LETTERS
PUBLISHED ONLINE: 7 AUGUST 2011 | DOI: 10.1038/NGEO1218

Ice speed of a calving glacier modulated by small
fluctuations in basal water pressure
Shin Sugiyama1 *, Pedro Skvarca2 , Nozomu Naito3 , Hiroyuki Enomoto4 , Shun Tsutaki1,5 , Kenta Tone1,5 ,Sebastián Marinsek2 and Masamu Aniya6Nwhere ub is the basal ice speed, τb is the basal shear stress, Pi and
Pw are ice overburden and basal water pressures, and k, p and q
are empirical parameters6,7 . As τb is primarily controlled by ice
thickness and surface slope, changes in basal water pressure playa critical role in short-term ice speed variations. Observations inmountain glaciers have shown rapid acceleration as basal waterpressure approaches ice overburden pressure8–10 , which confirms
the nonlinear dependence of the basal ice speed on the effective
pressure defined by Pe = Pi − Pw . The hydraulic head within acalving glacier is expected to be higher than the surface level ofthe proglacial water body, which maintains basal water pressure
closer to ice overburden. According to the inverse proportionalityof ub to Pe , small perturbations in Pw near Pi result in large ice
speed variations. Moreover, changes in Pi due to glacier thinning

500 m yr¬1

Glaciar Perito Morenoosel
al d anos
n
Ca mp
Te

GPS1 BH
GPS2
GPS3
GPS fix
AWS

co
Ri

(1)

2 km

zo

τb p
τb p
=
k
ub = k(Pi − Pw )q
Pe q

a

a
Br

b

600
Ice

400

Elevation (m)Ice flow acceleration has played a crucial role in the rapid
retreat of calving glaciers in Alaska1,2 , Greenland andAntarctica3,4 . Glaciers that calve in water flow much faster thanthose that terminate on land, as a result of enhanced basal
ice motion where basal water pressure is high5 . However, ascarcity of subglacial observations in calving glaciers limits amechanistic understanding. Here we present high-frequency
measurements of ice speed and basal water pressures from
Glaciar Perito Moreno, a fast-flowing calving glacier in Patagonia. We measured water pressure in boreholes drilled at asite where the glacier is 515 ± 5 m thick, and where more than
60% of the ice is below the level of proglacial lakes. We foundthat the mean basal water pressure was about 95% of thepressure imposed by the weight of the overlying ice. Moreover,changes in basal water pressure by a few per cent drove nearly
40% of the variations in ice flow speed. The ice speed wasstrongly correlated to air temperature, suggesting that glaciermotion was modulated by water pressure changes as meltwater
entered the system. We conclude that basal water pressure incalving glaciers is important for glacier dynamics, and closely
connected to climate conditions.
Acceleration of fast-flowing calving glaciers is the focus ofattention as it is responsible for the rapid retreat of large tidewater
glaciers in Alaska1,2 as well as the recent wastage of the Greenlandand Antarctic ice sheets3,4 . Calving glaciers flow much fasterthan those terminating on land as a result of basal ice motion
enhanced by high basal water pressure5 . A commonly usedbasal flow law states

Drilling siteoverbu
lev rdenelMean water level200

Lake level
Ice

0

Bed
¬200
10

8

6
4
2
Distance from the terminus (km)0Figure 1 | Satellite image and longitudinal cross section of Glaciar Perito
Moreno. a, GPS (filled circle, open circle), drilling (plus) and airtemperature measurement (box) sites are indicated with ice flow vectors
measured from 31 December 2008 to 7 January 2009. The image wastaken on 27 March 2002 (image courtesy of the Image Science andAnalysis Laboratory, NASA Johnson Space Center). b, The bed profile isdrawn by interpolation of observational data points (open circle16 ). Mean
borehole water level during the measurement period is indicated.

or thickening have a great impact on the ice speed as well. These
characteristics make calving glacier dynamics more susceptible
to external forcing than land terminating glaciers. Studying the
response of ice speed to the changes in Pe is thus crucial for
predicting the future evolution of calving glaciers. However, only
a very few water pressure measurements have been performed incalving glaciers and have never been reported in Patagonia.
The Patagonia Icefields cover an area of 1.70 × 104 km2 (refs 11,12), forming the second largest ice mass in the southern hemisphere.

1 Institute of Low Temperature Science, Hokkaido University, Nishi8, Kita19, Sapporo, Japan, 2 Instituto Antártico Argentino, Cerrito 1248, C1010AAZ

Buenos Aires, Argentina, 3 Hiroshima Institute of Technology, 2-1-1 Miyake, Saeki-ku, Hiroshima, Japan, 4 National Insitute of Polar Research, 10-3
Midori-cho, Tachikawa, Japan, 5 Graduate School of Environmental Science, Hokkaido University, Nishi5, Kita10, Sapporo, Japan, 6 University of Tsukuba,
1-1-1 Tennodai, Tsukuba, Japan. *e-mail: sugishin@lowtem.hokudai.ac.jp.
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1

NATURE GEOSCIENCE DOI: 10.1038/NGEO1218

10
5
0

1.0
27
03
Dec. 2008 Jan. 2009

2.5

15
2.0
10
5

1.5

0
1.0

c

20

26
05
Feb. 201012
19
Mar. 201026

2.0
Air temperature (°C)

15
2.0

1.5

b

20
Air temperature (°C)

Ice speed (m d¬1)

2.5

Ice speed (m d¬1)a

Ice speed (m d¬1)

LETTERS

1.8
1.6
1.4
1.2
5

02

10
15
Air temperature (°C)

20

Figure 2 | Ice speed and air temperature measured in 2008/09 and 2010 austral summer seasons. a,b, Ice speed at GPS1 (black) and air temperature(red) obtained by filtering hourly data (grey). c, Scatter plot of hourly ice speed and air temperature for the measurements in 2008/09 (blue)and 2010 (red).

Most of the glaciers in Patagonia have been retreating and thinningfor the past several decades12,13 , which contributes to a sea-level
rise of 0.042–0.105 ± 0.011 mm yr−1 (ref. 14). Warming climate isa likely cause of the ice loss13 , but changes in glacier dynamics isimportant, as more than 80% of major outlet glaciers discharge
ice into lakes and the ocean by calving14,15 . Glaciar Perito Moreno(GPM) is one such glacier, with a length of 30 km and an areaof 258 km2 , situated in the southeast of the Southern Patagonia
Icefield13 . In the region extending 8 km from the terminus,
approximately 50–70% of ice thickness is below the surface ofproglacial lakes, Brazo Rico and Canal de los Témpanos (Fig. 1;
ref. 16). In this region, ice speed is more than 400 m yr−1 and theglacier is 200–800 m thick along the central flowline16 .
In the austral summer 2008/09 and 2010, we operated three GPS(Global Positioning System) receivers on GPM at hourly intervals
to measure short-term ice speed variations and their relationshipwith air temperature and basal water pressure. The study site was
4.7 km from the terminus and the bed elevation was more than
380 m below the lake level16 (Fig. 1). In March 2010, we drilled
two boreholes with a hot-water drilling technique at 160 m southof GPS1 (ref. 17; Fig. 1a). According to the length of the hoseused for the drilling, ice thickness was 515 ± 5 m. The water levelin the boreholes dropped before the drill reached the bed, which
occurred at depths of 176 and 344 m for the first drilling and at
375 m for the second drilling. After the drilling, the water level
was measured every 10–30 min using pressure sensors suspended
in the boreholes. Details of the measurements are described in the
Methods section.
Ice speed showed diurnal and longer-term variations over
the observation periods (Fig. 2a,b). During the one-monthmeasurement period in 2010, ice speed varied by +71 to −26%
from the mean of 1.43 m d−1 . An intriguing feature is the strong
correlation between the ice speed and air temperature. Hourly
ice speed correlated positively to the air temperature, with acorrelation coefficient r = 0.76 for the entire observation period(Fig. 2c and Supplementary Table S1). A maximum correlation wasachieved when the ice speed lag was 1 h relative to the temperature(Supplementary Fig. S1). Linear regression indicates that the ice
speed deviated ±0.053 m d−1 for a temperature change of ±1 ◦ C.
A likely mechanism of the air-temperature-dependent ice speed
variation was the change in basal water pressure driven by meltwater
input. Melt production is commonly related to the positive degree
day (ref. 18) as we observed at GPM (Supplementary Fig. S2).
From 4 to 13 March 2010, mean water levels in the boreholes
were 440 (BH1) and 450 m (BH1) above the bed, that is, 110–120 m
higher than the proglacial lake level (Figs 1b and 3a). Thisobservation indicated that the basal water pressure was at 94–96%
of the ice overburden pressure and the effective pressure was only
190–280 kPa. The water level oscillated in a diurnal manner with2

an amplitude of 10–15 m from 5 to 10 March (Fig. 3a). The levels
were not exactly the same in the two boreholes, but the oscillations
were in phase. The ice speed at GPS1 and GPS2 was correlated
to the water level (Fig. 3a–c). The range of the ice speed variation
was 37% of the mean at GPS1, whereas that of the water level was
only 4–6% (Fig. 3c).
The borehole data show that the glacier bed is exposed toconsistently high water pressure, and large diurnal ice speed
variations are driven by small pressure fluctuations near the ice
overburden. These observations are consistent with equation (1),which relates the basal ice speed to the inverse power ofeffective pressure. A numerical ice flow model shows that 94%
of the observed ice speed at GPS1 is due to basal ice motion(Supplementary Fig. S3). Thus, we neglected the contribution ofinternal ice deformation to the total glacier motion, and performed
the fitting of equation (1) to the ice speed (GPS1) and pressure(mean of BH1 and BH2) data to find the dependence of basal ice
speed on the effective pressure. The least square fitting yields
ub = 0.91Pe −0.35

σ = 0.061(2)

where ub and the root mean square error σ are in m d−1 and Pe is in
MPa (Supplementary Table S2). The fitting curve clearly indicatesthat the basal ice speed of GPM is highly dependent on the effective
pressure within the observed pressure range (Fig. 3d).
Water pressure beneath a calving glacier was previously
measured at Columbia Glacier, an Alaskan tidewater glacier withdimensions similar to those of GPM (about 5 km wide and400–1,000 m thick). Basal water pressure was consistently high andthe effective pressure varied within a range between −100 and
300 kPa (ref. 19). These figures are similar to those we observed at
GPM, but ice speed at Columbia Glacier was not well correlatedwith water pressure nor with air temperature. It was argued thatbasal pressure was non-uniformly distributed and the boreholemeasurements did not represent spatially averaged pressure, which
is more important for ice speed variations20 . It is assumed thathorizontal patterns of subglacial hydraulic conditions at Columbia
Glacier are less uniform than those at GPM, as described below.
Our observation revealed a clear relationship between air
temperature and ice speed, which implies that basal water pressure
is controlled primarily by the meltwater production rate. However,previous studies showed that basal pressure is not simply a functionof water input, but that it is substantially influenced by theevolution of subglacial drainage conditions over a timescale of days
to months10,21 . For example, a melt event in the early ablationseason elevates basal pressure, resulting in a speed-up event22,23 .
The speed-up ceases as a drainage system develops and waterdrains downglacier more efficiently. Moreover, the response ofbasal pressure to surface melt is often delayed more than several

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NATURE GEOSCIENCE DOI: 10.1038/NGEO1218
a

460

400
Plake

300

450

200

440

100

430

0

b

03

08

13

03

08

13

420

GPS1

1.5

20

1.0

GPS2

10

0.5
GPS3
0

15

03

05

07

09
11
Mar. 2010

13

15

5

Air temperature (°C)

Ice speed (m d¬1)

470

Pi

Water level (m)Water level (m)500

LETTERS0c

Ice speed (m d¬1)

1.8

1.6

1.4

1.2

d

430

440
450
Water level (m)

460

1.5

Methods

1.0

Pi ¬ Plake

0.5

Pi

Ice speed (m d¬1)

2.00

conditions for maintaining the drainage systems in a more uniform
way as compared with other glaciers. Spatially uniform drainage
system configurations are suggested by other observations: theenglacial drainage of the borehole water during the drilling, very
similar water level variations in the two boreholes and synchronous
ice speed variations at GPS1 and GPS2. These observations contrast
to those reported for other glaciers, which suggested non-uniformly
distributed subglacial hydraulic conditions24,25 .
The observations in GPM have important implications forstudying the dynamic behaviour of calving glaciers. First, theborehole levels were significantly higher than the lake level, which
disagrees with a common assumption that equates the subglacial
hydraulic head to the surface of the proglacial water body26 . Thisis crucial, as the ice speed is greatly underestimated with suchan assumption. Second, we confirmed that the calving glacier
dynamics are very sensitive to small basal water pressure variations
near the ice overburden. Together with the close correlation of theice speed with the air temperature, calving glaciers are assumed to bevery susceptible to changing climate. If the ice speed increases under
the rising air temperature condition, ice dissipation by calving
increases, causing thinning of the glacier. The thinning in turn
reduces the effective pressure, which results in further accelerationof the glacier5,27 . According to equation (1), basal ice speed in GPM
is negatively correlated with ice thickness under the observed basal
water pressure conditions (Supplementary Fig. S4).
Recent studies using numerical models reproduced several
aspects of the dynamic behaviour of outlet glaciers in Greenlandby capturing calving processes28 . However, basal water pressure is
not well imposed in the model as described above. The foregoing
discussion suggests a positive feedback effect between thinning andacceleration of calving glaciers, which can be captured by modelling
only with realistic basal water pressure and an effective pressure
dependent basal flow law. Further investigations of subglacial
processes and the accurate treatment of basal conditions in a model
are thus crucial for predicting the future evolution of calving glaciersin Patagonia and Alaska, as well as marine-terminating outlet
glaciers in Greenland and Antarctica.

432
Pe (MPa)

1

0

Figure 3 | Ice speed and borehole water levels. a, Water levels relative tothe bed in BH1 (blue) and BH2 (red) for two different scales. Dashed lines
indicate ice overburden (Pi ) and lake levels (Plake ) in the left axis. b, Ice
speed at GPS1–3 (black) and air temperature (red). c, Hourly ice speed at
GPS1 versus water level in BH1 (blue) and BH2 (red) with curves obtainedby fitting equation (1) to the data (Supplementary Table S2). d, Ice speed
versus effective pressure with a fitting curve that represents equation (2).
Dashed lines indicate the effective pressures for water levels equal to zero
(Pe = Pi ) and to the lake surface (Pe = Pi − Plake ).

hours10 . At GPM, the consistently high correlation between the ice
speed and the air temperature suggests a swift transfer of surface
meltwater to the basal hydraulic system and a relatively constantdrainage efficiency. Presumably, the meltwater immediately drains
into crevasses which cover the greater part of the glacier, and it
reaches the hydraulic system within a short time, as the hydraulichead is only 60–80 m below the surface. High water pressure anda large amount of meltwater input all year round16 are favourable

Ice speed was measured at GPS1–3 (Fig. 1a) with dual frequency GPS receivers(Leica System 1200) from 27 December 2008 to 8 January 2009 and from25 February to 2 April 2010. We installed 2-m-long aluminium poles in theice and mounted GPS antennae at the top of the poles. The GPS receivers
were activated every hour for 30 min and the satellite signals were processedwith data recorded at a reference GPS station fixed on a stable rock on a sidemoraine (Fig. 1a). GPS processing software (Leica Geo Office) was used toobtain stake positions with an accuracy of 3–5 mm (ref. 10). We computed
horizontal ice speed after smoothing the data with a Gaussian filter having a
bandwidth of 1.5 h. This bandwidth was chosen so that the mean deviation of
the data from the smoothed curve was equivalent to the positioning accuracy.
The relative error in the computed hourly ice speed was 7–12% for the mean
speed at GPS1, and the actual error after the smoothing procedure was smaller
than this estimate.
Boreholes were drilled by means of a hot-water drilling system consisting of
two high-pressure hot-water machines (Kärcher HDS1000BE) at a mean drillingrate of 49 m h−1 (ref. 17). The weight of the drilling hose was monitored during thedrilling to detect the glacier bed. The ice thickness was determined as 515 ± 5 m
from the length of the hose used for the drilling and its extension due to its own
weight. Water levels in the boreholes BH1 and BH2 were measured with waterpressure sensors (Geokon 4500S and HOBO U20-001-03) installed at 98 ± 1 and
120 ± 0.1 m from the surface, respectively. The measurement ranges/accuraciesof the sensors were −0.1–2 MPa/ ± 200 mm (BH1) and 0–0.85 MPa/ ± 38 mm(BH2). The pressure data were corrected for atmospheric pressure recorded every
hour at the air temperature measurement site. The data availability was limited
to 4–12 March at BH1, as the borehole was isolated from the subglacial drainage
network afterwards, and 5–13 March at BH2, as we could not retrieve the data after
the sensor got stuck in the borehole.
Air temperature was measured near the glacier front at the shore of Brazo
Rico (Fig. 1a). A temperature sensor (Vaisala, HMP35AC) protected with a
non-aspirated radiation shield was installed on a mast at 2 m above the rocksurface. The temperature was measured every 10 s with an accuracy of ±0.4 ◦ C torecord hourly mean values in a datalogger (Campbell, 21X; ref. 29).

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NATURE GEOSCIENCE DOI: 10.1038/NGEO1218

LETTERS
Received 15 March 2011; accepted 28 June 2011; published online
7 August 2011

References
1. Krimmel, R. M. Photogrammetric data set, 1957–2000, and bathymetricmeasurements for Columbia Glacier, Alaska. US Geol. Surv. Wat. Resour.
Invest. Rep. 01–4089 (2001).
2. O’Neel, S., Echelmeyer, K. A. & Motyka, R. J. Short-term flow dynamics ofa retreating tidewater glacier: LeConte Glacier, Alaska, USA. J. Glaciol. 47,
567–578 (2001).
3. Rignot, E. & Kanagaratnam, P. Changes in the velocity structure of theGreenland ice sheet. Science 311, 986–990 (2006).
4. Pritchard, H. D., Arthern, R. J., Vaughan, D. G. & Edwards, L. A. Extensive
dynamic thinning on the margins of the Greenland and Antarctic ice sheets.
Nature 461, 971–975 (2009).
5. Meier, M. F. & Post, A. Fast tidewater glaciers. J. Geophys. Res. 92,
9051–9058 (1987).
6. Greve, R. & Blatter, H. Dynamics of Ice Sheets and Glaciers (Springer, 2009).
7. Bindschadler, R. The importance of pressurized subglacial water in separationand sliding at the glacier bed. J. Glaciol. 47, 3–19 (1983).
8. Iken, A. & Bindschadler, R. A. Combined measurements of subglacial
water pressure and surface velocity of Findelengletscher, Switzerland:
Conclusions about drainage system and sliding mechanism. J. Glaciol. 32,
28–47 (1986).
9. Jansson, P. Water pressure and basal sliding on Storglaciären, northern Sweden.
J. Glaciol. 41, 232–240 (1995).
10. Sugiyama, S. & Gudmundsson, G. H. Short-term variations in glacier flow
controlled by subglacial water pressure at Lauteraargletscher, Bernese Alps,
Switzerland. J. Glaciol. 50, 353–362 (2004).
11. Aniya, M., Naruse, R., Shizukuishi, M., Skvarca, P. & Casassa, G. Monitoring
recent glacier variations in the Southern Patagonia Icefield, utilizing remotesensing data. Int. Arch. Photogram. Rem. Sens. 29, 87–94 (1992).
12. Rivera, A., Benham, T., Casassa, G., Bamber, J. & Dowdeswell, J. A. Ice elevation
and areal changes of glaciers from the Northern Patagonia Icefield, Chile.
Glob. Planet. Change 59, 126–137 (2007).
13. Aniya, M., Sato, H., Naruse, R., Skvarca, P. & Casassa, G. Recent glaciervariations in the Southern Patagonia Icefield, South America. Arct. Alp. Res.
29, 1–12 (1997).
14. Rignot, E., Rivera, A. & Casassa, G. Contribution of the Patagonia Icefields ofSouth America to sea level rise. Science 302, 434–437 (2003).
15. Warren, C. R. & Aniya, M. The calving glaciers of southern South America.
Glob. Planet. Change 22, 59–77 (1999).
16. Stuefer, M., Rott, H. & Skvarca, P. Glaciar Perito Moreno, Patagonia: Climate
sensitivities and glacier characteristics preceding the 2003/04 and 2005/06
damming events. J. Glaciol. 53, 3–16 (2007).
17. Sugiyama, S. et al. Hot-water drilling at Glaciar Perito Moreno, Southern
Patagonia Icefield. Bull. Glaciol. Res. 28, 27–32 (2010).
18. Ohmura, A. Physical basis for the temperature-based melt-index method.
J. Appl. Meteorol. 40, 753–761 (2001).

4

19. Meier, M. F. et al. Mechanical and hydrologic basis for the rapid motion of alarge tidewater glacier: 1. Observations. J. Geophys. Res. 99, 15219–15229 (1994).
20. Kamb, B. et al. Mechanical and hydrologic basis for the rapid motion of a large
tidewater glacier: 2. Interpretation. J. Geophys. Res. 99, 15231–15244 (1994).
21. Gordon, S. et al. Seasonal reorganization of subglacial drainage inferred frommeasurements in boreholes. Hydrol. Processes 12, 105–133 (1998).
22. Iken, A., Röthlisberger, H., Flotron, A. & Haeberli, W. The uplift of
Unteraargletscher at the beginning of the melt season—a consequence of water
storage at the bed? J. Glaciol. 29, 28–47 (2001).
23. Mair, D., Nienow, P., Willis, I. & Sharp, M. Spatial patterns of glacier motion
during a high-velocity event: Haut Glacier d’Arolla, Switzerland. J. Glaciol. 47,
9–20 (2001).
24. Murray, T. & Clarke, G. K. C. Black-box modeling of the subglacial water
system. J. Geophys. Res. 100, 10231–10245 (1995).
25. Nienow, P. W. et al. Hydrological controls on diurnal ice flow variability invalley glaciers. J. Geophys. Res. 110, F04002 (2005).
26. Vieli, A., Funk, M. & Blatter, H. Flow dynamics of tidewater glaciers: A
numerical modelling approach. J. Glaciol. 47, 595–606 (2001).
27. Pfeffer, W. T. A simple mechanism for irreversible tidewater glacier retreat.
J. Geophys. Res. 112, F03S25 (2007).
28. Nick, F. M., Vieli, A., Howat, I. M. & Joughin, I. Large-scale changes inGreenland outlet glacier dynamics triggered at the terminus. Nature Geosci. 2,
110–114 (2009).
29. Stuefer, M. Investigations on Mass Balance and Dynamics of Moreno Glacier
Based on Field Measurements and Satellite Imagery PhD thesis, Univ.
Innsbruck (1999).

Acknowledgements
We thank the members of the field campaign at Glaciar Perito Moreno in 2008/2009
and 2010 for their help on the glacier. Hielo y Aventura S. A. offered logistic support
and Gendarmeria Nacional Argentina operated helicopter transportation. Drilling
equipment was constructed by the workshop of the Institute of Low Temperature
Science, Hokkaido University. Thanks are extended to R. Greve and H. Blatter forcomments on the manuscript. This research was funded by the Japanese Ministryof Education, Science, Sports and Culture, Grant-in-Aid 18251002 (2005–2009) and23403006 (2011–2014).

Author contributions
S.S., P.S. and M.A. designed the research. S.S., N.N., H.E. and K.T. drilled the boreholes.
S.S., P.S., N.N., S.T., K.T. and S.M. collected the water pressure and GPS data. S.S.
analysed the data and wrote the manuscript. The authors discussed the results andcommented jointly on the manuscript.

Additional information
The authors declare no competing financial interests. Supplementary informationaccompanies this paper on www.nature.com/naturegeoscience. Reprints and permissions
information is available online at http://www.nature.com/reprints. Correspondence andrequests for materials should be addressed to S.S.

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