model the estimates areapproximate and converge within a range assumptions arejustified tomake some first-order quanti- cited inother studies. Thenet volumeloss ofglacierstorage tativeestimates ofmeltwater impacton stream discharge calculatedfrom the annualhydrological balance amounts giventhe lackof anyother data. toabout 35% of the meanmonthly discharge for both of these glaciers(T able3 ).Thisestimate is closeto that from 5.2.Seasonality of tropicalmeltwater GlaciarChacaltaya in tropical Bolivia, where complete dis- appearanceof the glacierwould reduce averageannual Thediscrepancy in timing between maximum stream dis- stream dischargeby 30% (F rancouand others, 2000).The chargeand maximum precipitation within the Yanamarey isotopicmixing-model estimates spana largerrange, show- andU ruashrajuglacier catchments documents animportant ingthat the glacierice melt contributes 23^54%of pro- seasonalvariation in the hydrologicalbalance of these trop- glaciallake-discharge volume over the annualcycle. This icalglaciers. Previous research ontropicalglaciers suggested reflects the variancein the monthlyisotope value of precipi- that the maximumdischarge from the glaciersoccurs atthe tationused inthe mixingmodel, which was only averaged peakofthe wetseason, when both accumulation and ablation overthe years1995^97.Yet the upperrange of results arestill reachmaximum levels (e.g .Kaser,1999).However,the lackof similar tothose froman earlier mass-balance study of correlationbetween peak precipitation and peak discharge GlaciarY anamarey,citinga 50%contribution to average shownhere implies achangein glacier storage over the stream discharge(Hastenrath andAmes, 1995).Therefore, hydrologicalyear .Theearly months ofthe wet season consideringthe different methods andexpected range of (October^November)produce net glacierloss, associated variation,an averageestimate ofabout40% is areasonably withmaximum discharge, whereas the later months ofthe conservativeestimate ofthe contributionof glaciermelt to wetseason (J anuary^March)result innet accumulationof the dischargefrom the glaciercatchments. mass, asdischargedecreases andprecipitation levels continue Theresults ofthe annualhydrological balance of the torise. Giventhe inconsistencies inthe lengthof time series glaciercatchments canbe combinedwith the insights gained forthe different spatialscales compared( 1yearat the glacier fromthe hydrochemicalmixing models in the Querococha catchment scalecompared to 27 and 42 years at the Olleros watershedto estimate the overallglacier meltwater impact andQuerococha watersheds, respectively),these results defy to the RõÂoSanta.Assuming the mixingmodel shows a dilu- rigorousquantitative comparison. N evertheless, theyprovide tioneffect downstream,with waters mixingconservatively , importantevidence that is substantiatedby other research on the glaciercatchment discharge(Y AN)seems tocontribute the seasonalenergy balance of tropicalglaciers. about30% of the stream-flow atQuerococha (Q3) .The Thetropical climate duringmaximum glacier discharge well-separatedend-member groupswith two intermediate is conduciveto melting ice, emphasizingthe hydrological members seen onthe Piperplot give confidence that the significanceof these transitionalmonths betweenthe late waterleaving the glacierenvironment at YANis dilutedsuc- dryandearly wet seasons.Previous work on GlaciarZongo, cessivelyover distance to Q3 .This30% contribution thus Bolivia(F rancouand others, 1995b;Ribstein andothers, represents the impactof glacierwatershed discharge on dis- 1995;W agnonand others, 1999),andongoing work on both chargefrom the Querocochawatershed, having about 3% Bolivianand Ecuadorian glaciers (F rancouand others, glaciercoverage. This is evenless glaciercoverage than the 2000)show that this isaperiodthat features bothminimum entire Callejonde Huaylas, which drains via the R õÂo Santa surfacereflectivity (albedo) after the dryseason and toLa Balsa, but presents aconservativeanalog for this increasedatmospheric humidity ,whichcause more energy largerdrainage having about 8% glaciercoverage (T able tobe availableto melt glacierice. T otalnet radiationis the 1).Sincethe hydrologicalbalance and isotopic mixing primarysource ofenergyto the glaciersurface, and a sur- modelof the glacierwatersheds showsabout 40% (up to facethat is less reflective (loweralbedo) absorbs more 70%)of the annualvolume of the proglaciallake discharge energy;an increase inhumidity shifts the energybalance is contributedby glacier melt, then the totalvolumetric atthe glaciersurface towardsproducing more meltingthan impactof glacier melt tostream-flow atQuerococha is sublimation/evaporation.Finally ,there is alsoa slight about1 2^20%.Therefore, glacier meltwater canbe conser- increase intemperature duringthe australspring (T able2 ), vativelyestimated toprovide at least 10%ofthe annualdis- enhancingthe sensible heatflux to the glaciersurface. chargevolume to the R õÂoSanta,with larger relative Duringthe ensuingmonths ofthe wet season,there is ashift contributionsin the dryseason. fromnet meltingto net glacierstorage. While the annualhydrological balance describes amax- 5.4.Impact of tropicalglacierized area on regional imum volumeof glacier storage loss duringthe transitional runoff periodbetween wet anddry seasons, the relativeimpact of this contributionto total stream dischargeis largestduring Thegreater relative contribution of glaciermelt inthe dry the dryseason. The actual volume of glacier meltwater seasonunderlies the roleof glaciers as ` `buffers’’tothe 279 Downloaded from https://www.cambridge.org/core. 13 Oct 2020 at 17:07:17, subject to the Cambridge Core terms of use. Journal of Glaciology

regionalhydrology ,anobservationmade for glaciers world- logicalyear comes fromglacier melt. Therelative melt wide(Y oung,1982;Ro « thlisberger andLang, 1987;Chen and proportionof stream dischargeis greatest duringthe dry Ohmura,1990;K uhnand Batlogg ,1998).Glaciers holdprecipi- season,and also diminishes downstreamfrom the glacier tationin storage and compensate stream -flowin dry periods asprecipitation runoff and ground-water discharge are byproviding meltwater (e.g.Fountainand T angborn,1 985). mixedinto the streams. Nevertheless, ahydrochemical While allthe largertributary streams ofthe R õÂo Santa have mixingmodel of the lakeQuerococha watershed conser- annualhydrographs closely correlated to precipitation ,those vativelyestimates that atleast 10%,and potentially as watersheds witha greaterpercentage of glacierized area dis- muchas 20%,of the R õÂoSantadischarge in the Callejon playless variabilityin the magnitudeof discharge between the deHuaylas comes frommelting glacier ice that is not wetand dry seasons (Figs 2 and1 0a).Asthe glaciersin the replenishedby annual precipitation. watersheds shrink,the contrast betweenwet- anddry-season 3.Glacierized watersheds ofthe CordilleraBlanca feature stream dischargewill become greater ,since there willbe less enhancedannual dischargeand less variablerunoff than glacierizedarea to store the concentratedprecipitation at the non-glacierizedstreams. Thedegree of glacierization is heightof the wetseason, and less ice melt tobuffer the low directlyrelated to the magnitudeof these effects; asthe flowswhen precipitation decreases inthe dryseason. percentageof glacierized area within a watershed Thereis anoteworthycontrast inthe waythese tropical increases, specific dischargeincreases, the coefficientof glaciersinfluence stream dischargecompared to those at variationin discharge decreases, andthe ratioof max - higherlatitudes. Larger percentages of glacierized area in imum dischargeto mean discharge decreases (Fig.10). watersheds tend toaccompany higher levels of the specific While the former twoof these characteristics areconsistent dischargefrom streams inboth the CordilleraBlanca (Fig . withother glacier watersheds worldwide,the trend in 10b)andthe EuropeanAlps (KuhnandBatlogg,1 998).How- maximumto mean discharge ratio is oppositethose of ever,whenthe ratioof maximum monthly discharge to highernorthern latitudes, and implies adifferent response meanmonthly discharge ( max Q=mean Q)is regressed toseasonal climatic forcing. againstthe percentageof glacierized area for the same streams, the slopeof the lineis negative(Fig .10c),opposite 4.With continuedglacier recession, stream dischargeis that shownfor glacierized watersheds inthe Alps,which has likelyto decrease andbecome more variable,with larger apositiveslope of identical magnitude. This implies that volumesduring the wetseason and smaller amountsin maximumdischarge in streams ofthe CordilleraBlanca the dryseason. Thishas important implications for water tends tobe relatively more mitigatedby increased mean dis- resource managementin this developingnation where chargewith progressively greater glacierized area. One pos- glaciershave been receding throughout the past century, sible explanationfor this difference is that climate affects amplifyingstream dischargeas they melt. Progressive seasonalglacier melt differentlyin the tworegions. T ropical deglaciationhas been observed over the pastcentury glaciermelt moderates the contrasts ofa verystrong seasonal (Kaser andOsmaston, 2002),evenas temperatures have risen by 0.3Kper decade(V uilleand Bradley ,2000). precipitationregime, whilemid-latitude glaciersare ex- ¹ posedto greater temperature seasonality,andare less able Rivers likethe R õÂoSantahave had discharge augmented tomoderate extremes inrunoff. Seasonal glacier melting in byglacier melt duringa periodof growthin infrastruc- the mid-latitudes thus tends toincrease the amplitudeof run- ture, featuringconstruction of HEP plantsand urbaniz- offvariations, while meltwater fromtropical glaciers ation(Barry and Seimon, 2000) .Future publicwater smooths the seasonalvariation of runoff (Kaser andOsmas- supplies willdecrease andbecome less consistent than ton, 2002). the growinginfrastructure hasplanned for ,basedupon the higherdischarges of the past century. 6.CONCLUSIONS

Thisyear-longcase studyof Peruvianglacier hydrology and ACKNOWLEDGEMENTS hydrochemistryhas revealed four main points that should beconsidered in future investigationsinvolving more Themanuscript wasrefined by the carefulediting of R. Nar- detailedenergy-balance and runoff measurements. use (Scientific Editor),withhelpful comments fromA. G . Fountainand an anonymous reviewer ,forwhich we are 1.Theannual maximum glacier melt occurs duringthe grateful.R esearch wassupported by the U.S.F ulbrightCom- australspring for the glaciersin the CordilleraBlanca, mission andthe GeologicalSociety of America. J .Go ¨ mez, Peru.Stream dischargefrom glacierized watersheds J.``Pepe’’Ames, C.Hopkinson, B. Seavey and the Peruvian reaches amaximumin this seasonas a result ofthe NationalInstitute ofN aturalR esources (INRENA)officein enhancedmelt. Asglacier-mass changesare evaluated Huarazare acknowledged for help with fieldwork, and A. inrelation to global climate changes(Oerlemans, 1994; Ames forlogistical assistance. Wealsothank A. R odriguezof Haeberliand others, 1999;Dyurgerov and Meier ,2000), EgenorSA for his helpwith historical stream-discharge data. andin relationto changes peculiar to tropical highlands (Diaz andGraham, 1 996;Thompson, 2000; V uilleand Bradley,2000),specialattention should be paidto changes REFERENCES occurringduring this springseason when glaciers are most sensitive tomelting . Barry,R.G. and A. Seimon.2000. Research for mountain area development: climaticfluctuations in themountains of the Americas and their signifi- 2.A significantamount of annualstream dischargeto the cance. Ambio, 29(7),364^370. RõÂoSantathat drains the CordilleraBlanca is supplied Chen, J.andA. Ohmura.1990.Estimation of Alpine glacierwater resources andtheir changesince the 1 870’s. InternationalAssociation of Hydrological bymelting glacier ice. Atleast one-thirdof the waterdis- SciencesPublication 193(Symposium at Lausanne 1 990ö Hydrologyin chargingfrom the glacierwatersheds overthe hydro- MountainousRegions I :HydrologicalM easurements;the WaterCycle ),127^135. 280 Downloaded from https://www.cambridge.org/core. 13 Oct 2020 at 17:07:17, subject to the Cambridge Core terms of use. Mark and Seltzer:Tropical glacial meltwatercontribution tostream discharge

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MSreceived 4February 2002 andaccepted in revised form 9February 2003

281 Downloaded from https://www.cambridge.org/core. 13 Oct 2020 at 17:07:17, subject to the Cambridge Core terms of use.