J ournal
of Glaciology, Vol.
41, No . 137, 1995
Monthly balance and water discharge of an inter-tropical
glacier: Zongo Glacier, Cordillera Real, Bolivia, 16° S
BERNARD FRAf\C O U, PI ERRE RIBSTEI N,
L' Institut FranJais de Recherche Scientifiqlle pour le D eveloppement en Cooperation, CP 9214, La Pa;:., Bolivia
RONALD S ARA VIA,
Compania Boliviana de Energia E lectrica , CP 353, La Pa;:., Bolivia
ERI C TIRI AU
L' lnstitut FranJais de Recherche Scientifique pour le Developpement en Cooperation, CP 9214, La Pa;:., Bolivia
ABSTRACT . M eas urem ents of m ass bala nce we re performed eve ry m o nth on
Zongo Gl acier, Bolivia. Simulta neously, wa ter-discha rge, tempera ture a nd precipitation d a ta we re obtained. The first yea r of the survey, 199 1- 92, was m a rked by a n
ENSO (El Nifio- So uth e rn O scill a ti o n ) eve nt with hig h tempera tu re a nd low
precipitation , whilst th e following yea r, 1992- 93, was n o rm a l. R es ults p o int to the
earl y a nd la te we t seaso n (O ctober- D ecember and M a rch- M ay) as pl aying a criti cal
role in the d etermin a tio n of the annu al m ass balance . Th e we t seaso n is th e wa rmes t
period of the year a nd co nsequ entl y th e dura ti on orth e we t seaso n is a highl y rel eva nt
va ri a bl e in d etermining m ass balance . Bo th g laeiologica l a nd h ydrological m eth ods for
th e d etermin a tion of the m ass balance p rovid e simil ar r es ults. Our stud y co nfirms th a t
ENSO events have a m aj o r inOuence o n th e ra pid glacie r retreat currentl y a ffec ting
this pa rt of the And es.
INTRODU CTION
Very few studies h a ve b een perform ed on th e m ass
bala nce of glaciers situ a ted between the tropics . The only
studi es so fa r published a re of Lewis Glacier in K en ya
(Hastenra th , 1984, 1989 ) a nd of two g laciers in the
Curdill er a Bla nca, Peru (Ames, 1985; K ase r a nd others,
1990) . Th ese studies wer e co nducted b y collecting d ata
on a yea rl y basis and wer e not coupled with systema ti c
runoff m eas urements.
H owever, th e coincid en ce at low la titud es between the
wet seaso n, favo ura bl e to acc umul a tio n , a nd the wa rm
period , wh en a bla tion is m aximum , m a k es it difficult to
acquire kn owledge of glacier mass ba la n ce fr om simple
annu a l d a ta . Furth ermore , th e precipita ti o n distribution
is fairl y va ri a ble from yea r to yea r a nd th e dura ti on of the
rain y sea o n is ye t more unpredi cta ble. The glacier may
be ex p ected to react sig nifi ca ntl y to thi s va ri a bility.
Thu s, if we want to und ersta nd th e processes th a t
co ntro l g lacier mass b a la n ce in th e e regions, it is
necessa ry to conduct m ea surement on a m onthl y bas is
a nd fo ll ow up th e survey over a peri od o f seve ral yea rs.
With su ch a n approach , it is possible to a tta in a greater
und ersta nding of the main factors that control th e rapid
glacie r retreat prese ntly occ urring in thi ' p art of th e
And es (Lli bo u try a nd others, 1977 ; Ames , 1985; ] ord a n,
1991 ) , as well as in o th er low-I a ti tud e mo un ta in s
(Hastenra th a nd Kruss, 1992 ). Anoth er qu es ti on needing
to be addressed is the consequ en ce or an Er SO (El Niiio
So uth ern O scill a ti on) event fo r th e glacier m ass b a la nce .
Acco rding to th e stud y a t th e Qu elccaya I ce C a p , Peru
(Th om pson a nd others, 1984 ) , E N SO even ts co uld a ffect
mass ba la n ce b y d ecreasing sn ow acc umula tio n a t high
a ltitud e in th e And es. Neverthel e s, the ma rked reduction
in precipita ti on d uring th ese e pisodes occ urs during the
wa rm sea on , so th e effects o n th e mass bala nce co uld a lso
be associated with a strong a bl a ti on (Fra nco u, 1992 ) .
Our in vestiga ti on has co n ce ntra ted on two glaciers in
th e Cordill era R eal, Bolivia : Z o ngo Gl acier a nd Ch acaltaya Gl acier. This p a per presents the da ta collected on
Zongo Gl acier during th e first two hydrological yea rs of
th e stud y, fro m September 199 1 to Au g ust 1993 . It
repre ents a first a ttempt to prese nt a monthl y pattern of
glacier m ass b a la nce in th e trop ics a nd to a n a lyse the
effect of a n E N SO event.
AREA DESCRIPTION AND D ATA-GATHERING
METHODS
Th e glacier l S situ a ted in th e Hu ayn a P o toS! massif
(6088 m), a t 16° S, in th e Am azo n basin. Thi s vall ey-type
glacier is 3 km long and h as a surface a rea of 2.1 km 2
(Fig. I). Th e upper rea ches a re ex posed to the so uth
wh ereas the lo wer secti on faces ea t. Th e m axim um a nd
minimum eleva tions are 6000 a nd 489 0 m a .s.!. , resp ec61
Journal of Glaciology
•
Huayna Potosi 6088m
"
@- Huayna Potosi
165
~Cha~~ya
Lapaz~
6B W
1
• 3
o
1000 m
2
= = =---
D.
4
5
T 7
+6
R 8
**
Fig. 1. Huayna Potos!' and ,(ongo Glacier with the survey system in 1993. I, Princij)(il peaks; 2, Limits of basin;
3, Stakes; 4, Pits and crevasses; 5, Water-level recorder; 6, Rain-gauges; 7, Thermograph; 8, PyrarlOmeter.
tively. The accumul ation zo ne a mo unts to 60% of th e
tota l a rea under stead y-state co nditio ns (a nnua l mass
balance ap proaching ze ro), at a n eq uilibrium-lin e
altitud e (ELA ) of 5 100 m. U nd er similar co nditio ns,
avc rage precipitation a t the ELA totals 1.0 m a- I and th e
average tempera ture is - 2°C. Thus, it can be dedu ced
that this glacier is mainly temperate with a limited cold
surface nea r the summit.
A 15 stake network was pl aced in the a blation zon e in
order to determine the mass bala nce a nd the surface
velocity, a ugmented by a further three stakes and several
pits in the accu mul a tion zone fro m 5500 to 5800 m
(Fig. I). The 15 stakes in the abla tion zo ne were surveyed
ever y month a nd th e surface density of the snow a nd ice
was m eas ured in order to obtain th e a mount of
acc umula ti on and a bl ation in water equivalent. The
acc umula tion in the upper reach es of the glacier was
reco rd ed immediately before and after the wet seaso n , in
September and April. Annu al net balance was evalu a ted
between the beginning a nd end of the h ydro logical year,
September- August. For the ice falls not surveyed by the
stake network (th e 5050--4950 a nd 5500- 5300 m altitude
ranges), net balance was estim ated by linear interpolation. Mass balance was a lso computed independ ently by
means of a hydrological meth od (Ribstein a nd others, in
press), whereby we have the possibili ty of verifying the
m ass -b alance valu es obtain ed by the glaciologica l
me thod . In ord er for this to be done, a water-level
recorder was set at 4830 m (150 m below the glacier
terminus ), providin g continuous runoff meas urements.
Five storage rain-gauges, surveyed monthly, were p laced
near the glacier at 5200 and 4900--4850 m . These raingauges, with a cross-section of 2000 cm 2 , have proved to
62
be suita ble for meas uring snow precipitation a nd have
shown an increase of 20% over the precipitation levels
indicated by the lone classica l 3 14 cm 2 rain-gauge in place
since 1970 on Plataform a Zongo (4770 m ) .
In ord er to obtain estim ates of th e parameters
co ntrolling glacier ablatio n, thermographs with radiation shields were pl aced at 5200, 4830 and 4770 m. M ore
recen tly, in 1993 , pyra nometers capa ble of meas urin g
global radiation were sited at 5200 and 4830 m, with a
radiometer placed a t 5200 m so as to record the net allwave r adiation.
Every year sin ce 1991 , a topographic survey has been
condu cted in August o n the 15 ablation stakes and on the
terminus contour to give estim ates of the surface velocity
and of th e terminus flu c tu ation. During the first year of
the stud y, the measured glacier retrea t was 10- 20 m ,
whereas during the second year the glacier terminu s was
stationary .
The easy accessibility of the glacier all the year round
makes it possible to cl aim high levels of acc uracy for d ata
co llected during the 2 years. But the progressive adj ustment of th e survey network during this time does not
all ow us to use all the collec ted data in this paper.
MONTHLY NET BALANCE IN THE ABLATION
ZONE, 1991-93
Th e climates of 1991 - 92 and 1992- 93 present very different p a tterns. The first year was marked by maturephase ENSO conditi on s (Kousky, 1993 ) . V a lu es of the
so uth-oscill ation ind ex (the differe nce betw ee n the
standardised sea-level pressure anomali es at T a hiti a nd
Francolt and others: M onthly balance and water discharge
D a rwin ) were strongly nega ti ve, with a minimum - 2.2
sta nd a rdised valu e fo r th e 5 month running mean. This is
the lo wes t monthly valu e sin ce th e 1982- 83 episod e,
whi ch h as been d esc ribed as a ve ry strong event. Su ch a
situ a ti on in th e Pacifl c is associa ted with low precipita tion
levels in th e central And es . This o bservation was flrst
pointed o ut in the Quelccaya I ce Ca p b y Thompson a nd
oth ers ( 1984 ), and a n a lysed furth er b y Franco u a nd
Pizarro ( 1985) and T a pley and W aylen (1990), using
d a ta co llec ted fro m m eteorological statio ns.
Th e second yea r was quite norma l. T emperatures
reco rd ed O\'e r a period of 30 a a t the Chacaltaya sta tio n
(523 0 m ) , 10 km south of Zongo Gl acier , a nd precipita tion co llee ted over a 20 a period a t Pla taform a Zongo
(4 770 m ) ena ble us to calibra te each m o nth during both
obser ved years (Fig. 2) . Thus, a clear positive devia tion of
th e m aximum temper a ture co uld be o bse rved fro m
J a nu a r y to \-l ay during the first yea r , linked with a
high er ra dia ti on input a nd a defl cit o f ra infa ll. Durin g a
norma l hydrological yea r, 7 months h ave more th a n
50 mm of precipita ti o n , whereas in 199 1- 92 onl y 4
exceed ed this tota l.
of Z ongo Glacier
with April a nd M ay sh o wing limited abla tion. Th e
vari ation in the balance from 5200 m to the terminus for
every m o nth of th e flrst year of the stud y is presented in
Figure 4a. Imm edia tely b efore and a fter th e J a nuaryMarch period th e major pa rt of the glacier exp eri enced a
strong a bl a tio n regim e, even right up to hig h eleva tions.
On the oth er h a nd , during a normal year such as 1992- 93
(Fig.4b ), the monthl y ba la nce switched r a pidl y to a
positive valu e. Whi lst awaiting d ata ove r a lo nger period
of time, we can alread y stress that O cto ber- D ecember
and M ar ch M ay a re decisive in the determina ti on of th e
annu al m ass b ala nee. I n th e ce ntral And es , th ese are th e
months when va ri ability o f clim a te is a t a m aximum.
H oweve r, it is o bvious th a t th e co ld , dr y m o nth s of JuneAugust d o h ave a limited influ ence at yea r scale, since the
mass bala n ce during th ese m onth s approac h es ze ro.
ANNUAL NET BALANCE AND WATER
DISCHARGE
For each year , th e net ba la n ce f3n has bee n calcula ted by
integra ting th e mass bala nce over the to tal surface area S,
acco rding to Eq ua tion ( I ).
3 .0
(1)
2 .0
c
0
'"'5'"
where Se a nd Sa a re th e a reas co mputed for th e
acc umula tio n a nd a bla ti o n zo nes, respec ti vely.
1.0
(I)
"0
~
!ii
"0
0.0
"
;;
-1.0
a
-2.0
0.0 4
I
O.
>-
."
E
0
!ii
I
-0. 04
-0.08
b
-0. 12
S
0
N
1991
0
I
J
F
M A
M
J
O ND 1 J
1992
F
M AMJJA
1993
Fig . 2. Standard dev iation of (a ) maximum and minimum
1991- 93 monthly temperatures from the 1953 93 mean at
Chacaltaya (5230 m); ( b) monthly 1991- 93 precipitation
fro m the 1972- 93 mean at Platafa nna Zongo (4 770 m) .
As presented in Fi g ure 3, th e m onth ly evo luti on of
glacier ba la nce in th e a bla ti on zone is m o re co mplex th a n
may be inferred from th e simple we t season/dr y season
a ltern a tio n typi cal of th e tropi cs . Durin g th e ENSO event
of 199 1 92 a markedl y negati ve net ba la nce occ urred in
O cto ber Decem ber a nd M a rch- M ay, just before and
after a sh ort period of acc umu lation Ua nu a ry- Febru ary ) .
Durin g th e ave rage yea r 1992- 93, nega tive mass ba la nce
was limi ted to th e beginning of the h ydro logical year ,
O ve r th e two yea rs, th e spec ific net ba la nce (f3n ) per
year was:
In 199 1 92 : 1. 38 m of wa ter
in 1992 93 : + 0. 02 m of wa ter.
Th e ELA was situ a ted in 199 1 92 a t 53 00m a nd in
1992 93 a t 5 100 5 150 m (Fig . 5). In the sam e fi gure, we
note th a t: ( 1) th e curves a re m ore-or-I ess parallel from on e
yea r to th e o th er, as alread y obselTed on m a n y middlela titud e g lac iers, first by M eier a nd T ang bo rn ( 1965 ) and
LJiboutr y ( 19 74); (2) durin g the ENSO eve nt of 1991 - 92
th e amount o f acc umul a ti o n a t hi gh eleva ti o n is ve ry low
and this conflrms th e o bse rva ti ons of Tho mpson and
oth ers (J984) in Qu elecaya . In such a case, even a glacier
which lik e Z o ngo Gl acier ex tends o\-c r a g rea t altitud e
range may be a lm os t completel y submitted to a n a bla tion
regim e.
As indica ted in Figure 6, th e ENSO eve nt 199 1- 92
was cha rac te rised by hig h run off depths from O cto ber to
Dece mber , with tbe December maximum 100 mm (33% )
higher th a n during th e fo llo win g yea r. Furthermore, a
seco nd run off peak occ urred in M arch 1992 , with runoff
then rem a inin g high in April a nd M ay . This conflrms
th a t a mark ed d efi cit in th e precipita tion la te in th e wa rm
season, .r--.1a rc h Apri l 1992, a llows a rapid m elting of th e
snow cove r acc umu la ted in J a nu a ry- F e bruary. Th e
wa ter d isc h a rge a t the o utle t of th e 3 km :! basin was
5.38x 10 6 m 3 in the first year a nd 3.24 x 10 6 m 3 in th e
seco nd. C o nsid erin g th e ave rage precipit a ti on (P )
co ll ec ted in th e rain -ga uges, th e tota l precipitation
received b y th e glacier was 0. 9 16 m in 199 1 92 and
1.060 m in 1992- 93. We m a y ass um e that th e a rea outsid e
th e glacier, 0.9 km 2 , has a runoff coeffi cie nt of 0.8. This
coeffi cient is quite importa nt for a free g la cier ca tchment
63
Journal of Glaciology
0.2
~--~~--,----,---,----,---,----,---,----,---,----,---. 0 . 2
I\
,"\
0
~
'"~
Eo<
-0 . 2
Ii<
0
:>:
'"u
~
~
f\vj\~ ~I /
- 0.4
~
- 0.6
V
\~ //
- 0.2
0:
'"~
Eo<
\/
II
\V
I
i \/ \ ""
) , 1\
/
/~
/ 1\
!.
- 0.4
Ii<
0
:>:
,/
- 0.6
!Xl
- 0.8
- 0. 8
_ 1 ~-,~~.-+-,-~-,-+-,--~,-~-,~-,~--,-+-.-4--.-+-,-L _ 1
9 . 91
5 . 92
1. 92
11.91
3 . 92
9 . 92
7 . 92
1.93
11. 92
5 . 93
3 . 93
7 . 93
MONTH
[ -
STAKES 1·12 .... _.. STAKES 13-15
Fig . 3. Monthly balance from September 1991 to August 1993 in the ablation zone, calculated using two groups of stakes.
Stakes 1~12 are situated near the ELA (5200~5100m) and stakes 13~ 1 5 are near the glacier terminus (4900 m ). The
balance was estimated from a monthly stake reading and a density measurement of the snow /ice surface.
in this Andean region (Bo urges and others, 1992 ). H ence,
the water d ischarge corresponding to th is area was
0.66 x 10 6 m 3 in 199 1~92 and 0.76 x 106 m 3 in 1992~93.
Th us, the amount of melted snow and ice corresponding
to a 2. 1 km 2 surface area is:
in 199 1 ~ 92 : 2.25 m
in 1992- 93 : 1.18 m .
T he specific net balance ((3n ) and the precipitation (P )
being known, the total ablation (A ) is deduced from
Equation (2) :
a
5200
5100
5000
A
,.
,,900
fl
-.5
-1.
.5
.0
=
(2)
P - (3n '
m
In
1 99 1 ~92:
tn 1 992~93 :
5600
Z{m)
2.30m
1.04 m.
Area km 2
Z{m)
b
6000
0
t.
5
~~=:=::::=~~--r---'~--r--,
\\
5200
5500-1---.'
5100
5000
4900+------------,--------~--~----------~
-1.
:5
.0
.5
Pn
m wa t er
F ig. 4. Monthly net balance ((3m in m of water) as a
function of altitude ( Z ) during (a) 199 1~92, and ( b)
1992- 93. Numbers 1~ 1 2 refer to the months. Solid line:
S eptember, October, November, April, May. D ashed line:
Jun e, Ju ly, August. Dotted line: D ecember, January ,
February , March.
64
-7
-6
-5
-4
-3
-2
-1
0
1
2
3
Fig. 5. Annual net balance ((311 in m of water) as a
function of the altitude Z during 1991~92 and 1992~93. It
should be noted that the ELA was at 5300 m in 1991- 92
and at 5100 m in 1992~93.
Francou and others: M onthly balance and water discharge of Zongo Glacier
r--~============~-I'S
S60
a
SOD
(
mm
ext. terr. radia .
-
ra te of O. 74°C 100 m I calculated be twee n th e
two th e rmogra phs a t 4 770 a nd 5200 m , this 3°C
tempe r a ture is equiva lent to a positi ve temperature o ve r prac ti ca ll y a ll o f th e a bl a ti on zo n e;
(3) the precipita tion coll ected a t 4770 m: P in m
(T a ble I ; Fig. 6b ).
mean time T> 3 C
.......- 12
.
N
E 450
~
c
.
Jl
400
'6
i!!
350
With th e specifi c net bal a nce es tima ted in th e a bla tio n
zo ne of the g lacie r (f3na in m o f w a ter (T a ble I; Fig . 6 b )) ,
th e linear regressio n using th e m o nthl y d ata is:
300
0.'
b
[D
rainfall -
runoff
S
f3n a = - 0.07565(c1 T3 - 0.0032 9( H o) + 1.74P + 1.413
wit h 1'2 = 0 .84, n = 24,
(3)
,j;
0.
-1!
wh ere 1'2 is th e co rrela tion coeffi c ie nt a nd n is th e number
o f d a ta .
~
1ii
~
0.1
It m ay be n o ted th a t dT3 is th e b es t co rrela ted vari a bl e
(r1991
1992
1993
Fig . 6. ( a) Extraterrestrial radiation and mean of hours
per day with temperature above :ye, and (b) minfall and
monthly runoff depth, on Zongo Glacier during the 2years
of the stU{ly .
Conseq ue n tl y, it should b e n oted th a t th e g laciological
a nd h ydrological ba la nces give simil a r res ults. Th e loss by
sublim a tion see ms to be ve r y sm a ll , lower th a n 10 % of the
tOLa l a bl a ti o n . Consid e rin g th e d a ta acc uracy, th e
sublim a tion cann o t b e d e termin ed by m ea ns of th e
h ydrolog ic a l ba la nce . This is co nsistent w ith th e low
va lu es of sublim a ti on o bta ined by H as tenra th ( 1978) a t
th c summit of th c Quelccaya I cc Ca p.
FACTORS CONTROLLING THE GLACIER
BALANCE
Whilst a w a iting th e o ppo rtunity to m od el th e glacier
a bla ti on from recentl y insta ll ed tempera ture a nd radi ati on rccord s, we can make a prelimin a r y a ttempt to
asce rtain whi c h va riables a r e importa nt in es tim a ting th e
specific n e t-ba la nce evolution in th e abla tio n a rea of the
g lacier (i.e. b etween 5200 and 4890 m ).
A stepwise multiple lin ear regression was a ppli ed to a
la rge numbe r of possibl e ind epend ent va ri a bl es. From
a mongst a ll of th e ind e pend e nt va ria bles, th e best res ult
o btain ed pinpointed the foll o win g three as b eing th e mos t
criti cal:
( I) the a m ount of r a di a ti o n from th e Sun arri ving a t
th e outer ed ge of th e a tmosph ere, a lso called
extra -terre trial ra di a tio n: Ho in \V m 2. This is
consta nt a t year scale, a nd obvio usly h as a g rea t
inf1u e nce a t hig h a ltitud es a t thi s low la titud e.
Th e m onth ly radiatio n va lu es (Tab le I ; Fig. 6a )
we re ca lcula ted u sing th e equation s of Pa ltridge
a nd Pl a te ( 1976 );
(2) the ave rage d a il y dura ti on of a ir te mperature
g reater th a n 3°C a t Pl a ta form a Z o n go (4770 m ):
dT3 in h d I (T a ble 1; Fig. 6a ). Gi ve n th e lapse
= -0.72 ) .
With th e run off depth Q (in m ) as th e d e p end ent
va ri a ble (T a b le I ; Fig. 6b ) , th e b est regressio n was
ob ta ined using th e Ho a nd dT3 va ri a bles :
Q = 0.00128( Ho) + 0.023 58(c1T3) - 0.538
wi t h 1'2 = 0. 86, n = 24.
Thi s tim e, t h e va ri a bl e w ith
coefIi cient is Ho: l' = 0. 80.
(4)
th e bes t co rr e la ti o n
These res u lts suggest the fo ll owin g three co mm ents:
(I) T emper a ture a nd ra di a ti o n closely con tro l the
a bl a ti o n in th e lower pa rt o f Lh e glacier .
(2) Th e g reates t a mount of sola r ra di a ti on, leadin g to
th e melting of th e grea tes t qu a ntit y of sn o w a nd
ice, occ urs near th e tim e o f th e summ er sol sti ce.
Th e refore, we ca n begin to unde rsta nd w h y a
defi c it in p rec ipita ti o n , a nd h enee less fr eCju elll
cl o ud- cove r durin g this p e ri od , leads to the m os t
sig nifi ca nt m ell. Acco rdin g to th e obser va tio ns of
Ribste in a nd o th e rs (in press ), th e hi g h es t
disc ha rge reco rded a t t h e limnimet ri c sta tio n
occ urred a fter a 10 d dr y pe ri od durin g th e wa rm
season (O c to ber M a rc h ) . It is clea r th at dry
pe ri od s durin g th e w a rm seaso n a re m o re
frequ e nt durin g ENSO eve nts. It sho uld be
noted th a t th e a bla tio n produ ced durin g th e
dry seaso n (April- Septem ber ) ~\' i t h low ra di a ti o n
is no t ve r y sig nifi ca nt a t yea r sca le.
(3) Precipita ti o n appears to b e poo rl y correl a ted with
glacie r a bl a ti on a nd run off, since in ge n e ra l th e
precipita ti o n is of a solid fo rm . Acc umul a ted
snow a nd h a il have a melt time va rying acco rdin g
to th e clim a tic conditio n s. It might be possible a t
a la te r stage to determin e these co nditi o n s.
CONCLUSIONS
F o r th e first tim e, the coll ec ti o n of m o nth ly d a ta on a n
inte r-tropical g lacier a ll ows us to hig hlig ht the m os t
importa nt fac to rs th a t co ntro l m ass ba la nce a t lo w
la titud es. Th ese fac to rs a rc th e dura ti on o f th e we t
season within th e wa rm peri o d , and th e tempe ra ture
65
Journal oJ Glaciology
Table 1
Month
September 199 1
O cto ber 1991
1 ovember 1991
D ecem ber 1991
J a nu a ry 1992
Febru ary 1992
March 1992
April 1992
May 1992
Jun e 1992
Jul y 1992
August 1992
September 1992
O ctober 1992
November 1992
D ece mber 1992
J anu a ry 1993
February 199 3
March 1993
April 1993
M ay 1993
June 1993
July 1993
August 199 3
Balance I
R UIlOJJ2
Precipitation 3
dT 3
4
111
m
m
h
Wm- 2
0.378
-0.627
-0.528
-0.576
-0. 178
-0. 13 1
- 0. 507
- 0.671
-0 .6 24
- 0.008
- 0.162
0.000
- 0.165
- 0.388
- 0.329
- 0.144
- 0.009
- 0.222
- 0.089
- 0. 149
- 0.2 52
-0.090
-0. 130
+0.077
0.076
0.205
0.237
0.305
0.171
0.154
0.217
0.168
0.160
0.045
0.025
0.029
0.059
0.136
0.202
0.179
0.102
0.092
0.104
0.058
0.058
0.036
0.026
0.028
0.025
0.042
0.090
0.081
0.164
0.129
0.032
0.015
0 .001
0.018
0.021
0.045
0.021
0.060
0.090
0.123
0.249
0 . 100
0.146
0.063
0.013
0.003
0.004
0.046
4.9
7.4
8.3
8.8
5.2
4.4
8.3
9.7
11.0
5.9
6.0
3.7
5.1
4.7
5.9
3.4
3.5
6.1
4.0
6.4
8.1
8.5
6.1
5.2
403.8
439.2
455 .9
459.8
458.3
448.5
421.5
377.5
334.0
311.6
320.9
357.9
403.8
439.2
455.9
459.8
458.3
448.5
421.5
377.5
334.0
3 11. 6
320.9
357.9
Mean net specific glacier balance in the a blation zone 5200- 49 00 m.
Run off d epth related to the basin a rea of 3.0 km 2 recorded a t the
limnimetric station.
3 Precipitation at Pl ataforma Z o ngo, 4770 m.
4 D ail y mean of hours with temperature> 3°C at Plataform a Z o ngo, 4770 m.
5 Extraterrestrial radiation.
I
2
level during the sensitive period which preced es and
follows J a nu a r y-F ebru ar y, the h eig ht of the rainy
season .
H ence, it can be noted that ma s-balance d etermina tion i more co mpl ex for tropical glaciers than for their
cousins a t midd le latitud es , for which only the three
summ er months appear to be criti ca l. Since th e climate in
this part of the And es is so variable, a longer-term st ud y
wi ll be necessar y to better und erstand this subj ect.
Th e seco nd conclusion reached concerns the effect of
the ENSO events. In compa rison with the " norm a l"
1992- 93 hydrological yea r, 199 1- 92 (a n ENSO event)
w as cha rac terised by: (I ) a m a rked ly negative net
ba la nce, with a wate r depletion equi valent to twice the
a mount of the precipitation acc umul ated; (2) a n in crease
in th e elevation of the ELA by 200 m; (3) a reduction of
the AAR (acc umulation-a rea ratio ) from 86% to 58%;
(4 ) a very low acc umu la tion rate a t high altitud e.
The above effects a re a con seq uence of a m arked
incr ease of ablation du e to a rise in the m aximum
temperature during the summer p eriod , associa ted with
increased radiation input, a nd also linked to the d efi cit in
66
acc umulation ansll1g from a foreshorten ed season of 2
instead of the norm a l 4 months.
Thus the conclu sions reached by Th ompson a nd
o th ers (1984) from the stud y of the Qu elccaya I ce Cap
concerning a decrease in the acc umul ation during E:\fSO
events a re confirmed . Furthermore, we can ad d that the
ENSO effect modifies not only the acc umul a tion term in
th e m ass-balance eq uation, but a lso the ablation term,
p a rtic ula rly in the present case stud y. Th e observations
made during the 199 1- 92 ENSO event a re confirmed by
our analysis of water-level records from the Zongo Glacier
outlet over a 20 a period (Ribstein a nd others, in press) .
The records reveal that the highes t levels of runoff over
this p eriod were obtain ed during the three previous
ENSO events (1976- 77 , 1982- 83 a nd 1986- 87 ), with a
partic ula rl y high peak during the stron g 1982- 83 eve n t.
The co ntinu ation of the present stud y over a period of
sever a l years would clearly be benefi cia l in furthering our
und erstanding as to why tropi cal glaciers are currently
experien cing such rapid retreat. Of furth er use would be a
long-term stud y of m ass-balance data from variou oth er
Andean glaciers.
Francou and others: Monthly balance and water discharge of Zongo Glacier
It is a lread y widely acknowledged that g lobal a tmospheric warming is a cause of glacier retreat. H owever, oLlr
stud y sLlgges ts th a t another important factor is the
occurrence of short period s or warm , dry wea ther during
the rainy season, a combination that is frequ ently linked
to ENSO eve nts.
ACKNOWLEDGEMENTS
The glaciological prog ra mm e was initially supported by
Dr M. Servant (L' ln stitut Franc;:ais de R echerche Scientifique pour le Devcloppement en Cooperation ). For th e
fi eld m eas urements, th e eflorts or F. Quispe a nd th e
technicians of the Bolivian Power Company (COBEE )
are much a ppreciated. The comm ents ofDr L. R eynaud ,
Dr D. MacAyeal and an a nonymou s referee are also
gra tefull y ac kn owledged.
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67