4J

BulletinofGlaciologicalResearch20（2003）41−47⑥JapaneseSocietyofSnowandIce

lnfluence ofsurfacedebrjson summerablationin Unteraar・and Lauteraargletscher，
Switzer】andShin SUGIYAMA

InstituteofLowTemperatureScience，HokkaidoUniversity，Nishi−8，Kita−19，SapporoO60−0819Japan（ReceivedSeptember17，2002；RevisedmanuscriptreceivedOctober31，2002）

Abstract

In orderto evaluate theinfluence ofsurface debris on summer ablation on a glacier，Surface
ablation and air temperature were measured from May to Septemberin200lon Unteraar−andLauteraargletscher．Thepositivedegree−dayfactorobtainedforadebris−COVeredsurfacewas2．98mm
W．e．KLlday￣1，Smallerthanthoseobtainedfordebrisfreesurfaces，4．459．81mmw．e．K￣1day￣l．The

factorswereutilizedtocomputedailyablationineachaltitudinalrangeusingthepositivedegree−day
method．Resultsofthecomputationshowthatthedebrislayersignificantlyreducesthetotalablation
inJulyandAugustaftertheglaciersurfacechangeditsstatefromsnowtodebriscoveratthelower

reach．Thus，itisconcludedthatthedebrisTCOVeredsurfacesubstantiallycontributestothedecrease

inablationduringthemidsummerseason．ThetotalablationfromMaytoAugustwouldbelarger

by32％iftheglaciersurfacehadbeenassumedtobeentirelydebrisfree．

1．lntroduction

Glaciers are often covered by debris in theirablationareas．Rocksandsedimententrainedintoice

inthe accumulationarea aretransferredto thelower

reachoftheglacier，thenmeltoutonthesurfaceand
form a supraglacialdebrislayer．Because thermal
processes on a debris layer are quite different fromthose on snow orice，theinfluence of thelayer on
ablationiscrucialfortheglaciermassbalance．The
evolution of medialmoraines or supraglaciallakes，Oftenobservedondebris−COVeredglaciers，areCOnSid−

eredtobetheresultsofthethermalregimespecifictodebris−COVeredice surfaces．

Observationsandexperimentsonglaciersinthe
pastagreewiththatdebrislayersreducetheablation

as an efficient toolto evaluatelarge scale massbalancedistributiononadebris・COVeredglacier（Nak−awo gJαJ．，1999）．

One of the simplest but sufficiently accurate
SChemestoestimateablationonaglacieristheposi−
tivedegreeTdaymethod．Itbasesonlinearcorrelation
betweenablationandsumofdailymeantemperatures

abovemeltingpoint（PDD）duringaperiod．Thefactor

thatlinks the ablationto PDDis calledthepositive

degree−day factor．Braithwaite（1995）summarized

positivedegree−dayfactorsreportedforiceandsnow
Surfacesonvariousglaciers．Generallyasnowsurface
has a smaller factor thanice，but the differencesamong glaciers，Or eVen Within a glacier，are Oftenlargerandattributedtodifferentmeteorologicalcon−

ditions．Kayastha et al．（2000a）studied spatialand
Oficebeneath，unlessthelayerisverythin．Mattsonet temporalvariationsofdegree・dayfactorsonaglacieral．（1993）showed that ablation under debrisisless
intheHimalayas．Theyconcludedthatlargercontri・

thanthatonbareicewhenthedebrislayeristhicker
than30mm，Whileitis greater when thelayeris
thinner．Althoughthesimilardependenceofablation
On debris thickness hasbeen observedin other gla−
Ciers，thecriticalthicknessvariesineachglacierandSeemedtobecontrolledbythethermalpropertiesof

the debris（Rana et alリ1998）．Nakawo and Young（1981；1982）proposedandtestedamodelto estimate

ablation under a debris layer from meteorological
Variables，in which the thermalresistance of debris
WaS takeninto account byincluding debris surface
temperature．His modelwas combined with surface
temperature measurements from satellites and usedbutionsofshortwaveradiationtoablationresultsina

larger degree−day factor．The positive degree−day

method has not frequently been implemented todebris−COVeredsurface．InKhumbuGlacier，a debris−

COVered glacierin Nepal，eXperimentalwork was
Carried out to determine positive degree−day factors
forablationunderdebrislayerswithdifferentthickT
nesses（Kayastha et alリ2000b）．The factor obtained
for400−mmthickdebris−COVeredsurfacewas30％of
the value for bareice．

Previous works described above suggest thatthick debriscover on aglacier reduces ablation andtakes a key rolein the evolution of debris−COVered42

BulletinofGlaciologicalResearchglaciers．However，thetotalreductionintheamountisathick debrislayer that coversthemajorpart ofUnteraargletscher and some parts of Lauteraar andevaluatethetotaldecreaseinsummerablationdueto

SeVeralstripes of medialmoraines are formed andtheir heights measure up to about50m．Althoughlarge rocks are piled up to several meters on the
medialmoraineattheconfluencearea，thethickness
Of the debrislayer varies from 50 to 200mmin
general．
The glacier is frequently surveyed by aerial

Ofablationinaspecificglacierhasnotbeenstudiedso
far．The objective ofthis reportis to quantitatively

debris cover in Unteraargletscher by the positive

degree−day method．This attempt may display theimportance of debris cover in glacier mass balance
andgivesaninsighttothereactionofdebris−COVeredglacierstoclimatechanges．
2．Studysite
Unteraargletscherisatemperatevalleyglacierin

Finsteraargletscher（Fig．2）．On Unteraargletscher，

photogrametry by Glaciology Section，VAW−ETH，Zurich．Thedigitalelevationmap oftheglacier sur−face constructed from a surveyin1999（personal

the Bernese Alps，Switzerland．Together withits COmmunicationfromJ．Helbing，2002）wasusedinthis
tributaries，Lauteraar，FinsteraarandStrahleggglets− Study and the surface debris distribution determined

Cher，itcomposesaglaciersystemwithatotalareaof

28km2（Fig．1）．Characteristicfeatureoftheglaciersfrom aerialphotographsbySchuler et al．（2002）wasSuperimposed on the map．Figure3is the resulting
mapthatshowsdebrisdistributiononUnteraar−andLauteraargletscher．Because of thelocations of ourfield measurement sites，this study focuses on those
two glaciers and we defined the boundary between
Unteraar−and Finsteraargletscher at the altitude of
2400m a．s．1‥Fig．4is a hypsographicaldiagram ofdebris−COVered and debris−free surface area．Debris−

Fig．1 MapofUnteraar−，Lauteraarr，FinsteraarandStra−
hlegggletscher．The solid squares are the temperature
andablationmeasurementssites，andthesolidtriangle
istheablationmeasurementsite．PlussymboIsarethe
additional ablation measurement sites on the debris
covered surface．

E8■10・

E8●15・

Fig．3 Map of Unteraar．and Lauteraargletscher with
Surfacetopographyanddebrisdistribution．Shadedarea
iscoveredwithadebrislayer．

Fig．2 Photographs showing（a）Unteraargletscher from thelower reach of theValleyand（b）theglaciersurfaceconditionatabout2300ma．s．1‥

43Sugiyama

theglacierwithoutcorrections．
︵
N
∈
￡
巾
聖
空
空
竜
l
≡
＜

Ablation stakes were drilled into the surface atfour sites，TAl，TA2，TA3and A4，then measured
fromMay19toOctober2withintervalsof5−28days．

．
4

ThemeasurementatthelocationA4，the altitude of

2670ma．s．1．whichishigherthantheequilibriumline
altitudein2001（Fig．1），WaSStartedonJuly3．6．5−m
long，34−mmdiameteraluminiumpipeswereusedas

2
0 1900

2100

2300

2500

2700

2900

Altitude（m）

Fig．4 HypsometricdiagramofUnteraar−andLauteraar−
gletscher for debris−COVered（shaded）and debrisTfree
surface conditions．

thestakesandinstalledwithasteamdrill．Becauseof

Surfaceirregularities，individualmeasurements may

haveerrorsupto±10mmapproximately．Measured

Changesinthesurfaceheightwereconvertedtowaterequivalent ablation by densities of600kgm￣3for

SnOWand900kgmr3forice．
Transientsnowlinepositionwassurveyedonthe
glacierbyhandyGPS withintervals ofa week to a
COVered surface accounts for47％of the totalarea
andmainlyoccursin Unter去argletscher．Apart from month．Boundary between snow andice was fairly

thesurfaceareaofll．6km2showninFig．2b，Lauter− welldetermined and errorin the snowline altitude
aargletscher includes hanging glaciers on the steep
was estimated as about±20m．
Sidewallsinthealtitudehigherthan2900ma．s．1．．Map

issued by Swiss Federal Office of Topography indi‑
CateSthatthetotalareaofsuchglaciersismorethan

5km2．Surface conditions and ablation mechanisms，

3．2．斤g5〝Jね

Daily mean air temperatures at the three sites
throughoutthemeasurementperiodareshowninFig．

however，are COnSidered to be so complicated thattheywereexcludedfromthisstudy．

turesweregenerallypositiveuntiltheendofAugust，

3．Field observations

temperatures suddenly dropped and the snow line

3．J．〟g〟乙OdtheperiodfromAugust27toSeptember29．

Airtemperaturewasmeasuredevery15minutes
from May19to October2，200lon theglaciers at

5．Afterthestart ofmeasurementsinMay，tempera−
exceptforafewcoldperiodsinJune．InSeptember，altitude descended from2620to2400m a．s．1．during

three sites named TAl，TA2and TA3（Fig．1）．The ︵p︶ 0
altitudesofthesiteswere2230，2330and2450ma．s．1．，25respectively．TA2andTA3arelocatedondebrisfree
SurfaceswhileTAlisattheridgeofamedialmoraine
WhereaboutalOO−200mmthick debrislayercovers
theice．Allthree sites were covered withsnow when

WeStartedthemeasurementsinMay，thentheyturned

toice or debris−COVered surfacelaterin the summer．

⊃
l
匹
乳
∈
心
葡
じ
認
∈
倉
巾
q

Temperatureloggers（Hioki3633）wereinstalledin
200rmmlong，50−mmdiameterplasticplpeSfixedon
ablationstakesdrilledintoice．Thepipesweredirect−

edalongthevalleysothatthewindhelpsventilating

the sensors．Occasionally，the heights of theinstru−

mentswereadjustedtokeepthematlto2mabove
the glacier surface．To evaluate the validity of themeasurements，One Of theinstruments wasinstallednear the weather station at Lauteraarhut on the left
flankofUnteraargletscheranditsdatawerecompar−
ed with those measured by a ventilated thermistor0
50

May June Ju［y August September
2001

Fig．5 Dailymeanairtemperaturevariationsmeasuredon
theglacieratTAl（solidwithdatapoints），TA2（solid），
and TA3（broken）．

Temperature at TA1 was generally higher thanthose at the other two sites and the differencein，

CreaSedinJune．Detailexaminationofsubdailytem−

peraturevariationsimpliesthatthesurfacecondition（VaisalaTlO7）．16−daysmeasurementinJuneshowed at TAlturnedfromsnowto debris onJune7．After

fairlygood qualitative agreementin diurnalfluctua−
tions，but suggested that our instrument tend to

thetransition，debriscoveredsurfacewasintensively
Warmed upin the daytime and caused clear differ−measurelower daily mean temperature，0．5 Kinencesinthedailymeantemperatureagainstsnowand
average．Since theinfluence ofthe simplifiedinstru−

ment on temperature measurement seems to be not
identicalineachlocation，Iusedthedataobtainedon

ice surfaces．

The temperaturelapse rate on theglacier was
calculated from the data sets obtained at TA2 and44

BulletinofGlaciologicalResearchTA3，debris free measurements sites，from May to Positivedegree・dayfactorsobtainedforthesitesare
September．Intheprocessofthecalculation，thesur− giveninTablel．Thevalueondebris・COVeredsurface

atTAlisabout30％ofthoseonsnoworiceatTA2
and TA3，Whichindicates that theinsulation due tofacetransitionsfromsnowtoiceatthemeasurement

Siteswerenotconsidered．Arateγ＝2．89×10▼3Km￣1

WaSObtainedandusedtoestimateatemperaturefield

the debris cover reduced the ablation ofice beneath．

in the next section both for debris−COVer and debris−

free surface conditions．

The cold weather in September suggests that
ablationdecreasedandmajorpartofprecipitationfell
as snow on the glacier during that period．Because
SnOW preCipitation might have changed the surface
COnditioninanunpredictablemanner，itisdifficultto

determinetheablationinSeptember．Thustheanaly−

Sisofablationintheremainderofthisreportfocuses
OntheperiodfromMay19toAugust31．
The main objective of the field measurements

AtA4，thesurfacewascoveredwithsnowthroughout
thestudyperiodandthefactorismuchsmallerthan

thoseatTA2andTA3．Thesma11erdegree−dayfactor

atA4cannotbesimplyattributedtothesnowcoverin
thiscasebecauseTA2andTA3showedmuchlarger

Valueswhentheywerestillcoveredwithsnow．Plau−

Sibleinterpretationisincreaseinsurface albedo due

tofreshsnowcover．FromMaytoJuly，SnOWpreCipi−

tation took place from time to time on the glacier．
Albedowasexpectedtobehighafterthesnowfalland
SuCh condition may havelastedlongerin the upper
WaStOfindthepositivedegree・dayfactorthatrelates reachoftheglacier．Thereductionofenergyincome
ablation to PDD measured at each site．The total
throughradiationlikelykeptthedegree−dayfactorat
ablation，A，0Veraperiodisgivenby，
A＝かPDD，

A4smaller．

As the conditions of the debris layer are not（1）

Spatia11y uniform，itis crucialto evaluate the re−Wherekisthepositivedegree−dayfactorandPDDisthe sum of daily mean air temperatures above themelting point during the period．Figure6shows theplotsofcumulativeablationtocumulativePDDatthe

August23was measured at6sites shownin Fig．1．

reconstructedfromthedataatTA2andTA3withtheplotted to the altitudeinFig．7．Thereis no clear

measurement sites．For A4，air temperature waspresentativity of the degree−day factor obtained at

TAl．BesidethefrequentmeasurementatTAl，abla−tion under the debrislayer betweenJune 29 and

The degree−day factors obtained for those sites arelapserateobtainedabove．ForTAl，Onlythemeasure− dependenceonthealtitudeandtheablationispresum−

ments after the surface turned from snow to debris

COVeredwereplotted．Linearitybetweenthecumula−

tiveablationandcumulativePDDateachsiteisfairly
Well，andthechangesinthesurfaceconditionsfrom
SnOWtOiceatTA2andTA3arenotdistinguishable．
︵
．
の
こ
≦
∈
︶
u
O
竜
三
e
聖
葛
ち
∈
コ
0

ablycontrolledbythelocalconditionssuchasdebris

thickness，rOCksize，Surfaceslope，andsoon．Because

Of the frequent ablation measurement and reliabletemperature data，the degree−day factor at TAlis

assumedto representalldebriscoveredregioninthe

next section．

5
4

︵
l
ゝ
巾
P
T
￥
甲
≧
∈
∈
︶
一

4
3
2
1

0
0

200

400

600

2

2250

Fig．6 Cumulative ablation versuscumulative PDD from
June14toAugust28atTAl（solidsquares），fromMay
19toAugust28atTA2（opencircles），fronlMay19to
August27at TA3（solid circles），and fromJuly3to
August27atTA4（opensquares）．

2300

2350

Fig．7 Positivedegree−dayfactorsobtainedondebriscov−eredsurfacesbetweenJune29toAugust23．Solidline
indicates the value obtained at TAl．

Tablel．Positivedegree−dayfactorsobtainedforthemeasurementsites

Site k（mmw．e．K￣1day▲1）Altitude（ma．s．l．） Period Surfacecondition

TA1
TA2
TA3
A4

2．98
9．81
8．93
4．45

2400

A旧tudo（m）

CumulativePDD（Kday）

Jun14TAug28
May19−Aug28
May19−Aug27
Ju13・Aug27

DebrisSnow→Ice
Snow→Ice
Snow45

Sugiyama

4．Ablation calcu］ation
4．ノ．肋〟わdPositive degree・day factors obtained at TA2，

TA3，A4 areinterpolated to obtain an altitudinal
distribution on snow andice surfaces between2230

and2670m a．s．1．．The values at TA2and A4were

given as constants for the regions lower and upper

thanthe aboveregion，andinfluence ofthis assump−tionislessimportant because the surface area oftheseregionsislimited．Foradebris−COVeredsurface，

︵
∈
︶
￠
P
⊃
雲
巾
￠
U
；
き
0
∪
め
l
U
O
届
∪
巴
ト

3000
2800
0

260
0

0
0
0

2400
0

0

つつ
ー
︵
2U

May JuneJu［yAugust2001

18

the factor was assumed to be constant and the value
ObtainedforTAIwasusedatanyaltitude．
Daily mean temperature at each altitude wasdetermined from the measured temperature and theFig．8 0bserved and calculated transitions of transient
snowline altitude．

ture at TA2 and TA3 are used for snow and ice

︵
T
訂
p
c
∈
も
こ
u
O
選
q
＜

lapserateobtainedintheprevioussection．Tempera−

Surfaces，andthatatTAlisusedforthedebriscov・
eredsurface．BeforeJune7，Whenallthetemperatureloggerswereabovesnowsurfaces，temperatureOna
debris，COVeredsurfacewasestimatedbytheempirical
relationship between the temperatures on the debris
COVeredanddebrisfreesurfacesfoundafterJune7．
Tospecifythesurfaceconditionateachaltitude

4
3
2
1
0

1900

2100

2300

2500

2700

2900

Altitude（m）

OneaChday，thesnowdepthdistributionatthebegin−ningofthemeasurementsmustbeprescribed．Snow− Fig．9 Altitudinaldistributions of daily ablation calcu−

icetransitiondatesatTAlandTA2，andsnowdepth

measurement atTA30nJune15were usedto calcu・
1atetheinitialsnowdepth．Theresultsandtheinitial
SnOWline altitude werelinearlyinterpolated，and
extrapolatedtotheupperreachoftheglacier．
Thedailyablationataltitudehonthedaytwas
Calculatedby，α（ゐ，′）＝斤。（ゐ）n（ゐ，り（5。（ゐ）＋S。（ゐ））for∑α（ゐ，J）＜晶（ゐ），（2）′＝0α（ゐ，り＝ん（ゐ）n（ゐ，′）5。（ゐ）＋斤。（ゐ）7も（ゐ，オ）S。（ゐ）ffor∑α（ゐ，f）≧d（ゐ），（3）1atedforMay30（thicksolid），June25（dotted），July22（broken），andAugust23（solid）．

temperatureconditions．The ablationwascalculated
every20mwiththesurfaceareadistributionshownin

Fig．4．Daily mean temperatures averaged between
TA2andTA3are6．35，6．39，6．58，and6．130Cfor
30，June25，July22，andAugust23，reSpeCtively．Itis
obvious that the ablation was reduced at the altitude

Of2100−2300m a．s．1．from May toJune，and at thealtitudeof2300−2400ma．s．1．fromJune toJuly．The

minorchangefromJulytoAugustisaccordantwith
the facts that the debris layer exists mainly below

2400m a．s．1．（Fig．4）and the snowline passed thisWhere T，k，S，and（あaredailymeanairtemperature，altitudeinJuly（Fig．8）．Totalablationwasreducedby
positive degree−day factor，Surface area，andinitial lO％fromMaytoJuneand21％fromMaytoAugust．
f＝O

SnOWdepthwithsuffixes cfordebrisfree and d fordebriscoveredsurfaces．a（h，t）＝Owhen T（h，t）≦OOC．
4．2．月β5〟ぬDaiIyabiationatevery20maltitudinalrangewas

Toevaluatethetotalreductioninablationduetothedebrislayerduringthestudyperiod，COmputation
WaS repeated with an assumption of entirely debris

free surface．In this experiment，Eq．2 was usedregardlessofthesurfacecondition．Altitudinaldistri−COmputedfromMay19toAugust31byEq．2and3．In
butionsofthetotalablationfromMay19toAugust31
Fig．8，tempOralchange ofsnowline altitudein the are shownin Fig．10．They sugge§t that ablationis

COmputationiscomparedwithfielddata whichwere
measuredindependentlyofthoseusedtosettheinitial

reducedsubstantiallyintheregionlowerthan2400m

tation and the observation indicates that the surface
conditions are wellreconstructed．
Figure9shows altitudinaldistributions of abla−tiononfourdaysindifferentmonthsbutwithsimilar

Theinsulation effect of the debrislayer becomes
noticeableinlateJuneandtakesanimportantrolein
July and August during the mid summer．The total
COmputed ablation of the glacier throughout thea．s．1．duetothe debris−COVeredsurfacecondition．The

SnOWdepths．Thegoodagreementbetweenthecompu−SeaSOnalvariationsofablation areshowninFig．11．

46

BulletinofGlaciologicalResearch

altitude of1900m a．s．1．This altitude was compared

5

︵
の
∈
ト
○
こ
u
O
葛
−
q
巾
空
室
⊃
∈
⊃
U

4

︵
M 2
∈
も
こ
u
O
毒
q
＜

3
2

1900

2100

2300

2500

2700

2900

A肌ude（m）

Fig．10 AltitudinaldistributionoftotalablationfromMay
19 to August 31 calculated with the consideration ofdebris cover（grayline）and with the assumption ofentirely debris−free surface（blackline）．Ablation ateach altitudeis cumulated and plotted with the solid
Circles（debriscoverconsidered）andopencircles（debris
freeassumed）fortheordinateintheright．

with terminus altitudes of the otherl16glaciersin
Switzerland that were reported byIAHS（ICSI）・
UNEP−UNESCO（1998）．1900m a．s．1．is muchlower
thanthe totalmean of2310m a．s．1．（q＝320m）and

Only13glaciersterminateataltitudeslowerthanthe
terminusofUnteraargletscher．Terminusaltitudeofa
glacier can be related to the glacier size，but themedianaltitudeofUnteraargletscher（2660ma．s．1．）is
Stilllowerthanthemean（2890ma．s．1リ0・＝260m）of

thel16glaciersanditiseighteenthfromthelowest．
ThesignificantlylowerterminusofUnteraargletscher
infers that its retreat rate has been reduced by theinsulation effect of the surface debris at the lower

reachoftheglacier．

The positive degree・day factor obtained fordebris−COVered surfacein this study，and the values

reportedforthreeglaciersinHimalayas（Kayasthaetal．，2000b）arelistedin Table2．Itis not straight−

5
︵
ゝ
召
の
∈
も
L
︶
u
O
葛
一
q
＜

6

3
4

2

つ
0
︵
U
O
O
O
O

︵
の
∈

． ト
4 ○

8

May June

July August
2001

1

こ
u
O
葛
一
q
冨
＞
焉
一
⊃
∈
⊃
U

0

Fig．11Theresultsofthesamecalculationas Fig．10but
Showingtheirtemporalvariations．

forwardtocomparethefactors，buttheratiobetween

factors for debris−COVered and debris−free surfaces

maygive anidea toevaluatetheeffect ofdebris on
eachglacier．ThesmallestfractiononUnteraargletsT
Cherindicatesgreaterinsulation ofdebrislayer，and
this may be attributed to the thicker debrislayer．
ThermalreglmeCharacteristictoeachglacierisalso

Veryimportant．Oneoftheexamplesisgreatercontri−

bution of net radiation to energy balance on
Himalayanglaciersathigheraltitude．Underidentical
air temperature conditions，debris surface tempera−

turemaybehigherwhenthelocalenergybalanceis

dominatedbyradiationratherthanby othercompo・

periodis3．56×107m3when the debrisis considerednents because ofitslow albedo．Ifitis assumed thatwhileitis4．70×107m3when the surfaceis assumed
ablationunderdebrislayerisrelatedtodebrissurface
entirelydebrisfree．Theresultspredict32％increase temperature，Smaller insulation effect of debrisis

inablationifUnteraar−andLauteraargletscherwere
notcoveredwithdebrislayer．
5．Discussion

Quantitative analysis of ablation in this study
revealedthat thesurface debrislayeris acting as a

goodinsulator on Unteraar−and Lauteraargletscher．

Suchinfluenceofdebrisshouldhavetakenakeyrole

inthelong−termeVOlutionoftheglaciers．Unteraarg−

letscher has been continuously retreating since 1871
（Haefeli，1970）and the current terminuslies at the

expectedundergreaterradiation．
Validity of the positive degreeTday method wasassessedbylookingatthesnowlinealtitudeinFig．8．
Thelimited number of measurement sites，however，

introduces errors in the estimations of the positive
degree−dayfactorandtemperaturedistributions．Fig−

ure 7indicates that detailspatialdistribution ofdegree day−factor on the debris−COVered surfaceis

requiredtoimprovethequalityofthecalculation．Itis
alsoneededtoverifytheconstantlapserateassumed
allover theglacier．Character of the temperature
measurementusedinthisstudyshouldbealsodiscus−

Table2．Positivedegree−dayfactorsobtainedforvariousglaciers．kdandkcarefactorsobtainedfordebriscovered
anddebrisfreesurfaces．OriginaldatainLirungandRakhiotGlacierswererecalculatedbyKayasthaetal．（2000b）．

Unteraargletscher
KhumbGlacierLirun女Glacier
RakhiotGlacier

AltitudeDebriskd
kc（ma．s．1．） thickness（m）（mmw．e．K￣1day▼1）
2230
0・1−0・2
2．98
9．81
5350
0．1
11．1
16．9
4350
0．1
5．5
6．6
3350
0．1
3．5
6．6

Reference
0．30
0．66
0．83
0．53

This study
Kayasthaetal．（2000b）
Ranaetal．（1998）
MattsonandGardner（1989）

47

Sugiyama

sed．Accordingto Ohmura（2001），the close relation・

shipbetweenairtemperatureandablationisbasedon
themajorcontributionoflongwaveatmosphericradi−
ationtosurfaceablation．Itimpliesthatthelocation

Oftemperaturemeasurementforthepositivedegree・

day method should be determined considering thedetails in the processes of energy transfer on eachindividualglacier．Thus，further study onthe energybalance overdebrisisneededforbetterimplementa−

tionofpositivedegree・daymethodtodebris−COVeredglaciers．

6．Conclusion

Theinfluenceofasurfacedebrislayeronsummer
ablation in Unteraar‑ and Lauteraargletscher was
Studiedbyfieldmeasurementsandpositivedegree−day

method．The obtained positive degree−day factors

showedinsulationeffect ofthedebrislayerwhichis
more efficient than those reported in Himalayan
glaciers．The reconstruction of summer ablationin
2001indicated that ablationis reduced substantially
duetothedebrislayerattheregionlowerthan2400

ma．s．1‥Thereductionwasenl1anCedasthesnowline

altitudeascendstheglacierinJulyandAugustduring
theintensive ablationseasonintheSwissAlps．Itis
predictedthattotalsummerablationwouldincrease

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funded by The Swiss National Science Foundation

throughgrant No．2100−063770．00 and TheInoue

ScientificFieldStudyFoundation．

−46．