Glacier inventory of the upper Huasco valley, Norte Chico, Chile: Glacier characteristics, glacier change and comparison with central Chile

Abstract

Results of a new glacier inventory of the upper Huasco valley, which lies within the arid Norte Chico zone of the Chilean Andes, are presented for 2004. Despite the high altitude, the glaciation in this region is limited in extent and is not classical mountain glaciation, which poses difficulties in completing standard inventory attribute tables. Small cornice-style ridgeline features constitute a large number of the non-transient ice bodies identified, and glaciers with surface areas <0.1 km2 comprise 18% of the glacierized area and 3% of the water resource stored as glacier ice within the Huasco valley. Rock glaciers are an important component of the cryosphere, comprising 12% of the total water volume stored in glacial features. Changes in glacier area over the last ∼50 years are in line with those for glaciers in central Chile despite the contrasting climate conditions. Projections of glacier area change based on glacier hypsometry and zero isotherm shifts predicted using the PRECIS regional model temperature change for IPCC scenario B2 conditions suggest that the survival of 65% of glacier area and 77% of active rock-glacier area will be threatened under forecast conditions for the end of the 21st century.

Full text

Glacier inventory of the upper Huasco valley, Norte Chico, Chile:
glacier characteristics, glacier change and comparison with
central Chile
Lindsey NICHOLSON,
1*
Jorge MARI
´N,
1
David LOPEZ,
1
Antoine RABATEL,
1
Francisca BOWN,
2
Andres RIVERA
2,3
1
Centro de Estudios Avanzados en Zonas A
´ridas (CEAZA), Benavente 980, Casilla 599, La Serena, Chile
E-mail: linznix@gmail.com
2
Centro de Estudios Cientı´ficos (CECS), Av. Arturo Prat 514, Mailbox 1469, Valdivia, Chile
3
Departamento de Geografı´a, Universidad de Chile, Portugal 84, Casilla 3387, Santiago, Chile
ABSTRACT. Results of a new glacier inventory of the upper Huasco valley, which lies within the arid
Norte Chico zone of the Chilean Andes, are presented for 2004. Despite the high altitude, the glaciation
in this region is limited in extent and is not classical mountain glaciation, which poses difficulties in
completing standard inventory attribute tables. Small cornice-style ridgeline features constitute a large
number of the non-transient ice bodies identified, and glaciers with surface areas <0.1 km
2
comprise
18% of the glacierized area and 3% of the water resource stored as glacier ice within the Huasco valley.
Rock glaciers are an important component of the cryosphere, comprising 12% of the total water volume
stored in glacial features. Changes in glacier area over the last 50 years are in line with those for
glaciers in central Chile despite the contrasting climate conditions. Projections of glacier area change
based on glacier hypsometry and zero isotherm shifts predicted using the PRECIS regional model
temperature change for IPCC scenario B2 conditions suggest that the survival of 65% of glacier area and
77% of active rock-glacier area will be threatened under forecast conditions for the end of the 21st
century.
INTRODUCTION
In the semi-arid Norte Chico region of Chile (27–338S), the
evolution of the cryosphere is of concern for local
populations because of its role in local water resources.
Although the region contains peaks in excess of 6000 m
a.s.l., low precipitation and strong ablation, a large portion
of which can be sublimation (Ginot and others, 2002), mean
that glaciers are limited in both number and extent (total ice
coverage in Norte Chico administrative regions of 73.85km
2
(Garı´n, 1987; Rivera and others, 2000)). Despite the limited
glacier extent, glacial meltwaters, when combined with
snowmelt, are an important component of the hydrological
resources (Favier and others, 2009) providing water that is
vital for the regional economy.
The mountain climate is characterized by persistent low
temperatures and humidity levels and generally clear skies.
Meteorological conditions measured over 7 years at the
Pascua-Lama mine exploration site (298S, 708W, 5000 m
a.s.l.) show that monthly mean temperature remains nega-
tive year-round, monthly mean relative humidity is 34–54%,
precipitation is concentrated in the winter season between
May and August, and monthly mean wind speed remains
above 4 m s
–1
. Synoptic-scale winds are predominantly
westerly but are deflected southward along the Andean
range (Kalthoff and others, 2002). Prevailing wind direction
at the Pascua-Lama meteorological stations is from the
northwest to north-northwest.
The glaciers of northern Chile (18–328S), including Norte
Chico, were inventoried using aerial photographs from 1955
and 1961 (Garı´n, 1987). The report for this inventory
highlights that in northern Chile: (1) it is not usually possible
to separate permanent accumulations of snow from ice
because glaciers show few surface signs of flow;
(2) permanent snowpatches were likely to be a significant
hydrological resource; (3) the glacial group at Los Tronquitos
(28832’ S) is the most significant concentration of ice in
northern Chile; (4) clearly defined moraine forms were
absent; and (5) rock-glacier features present in Norte Chico
were difficult to identify as some did not show surface flow
features. Rivera and others (2000) noted that as this
inventory was not well associated with watersheds, more
detailed work was required to make better hydrological
assessments.
This paper presents the findings of a new inventory of the
upper Huasco valley (Fig. 1) based on Advanced Spaceborne
Thermal Emission and Reflection Radiometer (ASTER)
images from 2004, presents an assessment of glacier type
and glacier evolution in this watershed, and compares the
results with those from a similar study in the upper
Aconcagua valley (338S), central Chile (Bown and others,
2008).
METHODS
Images and image treatment
All the materials used in this study are presented in Table 1.
The inventory work was carried out using ASTER images
from 2004 (Fig. 1), but the orthorectification process used
images from 2002 and 2003 as well. A 15 m digital terrain
model (DTM) was generated from the ASTER images using
the ASTER DTM module for ENVI. The same module was
Annals of Glaciology 50(53) 2009
*Present address: Center for Climate and Cryosphere, Department of
Geography, University of Innsbruck, Innrain 52, A-6020 Innsbruck, Austria.
111

used to orthorectify the images using an additional vectorial
input of the 1 : 50 000 scale drainage network produced
from the cartography of the Instituto Geogra´fico Militar
(IGM). The rectified images were mosaicked and colour
balanced using ENVI software. The catchment boundaries
were determined from the 1:50 000 contours (IGM). Near
the Argentine border, there is some discrepancy between the
catchment boundaries determined this way and the ridge-
lines evident in the rectified ASTER mosaic. The DTM is
expected to have a maximum of 10% difference to reality,
and horizontal and vertical accuracy of 30 and 15m,
respectively (Toutin, 2008). Additional differences may be
introduced because: (1) the ASTER images in this zone were
not rectified with the use of further ASTER images to the east;
(2) the drainage network vector used to optimize the
rectification process covers the Chilean territory only, so
does not cover all of the easternmost ASTER images; and (3)
the catchment boundary follows the ridgeline and the
rectification procedure is less accurate in high-relief areas.
In places, the eastern catchment boundary was corrected
manually to ensure glaciers draining to Chile are included in
the inventory. Aerial photographs taken between 1996 and
1999 were used as a reference for feature identification
rather than delimitation and also as a means of determining
the non-transience of features to be included in the
inventory.
Glacier delimitation
Snow and ice features identified in both the aerial
photographs from the late 1990s and the 2004 ASTER
images were considered as perennial and were included in
the inventory, although some of these bodies are of very
Fig. 1. Location of the Huasco valley showing the El Transito and El Carmen sub-catchments. ASTER images used for glacier delineation are
indicated in grey.
Table 1. Images and spatial data used in the study
Image/material Source Years Resolution/scale
ASTER ASTER 2002–04 15 m pixel
Aerial photographs GEOTEC flight 1996–99 1 : 50 000
Contour lines Instituto Geogra´fico Militar, Chile Based on air photography from 1955 Hycon flight 1 : 50 000
Huasco drainage network Direccio´n General de Aguas, Chile 1 : 50 000
Nicholson and others: Glacier inventory of the upper Huasco valley112

small surface area and it is not clear from the images
whether they would be classified as ne´ve´s, glacierets or
glaciers. The term ‘glacieret’ defines an ice body that is
formed primarily by blowing or avalanching snow, which
usually shows no surface signs of flow (Mu
¨ller and others,
1977; Rau and others, 2005). Additionally, this inventory
includes rock glaciers due to their prevalence in the basin
and consequent role in hydrological resources.
Snow and ice feature limits were mapped manually from
ASTER images taken on 14 February 2004, which is late
summer and so should present the most snow-free condi-
tions. Where transient snowpatches were identified to be
merging with permanent snow and ice features, the transient
portion was excluded from the inventoried area.
Only rock glaciers deemed to be active were included in
the inventory, as inactive rock glaciers may in fact no longer
contain ice and thus would not form part of the hydrological
resource of the catchment. Rock glaciers were determined to
be active if they showed: (1) a colour contrast between the
frontal slope and upper surface, as this was considered to be
indicative of ongoing frontal activity; and (2) evidence of
surface flow features. There are numerous fossil rock-glacier
features and valley fill deposits that may still contain some
quantity of ice, but as this is unknown such features were not
included in the inventory. Upper limits of rock glaciers were
taken to be the break of slope above the highest surface flow
features.
Database
The characteristics of mapped glaciers were entered into a
database to include the data requirements of both the
Chilean Direccio´n General de Aguas (DGA), which is based
on the World Glacier Inventory (WGI) database, and also
the Global Land Ice Measurements from Space (GLIMS)
database. Thus two identities were generated for each
glacier mapped, a UNESCO (WGI) name, based on country
and catchment, and a GLIMS ID, based on latitude and
longitude. Glaciers were classified using both the UNESCO/
WGI and expanded GLIMS system. The glaciers in this
region do not have classical altitude-delimited accumu-
lation and ablation areas, and instead wind patterns
determine the spatial distribution of accumulation. Addi-
tionally, the whole surface can be an accumulation or
ablation zone depending on the interannual climate vari-
ation (A. Rabatel and others, unpublished information), so
the field referring to dimensions of the zonation of the
glacier surface was filled with ‘not applicable’. Similarly, it
was not possible to identify snowlines. The WGI database
requests mean glacier width and length, but as the ice
bodies mapped in the upper Huasco valley are small, and
do not generally have a ‘tongue’ form the width of which
can be measured, or multiple tributary flowlines, these
fields were filled with maximum glacier dimensions in-
stead. Maximum length and width were measured as
straight line vectors parallel to and perpendicular to the
main contour trend of the ice body given by the IGM
1:50 000 contours. Maximum and minimum elevations
were determined from the polygon outlines and elevations
derived from the DTM. Glacier orientation was determined
automatically using the Landscape Management System
(LMS Tools) extension for ArcView 3.3; this extension
determines the predominant orientation and slope in a
polygon using a gridded DTM, in this case taken from the
IGM 1 : 50 000 contours.
Mean glacier thickness was determined in accordance to
an area–thickness relationship determined for glaciers in the
Maipo valley of central Chile (Marangunic, 1979). Analysis
of four ice bodies in the upper Huasco valley for which
ground-penetrating radar (GPR) thickness data are available
shows that the only glacier of the group is a good fit to the
relationship, but of the three glacierets, two are outliers,
with the positive outlier being a cornice glacieret (Fig. 2).
From this we assume that the relationship applies to glaciers
in this area as well, but it is less applicable to glacierets and
may be likely to underestimate the thickness of cornice
accumulations. Ice volumes were then estimated from the
mapped glacier area and approximated mean ice thickness.
Ice volume of rock glaciers was estimated following the
method of Brenning (2005), which assumes the average ice-
rich thickness of all rock glaciers to be 30 m and the
volumetric ice content of this layer to be 50%. It should be
noted that in the absence of bedrock topography and a more
extensive program of GPR surveying on glaciers in the
region, the ice volume estimates should be considered
approximations only and it is impossible to provide error
estimates on them. Water equivalents were calculated from
these ice volumes using ice densities of 0.9 kg m
–3
for the
glaciers and 0.8 kg m
–3
for the rock glaciers (Brenning,
2005).
RESULTS
Glaciation is limited in the upper Huasco valley, and many
perennial features mapped are very small and are likely to
be glacierets, ne´ve´ patches or perennial cornices. In the first
instance we consider all solid water features that were
identified as perennial, and subsequently we consider
thresholds for exclusion of smaller bodies. In the following
discussion, all snow, firn or ice bodies are referred to as
‘glaciers’ for simplicity.
Fig. 2. Relationships between glacier surface area and mean
thickness. Grey diamonds and equation indicate the relationship
given by Marangunic (1979); black squares represent three
glacierets and one glacier in the Huasco valley computed from
GPR measurements made by Golder Associates as part of the
glacier monitoring program of the future Pascua-Lama mining
project (Barrick Gold Corporation).
Nicholson and others: Glacier inventory of the upper Huasco valley 113

Of 152 glaciers identified, 111 were clean-ice glaciers, 1
was identified as having a debris-covered terminus and so
was termed a debris-covered glacier and 40 were active rock
glaciers. This population was considered as two groups:
‘glaciers’ (which includes the single glacier with a debris-
covered terminus) and ‘rock glaciers’. These two groups
constitute a total area of 23.17 km
2
of glacial features, of
which 16.86 km
2
are glaciers and 6.30 km
2
are rock-glacier
surfaces. Table 2 shows the distribution of glaciers within the
sub-catchments shown in Figure 3. In comparison, in his
inventory based on images from 1955 and 1961, Garı´n
(1987) identified 15 glaciers and snowpatches in the Huasco
valley, ranging from 0.01 to 5.60km
2
, although the latter
area refers to a large snowpatch that was covering the
location of present-day glaciers. The total ice-covered area
given by Garı´n was 20.16 km
2
, which is 3.30 km
2
more than
the inventoried glacier area presented here for 2004.
However, it should be noted that the two inventories are
not directly comparable due to different techniques, imagery
and scale of mapping used. The total estimated water
volume of the glacier and rock-glacier component of the
cryopshere in the upper Huasco valley in 2004 is
614.74 Mm
3
, of which 12% consists of rock glaciers.
Glacier distribution is strongly controlled by aspect, with
the result that over 80% of all the glaciers are on slopes
orientated towards the southeast, south and southwest
sectors. This preference is likely to be due in part to solar
influences and in part to preferential lee-slope snow
accumulation. There is no strong relationship between
glacier size and latitude or longitude, as the glaciers are
focused along high ridgelines. The hypsometry shows the
majority of clean-ice area between 5000 and 5200 m a.s.l.,
and the majority of rock-glacier surface area is found
between 4000 and 4400 m a.s.l. (Fig. 4). To elucidate this
elevation distribution of glacial features in terms of air
temperatures, the current zero-degree isotherm altitude
(ZIA) was computed from two weather stations (3975 and
4927 m a.s.l.) at the Pascua-Lama exploration mine site for
6 years (2001–2007). Annual ZIA is 4112 m a.s.l., and
summer (December–March) ZIA is 4718 m a.s.l. This annual
value is higher than that predicted from radiosonde
measurements from coastal stations spanning 30–388S
(Carrasco and others, 2005), which would suggest a ZIA of
about 3200 m a.s.l., but is considered a more accurate
representation for the upper catchment. Using these values,
glacier surface area can be seen to exist above the summer
ZIA, while rock glaciers are found between the annual and
summer ZIA.
A recommended smallest size of 0.1 km
2
for features
included in a glacier inventory is often used on the basis of
optimizing comparability between inventories using differ-
ent image types with different pixel resolution, for example
if Landsat Thematic Mapper (25m pixel) images are being
used. Applying this cut-off point to our data removes 70% of
the glaciers counted, which represent 18% of the glacier
area but only 3% of the estimated glacier volume. In the
case of rock glaciers, this size threshold results in the
rejection of 50% of those counted, representing 15% of the
rock-glacier area and volume (Table 3).
Many of the clean-ice glaciers identified were associated
with ridgelines and are likely to be permanent or semi-
permanent snow cornices. These cornice features tend to be
wider than they are long (where length is defined in the
downslope direction) and were identified numerically using
the ratio of glacier length to glacier width. Of the 112
glaciers, 40% have a length/width ratio <1, suggesting they
are ridgeline or cornice features. Eight of the 34 glaciers with
surface areas >0.1 km
2
were wider than they are long (24%),
while 37 of the 78 glaciers with surface areas <0.1 km
2
were
classified as ridgeline or cornice features (47%). Given that
the area to average thickness relationship we used for glaciers
appears to underestimate the volume of cornice features
(Fig. 2) and that almost half of the small features excluded by
this cut-off threshold are identified as cornice features, this
suggests that the ice and water volumes represented by
features <0.1 km
2
in Table 3 are a minimum estimate.
Table 2. Glacier areas in terms of WGI rank sizes, separated by sub-catchments (see Fig. 3) and by glacier type
Rank 1 (0.01–0.1 km
2
) Rank 2 (0.1–1 km
2
) Rank 3 (1–10 km
2
) Rank 4 (>10 km
2
)
Number Area Number Area Number Area Number Area
km
2
km
2
km
2
km
2
Glaciers (Debris-covered glaciers)
Laguna Grande 6 0.26 8 1.82 1 1.83
Valeriano 17 0.81 7 1.53
Cholloy 12 0.42 2 0.46 1 1.50
Plata
Potrerillo 25 0.94 12 (1) 4.49 (0.19) 1 1.92
Carmen 17 0.58 1 0.10
Socarron 1 0.03
All glaciers 78 3.03 30 (31) 8.40 (8.59) 3 5.24
Rock glaciers
Laguna Grande 1 0.04 4 1.25
Valeriano 5 0.29 3 0.96
Cholloy 9 0.35 9 2.42
Plata 1 0.19
Potrerillo 5 0.24 2 0.28
Carmen 1 0.26
Socarron
All rock glaciers 20 0.93 20 5.38
Nicholson and others: Glacier inventory of the upper Huasco valley114

DISCUSSION AND CONCLUSIONS
Type of glaciation in the upper Huasco valley
No classical valley glaciers are present in the upper Huasco
valley, and instead glaciation exists in the form of rock
glaciers and glaciers/glacierets of limited extent. Glacier
distribution is limited to ridgelines and shallow cirques and
shows no regional gradients in glacier size. Even on high
ridgelines, glaciers are limited to south-facing lee slopes,
suggesting that shelter from wind ablation is an important
control of glacier survival.
Methodological considerations
For the semi-arid Andes it is difficult to apply a number of the
standard glacier inventory approaches suggested by WGI and
GLIMS, as the glaciers here do not generally conform to the
classical alpine glacier upon which inventory database par-
ameters were previously based. For the upper Huasco valley
a number of WGI attributes could not be determined. Due to
the small size of ice features in the upper Huasco, maximum
length and width parameters are more applicable than mean
dimensions, and fields on snowline, accumulation and
ablation zones are not relevant to this style of glaciation.
In this region of the Andes, rock glaciers are a significant
water store (Brenning, 2005) and they may become more
hydrologically important under the influence of a warming
climate. Currently, there are no WGI-approved criteria for
mapping rock glaciers. In particular, it is necessary to
develop a standard procedure for identifying the upper limits
and the activity status of rock glaciers, which are both
important factors for determining the area of the feature that
Fig. 3. Mapped glacier features and sub-catchments of the upper
Huasco valley.
Fig. 4. Area–altitude distribution of glaciers and rock glaciers in the
upper Huasco catchment. Horizontal lines show the elevation of
measured (2001–07) annual and summer ZIAs and PRECIS-mod-
elled (2071–2100) annual and summer ZIAs for IPCC scenarios B2
and A2.
Nicholson and others: Glacier inventory of the upper Huasco valley 115

may be ice-bearing. Inclusion of rock glaciers and the
numerous small (<0.1 km
2
) ice bodies in the upper Huasco
is required for an accurate hydrological estimate of the
glaciological water stores.
Comparison of Norte Chico to central Chile
A recent inventory of the glaciers in the upper Aconcagua
valley (Bown and others, 2008) provides an opportunity to
compare the results of two Chilean glacier inventories in
catchments north and south of the South American arid
diagonal.
Most differences between these two regions can be
attributed to the climatic setting, as the Huasco basin lies
within the semi-arid conditions that prevail north of 328S,
whereas the Aconcagua basin has a more temperate
environmental setting, with higher snowfall especially
during winters affected by El Nin
˜o/El Nin
˜o–Southern
Oscillation (ENSO) events, when precipitation well above
normal is detected in the Andes of central Chile (Escobar
and Aceituno, 1998).
The outstanding difference between the two catchments
is that there are essentially no debris-covered glaciers in the
Huasco inventory, and rock glaciers constitute almost a
quarter of the total accounted number of features (Table 3),
whereas no rock glaciers were reported by Bown and others
(2008) in the upper Aconcagua valley basin, and a third of
the mapped ice area was debris-covered ice. In the Huasco
basin, spatial distribution of glaciers shows a strong
topographic control, with glaciers existing along ridgelines,
and rock-glacier distribution appearing to be random,
which is suggestive of very local topo-climatic controls and
varying debris supply. In the Aconcagua region, the spatial
pattern of ice distribution shows debris-covered glacier
areas are concentrated in the southern sub-catchments of
the upper Aconcagua valley and a significant increase of
bare ice towards sub-catchments Juncal and Blanco, where
most of the ice storage is located in deeply eroded high-
elevation cirques.
The two basins also differ significantly in terms of glacier
distribution among the size classes. The upper Huasco valley
contains many of the smallest size class of glaciers, which
account for a non-negligible part of the net ice surface of the
catchment, while in the Aconcagua region, where inventor-
ied ice is fivefold larger (Table 4), ice bodies <0.1 km
2
play a
marginal role.
In the central Chilean Andes, active rock glaciers
generally exist between 3500 and 4250 m a.s.l. in relation
to an annual ZIA of approximately 3600 m a.s.l. (Brenning,
2005). This relationship between ZIA and rock-glacier
distribution is similar to that found in the upper Huasco,
where the lower limit of active rock glaciers coincides
(within 100 m) with the annual ZIA and rock glaciers exist in
a range of 1000 m upwards of this level.
Glacier changes in the north-central Andes 1955–
present
In Norte Chico, the surface area evolution of Glaciar
Tronquitos (28832’ S, 69843’ W) has been reconstructed
using aerial photographs, showing a frontal retreat rate of
14 m a
–1
between 1955 and 1984 up to 23 m a
–1
between
1984 and 1996 (Rivera and others, 2002). Expanding on this
work, Figure 5a shows the comparison of glacier surface
area evolution between 1955 and the early 21st century for
six glaciers located between 288S and 338S. Their surface
evolution has been reconstructed using aerial photographs
and satellite images. Estrecho and Guanaco glaciers are
located in the upper Huasco. Data from Rivera and others
(2002) on Glaciar Tronquitos have been updated with the
2004 ASTER images used for this study. Data from Glaciar
Juncal Norte are presented in Bown and others (2008) and
from Juncal Sur and Olivares Gamma glaciers in Masiokas
and others (2008). A good consistency between changes in
Table 3. Summary of upper Huasco inventory showing the area and estimated volume of glaciers and rock glaciers and the same values
considering only features over and under a 0.1km
2
threshold
All identified features Features >0.1km
2
Features <0.1 km
2
Percent features <0.1 km
2
Glaciers Rock glaciers Glaciers Rock glaciers Glaciers Rock glaciers Glaciers Rock glaciers
Number 112 40 34 20 78 20 70% 50%
Total area (ha) 1686.18 630.38 1383.09 537.56 303.09 92.82 18% 15%
Estimated ice volume (Mm
3
) 599.00 94.56 583.84 80.63 15.15 13.92 3% 15%
Estimated water volume (Mm
3
) 539.10 75.65 525.46 64.51 13.64 11.14 3% 15%
Table 4. Comparison of inventory data of Huasco and Aconcagua valleys
All glaciers 0.01–0.1 km
2
0.1–1 km
2
1–10 km
2
>10 km
2
Number Area Number Area Number Area Number Area Number Area
km
2
km
2
km
2
km
2
km
2
Huasco glaciers 112 16.86 78 3.03 31 8.59 3 5.24
Huasco rock glaciers 40 6.30 20 0.93 20 5.38
Huasco all glaciers 152 23.17 98 3.96 51 13.96 3 5.24
Aconcagua glaciers 159 121 24 1 112 43 22 52 1 24
Nicholson and others: Glacier inventory of the upper Huasco valley116

these six glaciers is observed. Their evolution over this
50 year period is homogeneous, although a slight accelera-
tion could be suggested for some glaciers over the last
decade (Juncal Norte, Olivares Gamma). Although some
local conditions (e.g. topographical constraints (Bown and
others, 2008) or mining activity (Brenning, 2008)) may
modify glacier retreat trends, climate should remain the
main driving factor in glacier behavior.
Figure 5b presents a comparison between glacier loss
during the last 50 years and their initial surface, showing a
good relationship between surface lost and initial surface
area. The small glaciers of the subtropical Andes (i.e.
Estrecho, Guanaco and Tronquitos glaciers) experienced a
greater retreat (in percent of their initial surface) than did the
biggest glaciers of the central Andes of Chile (Juncal Norte,
Juncal Sur and Olivares Gamma glaciers). This size depen-
dency of response to climate also holds true within each
climatic regime, with smaller glaciers within both the Huasco
and the Aconcagua valleys showing a greater reduction in
surface area compared with the climate response of the upper
size-class glaciers within the same basins, which have been
less affected proportionally by climate change.
Glacier change in future climate scenarios
Mid-troposperic warming has been highlighted as a cause for
glacier retreat in central Chile, with changes in air tempera-
ture being more important in causing increases in glacier
equilibrium-line altitude than are changes in precipitation
(Carrasco and others, 2005). On this basis, a simple
prediction of future glacier area change can be made using
projected temperature shifts and the relationships established
between the hypsometrical distribution of glacier surface
area and the ZIA, to generate estimations of future glacier
extent in the Huasco valley. Climate projections for 2071–
2100 using Intergovernmental Panel on Climate Change
(IPCC) scenarios B2 and A2 have been computed by the
Departamento Geofisica of the Universidad de Chile using
the PRECIS regional climate model (model description can
be found at http://precis.metoffice.com/new_user.html and
outputs are available at http://mirasol.dgf.uchile.cl/conama/).
Modelled 2 m air temperatures for 29.258S and 70.008E
were used to compute the temperature change expected
between average baseline (1961–90) and average future
(2071–2100) conditions under the two climate scenarios.
The temperature change was applied to the mean annual and
summer air temperatures measured at 3975 m a.s.l. over the
time period 2001–07. The modern-day (2001–07) lapse rate
was used in conjunction with the projected temperatures to
compute annual and summer ZIA for the B2 and A2
scenarios. Annual (summer) ZIA changes from the modern-
day (2001–07) elevation of 4112 (4718) m a.s.l. to 4497
(5125) m a.s.l. under the conditions of scenario B2, and to
4718 (5400) m a.s.l. under the conditions of scenario A2 for
the period 2071–2100. From the modern-day inventory we
can approximate the lower limit of glaciers as the summer
ZIA and the lower limit of rock glaciers as the annual ZIA
(Fig. 4). Assuming glacial features below these limits will not
be in equilibrium and thus will no longer exist under the
projected conditions, scenario B2 suggests a loss of
11.00 km
2
(65%) of current glacier area and 4.41 km
2
(70%) of current active rock-glacier area, and scenario A2
suggests a loss of 16.07 km
2
(95%) of current glacier area and
5.72 km
2
(91%) of current active rock-glacier area. However,
it should be noted that these projected changes in glacierized
area in the Huasco valley do not account for changing
precipitation. PRECIS-modelled precipitation for the 2071–
2100 period at the study site shows no significant change in
precipitation amount or trend from the baseline period, but
an increase in interannual variability, which will have an
impact on how glacier mass balances respond to future
climate change.
The glacier-derived water resources from the arid Andes
contribute to the water supply downstream and are particu-
larly important in maintaining summer base flow and water
levels in drought periods. The results of this inventory reveal
that the limited glaciation in the upper Huasco basin
represents approximately 615 10
6
m
3
of stored water. At
present, the partitioning of glacier ice ablation into melt-
waters and that lost to the atmosphere through sublimation
remains poorly understood, as does the runoff contribution
of rock glaciers in this region. Understanding the interaction
of these ice masses with climate and projected climate
change is likely to be important for long-term water resource
management for the region.
Fig. 5. (a) Surface evolution of sample Chilean glaciers (approximate central coordinates are given) and (b) glacier surface evolution as
percent of the initial (1955) surface area. Data are from air photographs until 2003 and ASTER images thereafter; details of source data and
mapping treatment are in the source articles.
Nicholson and others: Glacier inventory of the upper Huasco valley 117

ACKNOWLEDGEMENTS
Images used in this study were procured with funds provided
by Barrick Gold Corporation as part of a glacier monitoring
program in the arid Andes. We thank three reviewers for
their helpful comments.
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