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Effects of debris on ice-surface melting rates: an
experimental study
Natalya REZNICHENKO,
1
Tim DAVIES,
1
James SHULMEISTER,
2
Mauri McSAVENEY
3
1
Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand
E-mail: natalya.reznichenko@pg.canterbury.ac.nz
2
School of Geography, Planning and Environmental Management, University of Queensland, St Lucia, Queensland 4072,
Australia
3
GNS Science, PO Box 30368, Lower Hutt, New Zealand
ABSTRACT. Here we report a laboratory study of the effects of debris thickness, diurnally cyclic
radiation and rainfall on melt rates beneath rock-avalanche debris and sand (representing typical highly
permeable supraglacial debris). Under continuous, steady-state radiation, sand cover >50mm thick
delays the onset of ice-surface melting by >12hours, but subsequent melting matches melt rates of a
bare ice surface. Only when diurnal cycles of radiation are imposed does the debris reduce the long-
term rate of ice melt beneath it. This is because debris >50 mm thick never reaches a steady-state heat
flux, and heat acquired during the light part of the cycle is partially dissipated to the atmosphere during
the nocturnal part of the cycle, thereby continuously reducing total heat flux to the ice surface
underneath. The thicker the debris, the greater this effect. Rain advects heat from high-permeability
supraglacial debris to the ice surface, thereby increasing ablation where thin, highly porous material
covers the ice. In contrast, low-permeability rock-avalanche material slows water percolation, and heat
transfer through the debris can cease when interstitial water freezes during the cold/night part of the
cycle. This frozen interstitial water blocks heat advection to the ice–debris contact during the warm/day
part of the cycle, thereby reducing overall ablation. The presence of metre-deep rock-avalanche debris
over much of the ablation zone of a glacier can significantly affect the mass balance, and thus the
motion, of a glacier. The length and thermal intensity of the diurnal cycle are important controls on
ablation, and thus both geographical location and altitude significantly affect the impact of debris on
glacial melting rates; the effect of debris cover is magnified at high altitude and in lower latitudes.
INTRODUCTION
The role of debris cover in modifying glacier behaviour is
well known, but poorly understood as yet. Small-scale field
studies have described enhanced (for very thin cover) to
significantly reduced (for thicker cover) ice-surface ablation
(e.g. Østrem, 1965; Lundstrom and others, 1993; Mattson
and others, 1993). These field measurements are comple-
mented by theoretical studies based on heat-flux calculations
(e.g. Nakawo and Young 1981, 1982; Bozhinskiy and others,
1986). Broader field studies have shown that debris cover
resulting from extensive, deep (metre-scale) dominantly fine-
grained rock-avalanche deposits can significantly affect mass
balance of the glacier and cause it to advance (Post, 1968;
McSaveney, 1975; Kirkbride, 1989; Hewitt, 2009) on
decadal to centennial timescales in the absence of climate
forcing. Thus not all terminal moraines result from climate
alterations. Between these theoretical calculations and field
observations, however, there is a dearth of information on the
processes by which debris affects melt rates. Herein we
describe laboratory experiments to investigate these pro-
cesses, focusing on the roles of diurnal cyclicity and debris
thickness and permeability in modifying melt rates under
debris. These experiments aim to clarify the process details,
rather than attempting to replicate field conditions in which
other factors play a part. The objective is to isolate the effects
of debris thickness and permeability on radiation- and
rainfall-induced ice-surface melting.
A thin debris cover accelerates the melting rate of the
underlying ice through increased absorption of solar energy
by low-albedo debris and rapid transmission of this heat to
the ice surface (Clark and others, 1994). Increasing debris
thickness causes a corresponding decrease in the ablation
rate; this was recognized in very early studies of glaciers by
Agassiz (1840). Debris cover thicker than a particular depth
acts as a barrier to heat transfer and decreases ice-melting
rates with increasing debris thickness, leading to a reduction
of the total underlying ice-surface ablation. From obser-
vations at many glaciers, a general relationship between
debris-cover thickness and ablation has been developed
(Mattson and others, 1993; Clark and others, 1994). In
comparison with bare ice, mean daily ablation decreases
exponentially with increasing debris thicknesses above the
‘critical thickness’; this is defined as a debris depth at which
melt rates are identical to those on adjacent bare ice
(Adhikary and others, 1997; Nakawo and Rana, 1999).
Several numerical models have been proposed for
calculating the insulating effect of debris cover (e.g.
Khodakov, 1972; Nakawo and Young 1981, 1982; Bozhin-
skiy and others, 1986; Kayastha and others, 2000; Konova-
lov, 2000; Han and others, 2006; Nicholson and Benn,
2006). These are generally based on an energy balance for
the debris layer. Ablation under the debris cover is usually
calculated on the basis of heat conduction through the
debris layer as a function of its thickness and the physical
properties of the debris, or by comparison with the surface
ablation of adjacent bare ice under the same conditions.
McSaveney (1975) developed methods for calculating long-
term ablation rates beneath thick debris, by analysis of the
thermal conductivity of the debris layer as a function of
Journal of Glaciology, Vol. 56, No. 197, 2010384
debris thickness, thermal gradient, length of the ablation
season and rainfall. None of these methods explicitly
considers the effect of temporal variation of radiation,
though many use a degree-day approach to estimate time-
averaged ablation.
Relatively few studies have considered the effect of
rainfall on ablation rates, where the presence of percolating
water can affect the heat flux through the debris layer itself
and thus modify the ice-surface melt rate (McSaveney, 1975;
Bozhinskiy and others, 1986; Ro
¨hl, 2008). It is established
that rain and snowmelt contribute negligible heat to debris-
free temperate glaciers (Paterson, 1994); however, different
thicknesses and properties of the debris cover will alter the
contribution of rainfall to ablation beneath it. Thus the effect
of the heat advection by rainfall through different debris
depths may be significant and still needs detailed investi-
gation. This study demonstrates the role of debris permea-
bility in modulating ice-surface melt rates under rainfall.
All previous data describing ice ablation rates under
debris cover have been derived from observations or
experiments conducted on glaciers, and thus involve site-
specific physical parameters that may affect ablation. These
observations were mostly obtained for small (and spatially
varying) debris thicknesses, and the resulting models were
based on parameters obtained from field observations or
adjusted according to specific site conditions. Such ap-
proaches inevitably leave considerable uncertainty about the
physical processes involved in ice-surface ablation beneath
debris covers of different thicknesses. Herein we present the
results of laboratory experiments on the effect of debris cover
on underlying ice-surface ablation. This appears to be the
first experimental approach to this problem; it directly
measures the effect of a debris cover of known depth and
physical properties on ice melt under controlled conditions,
and provides potentially useful insights into the effects of
debris on ablation rates under different insolation regimes.
We designed the experiments to test two specific
hypotheses:
1. That the insulating effect of debris on ice depends on the
occurrence of cyclic radiation input;
2. That the effect of rainfall on ice-surface ablation beneath
debris is affected by the permeability of the debris.
SUPRAGLACIAL DEBRIS
Supraglacial ablation-zone debris cover has two distinct
sources: englacial and extraglacial. The commonest source
of ablation-zone debris is from meltout of englacial debris,
which may itself be subglacially or extraglacially sourced
(e.g. rockfall in the accumulation zone which is buried by
snowfall (Sharp, 1949; Rogerson and others, 1986)). The
resulting cover is thin (up to 10 cm thick and very variable in
depth); it comprises usually angular clasts typically up to a
few tens of cm in size (Fig. 1a). Externally sourced material is
from rockfall onto the ablation-zone ice surface. Small
rockfalls leave deposits that may appear similar to englacially
derived cover, but are less areally uniform in depth. Large
rockfalls (of the order of hundreds of thousands or millions of
cubic metres, known as rock avalanches), however, leave
distinctly different debris cover. This is usually one to several
metres thick, has always been supraglacial, and comprises
high proportions of very fine material below a ‘carapace’ of
coarser material, giving it lower permeability than ‘normal’
(englacially derived) debris (Fig. 1a and b).
While there are certainly intermediate members, our
experience on debris-covered glaciers in New Zealand is
that these two types represent the majority of debris-cover
occurrences. Most of the area of debris-covered glaciers is
covered by the first type of debris. Rock-avalanche debris is
less common. We do not consider terminal, medial or lateral
moraines herein, as they occupy a relatively small propor-
tion of glacier area.
In the laboratory, we used well-sorted medium grey-
wacke river sand to represent normal (englacially derived)
supraglacial debris; like normal supraglacial debris the
material is highly permeable (Drewry, 1986) and has
relatively low thermal inertia. To represent rock-avalanche
debris we took material from below the carapace of the
Coleridge rock-avalanche deposit, New Zealand (Lee and
others, 2009). This material is typical of rock-avalanche
Fig. 1. (a) Meltout debris on Tasman Glacier, New Zealand. Note the thickness of the debris layer (a few cm) and the relative lack of fines.
(b) Rock-avalanche debris at the Mueller Glacier terminus, New Zealand, about 2–3m thick. Fines are plentiful, and there is wet material at
the ice contact.
Reznichenko and others: Effects of debris on ice-surface melting rates 385
debris: very widely (fractally) graded, with low permeability,
and with coarser fragments embedded in pulverized matrix
and containing abundant ‘powder’ (Hewitt and others,
2008). Being poorly sorted, it has a higher bulk density
and thermal inertia than the sand. The contrast in properties
between the sand and the rock-avalanche debris is similar to
that between the two main types of supraglacial debris, so
the difference in their effects on ice-surface ablation will be
similar to that of the two types of supraglacial debris.
EXPERIMENTAL METHODS
The objective of the experiments was not to reproduce
conditions on a glacier, but to study the processes by which
the presence of overlying debris affects ice-surface melting
due to incident radiation and rainfall. Thus we use radiation
that may not correspond to specific glacial conditions in, for
example, wavelength or incident angle, but we do use a
realistic timescale for radiation cyclicity. Similarly the
experimental debris in one case (sand) does not correspond
in size range to supraglacial debris; we use two types of
debris with identical lithology but very different grading and
permeability, to represent the relevant differences between
rock-avalanche debris and ‘ordinary’ supraglacial debris.
In the experiments, blocks of bare ice and identical
blocks of ice with different thicknesses of debris cover were
exposed to identical radiation (either constant or diurnally
cyclic), and the differences in ice surface ablation rate were
measured both with and without precipitation. The ice
blocks (approximately 450 350 260 mm
3
) were made by
freezing approximately 35 L of water in insulated containers;
these containers had basal drains which were opened once
freezing was complete. To simulate solar radiation, incan-
descent light bulbs with different light spectra (short- and
longwave radiation of 350–750 nm wavelength using
incandescent and warm white electric bulbs) were used
simultaneously. Two types of debris were used as noted
above, specific thicknesses of which (10, 50, 90 and
130 mm) were uniformly placed on the ice surface.
Because the objective was to compare melt rates between
bare and debris-covered ice under identical conditions, it
was not necessary to monitor or control ambient conditions
in the laboratory; however, debris-surface air temperatures
gave a good indication of variations in ambient temperature.
Air temperatures at the debris surface, within the debris layer
and at the ice–debris interface were measured to 0.58C
with thermocouples and recorded on a Campbell Scientific
21X micrologger connected to a computer (Fig. 2). Heat-flux
sensors measured the vertical local heat flow (W m
–2
) at the
same positions as the thermocouples. All data were
recorded at 10 min intervals. Ice-surface levels were meas-
ured at intervals of 12 hours with 2.5 mm accuracy, using a
system of vertical strings frozen inside the ice blocks. Water
discharge rates (mL h
–1
) of melted ice were recorded, which
included ice-surface melting plus the (minimal) melting
adjacent to the insulated sides and bottom of the container.
To examine the influence of debris cover on ablation rates,
‘bare-ice’ rates were compared with those under debris
thicknesses of 10, 50, 90 and 130 mm. Two sets of
experiments were conducted. In the first, bare and debris-
covered ice blocks were exposed to identical steady
continuous radiation until melting was complete. This we
refer to as the ‘steady-state’ experiment, where the main aim
was to see how the debris functions as an insulator. The
second set of experiments examined the effects of diurnal
temperature and radiation cycles, which were generated by
alternately cooling the ice in chest freezers for half of the
cycle and exposing it to radiation for the same length of time.
The experiments to study the effect of rainfall were
conducted in a similar manner to the cyclic experiments, but
with 10 mm depth of water sprayed onto the experiment
each day over the course of 1hour during the radiation part
of the cycle. We used only the shortwave radiation bulb
during this hour to eliminate longwave heating and replicate
the cooling effect of cloud cover. We carried out two sets of
rainfall experiments, one set with low-permeability rock-
avalanche debris and the other with high-permeability sand.
Note that rock-avalanche debris was not used in the non-
rainfall tests, because using small depths of material (up to
130 mm) would have been very unrealistic; rock-avalanche
deposits on glaciers are rarely <1 m thick (Shulmeister and
others, 2009). We expect that the effect of rock-avalanche
debris on ablation in the absence of rainfall would be similar
to that of sand, with the higher thermal inertia due to the fines
content increasing the delay before steady-state conditions
are achieved. Considering the depth of rock-avalanche
debris, we expect that, in reality, dry rock-avalanche debris
would effectively cause ice-surface ablation to cease.
The data collected are presented as graphs of temperature
profiles, heat fluxes, ice surface reduction rates, water
discharge rates and coefficient of ice surface melting (k)
against time, for each debris-cover thickness. kwas defined
as the ratio of the melting rates of debris-covered ice and
bare ice:
k¼Vd=Vb,
where V
d
and V
b
are the melting rates of the debris-covered
ice surface and the bare ice surface respectively.
EXPERIMENTAL RESULTS
Steady-state conditions
The first series of experiments with constant radiation and
narrowly graded sand showed that previously reported rates
of ice ablation, as a function of debris-cover thickness (e.g.
Fig. 2. The experimental arrangement for tests with diurnal cycles:
(a) melting blocks of bare and debris-covered ice exposed to
identical radiation during the 12 hour ablation period (attained by
two electric bulbs with short- and longwave radiation, shown by
grey lines) and cooling by freezer during the 12 hour night (shown
by black lines); (b) Campbell Scientific 21X data logger and PC,
which recorded temperature and heat-flux profiles.
Reznichenko and others: Effects of debris on ice-surface melting rates386
Østrem, 1965; Lundstrom and others, 1993; Mattson and
others, 1993), corresponded in general with laboratory data
at the start of the experiment; ice initially melted faster
under 10 mm debris cover, and more slowly with thick-
nesses 50 mm, than with bare ice. The reduction of
melting rate under the thicker debris cover was primarily
caused by the absorption of heat by the initially cold debris
cover at the beginning of the experiment, which delayed
and retarded the melting of the underlying ice. Once heat
conduction through the debris achieved a steady state, and
the melting rate became constant, the ice-ablation rates
under different debris-cover thicknesses and with bare ice
were fairly similar (Fig. 3); the melting rate with bare ice
was about 10% less than with 10 mm of debris, presumably
an albedo effect.
The effect of the debris in slowing the melting rate prior to
stabilization varied in direct relationship to its thickness.
Additionally, thicknesses greater than 90 mm delay the onset
of melting significantly. The onset of ice melting under
130 mm of debris was delayed >12 hours and the system
stabilized only after 60hours (Fig. 4). The duration of
retarding of the melting rates depends on the initial
condition of the system (e.g. debris thickness, its physical
properties and the initial debris temperature) and on the
radiation intensity.
These results show that, under constant radiation, signifi-
cant debris cover delays the occurrence of steady-state
ablation, but once this occurs, the debris does not affect it.
In particular, under steady-state conditions it took >12 hours
to establish constant heat conduction and to achieve the
same ablation rates as those for bare ice with a debris layer
thicker than 50 mm (Fig. 4). This suggests that under
diurnally cyclic radiation, steady-state heat transmission
may never occur through debris of this thickness. We
therefore investigated the ablation of ice beneath debris
layers under diurnally cyclic radiation and cooling.
Diurnal cycles
Under diurnal cycles, the time-averaged melt rate was
slower, so it took longer to melt the ice than under steady-
state conditions (Fig. 5). As suggested by the steady-state
tests, with debris thicknesses >50 mm, a stable heat-
conduction profile, in which the debris thickness does not
affect melting rates, was never achieved under diurnal
cycles (Figs 6 and 7).
Heat-flux and temperature-profile changes through the
debris layer clearly show the effect of the debris layer on heat
conduction (Fig. 6). After the ablation period of the cycle,
when radiation ceases and cooling begins, the system tem-
perature decreases. However, due to the heat capacity of the
debris, it takes longer than the cooling part of the cycle to
fully cool the debris. With every cycle these processes recur,
and consequently the system never reaches a steady state.
Figure 6 shows examples of the temperature profiles and heat
fluxes for one cycle through different debris thicknesses in
comparison with noncyclic experiments, where with 90 and
130 mm of debris layer the overall melting of ice is
significantly slower under diurnal conditions.
During all experiments the basal part of the debris layer
consistently became saturated with meltwater; and in
steady-state experiments, depending on the debris thickness,
the whole debris cover became saturated after a time. In the
Fig. 3. Ice-surface lowering of bare ice and ice under 10, 50, 90 and 130 mm debris cover in steady-state conditions. The almost parallel
lines after the initial period of heat-conduction stabilization through the debris and almost constant melt rates indicate the similar effect of
different thicknesses of debris cover on ablation rates.
Fig. 4. Water-discharge rates for bare ice and ice under 10, 50, 90 and 130mm debris cover in steady-state conditions, where the arrows
indicate the end of melting for bare ice and ice under debris of different thicknesses (note also the different initial rates of melting under
different thicknesses of debris). The decrease of melt rate with time is caused by the increasing distance of the ice or debris surface from the
radiation source as melting proceeds.
Reznichenko and others: Effects of debris on ice-surface melting rates 387
diurnal-cycle experiments, this process was significantly
retarded and it took of the order of days to saturate the debris
layer to >50 mm from the ice surface.
The results from all non-rainfall experiments are summar-
ized and presented in Figure 8. For each experiment the
average coefficient of ice-surface melting (k) was calculated
and plotted according to the debris layer thicknesses.
Although the average kfor the steady-state conditions was
calculated from data after stabilization of the heat flux
through the debris and melting rates, the average kfor the
ice under cyclic conditions remains significantly lower.
Effect of rainfall
Rainfall experiments were conducted with two different
debris materials: one of these was narrowly graded medium
sand with high permeability (1.6 10
–3
ms
–1
), used in the
other experiments; the other was rock-avalanche material
with lower permeability (1.8 10
–7
ms
–1
). Rain percolated
through the medium sand and reached the debris–ice
interface in about 1 hour, and caused the melting rate under
the debris cover to increase (Fig. 9). Melting rates under the
debris were higher than those with no rainfall because
percolating rain advects heat from the warm debris to the ice.
The sand used in our experiments is intended to represent
thin supraglacial material, which also has a high permea-
bility, and has a similar effect on ice-surface ablation.
Because rainfall accelerates the ablation of the bare ice to
almost the same degree as with debris cover (Fig. 9), it could
be concluded from our experiments that the rainfall has the
same effect on a clean glacier as on a glacier with a thin
supraglacial layer of highly permeable debris cover.
The experiment was then repeated using 90mm depth of
rock-avalanche material instead of sand. Rock-avalanche
material differs from ordinary supraglacial debris by the
much greater proportion of very fine (micron-scale) material,
which remains preserved within the avalanche deposit
below the noticeably coarser carapace (McSaveney and
Davies, 2007; Shulmeister and others, 2009), and, as a
result, it has low permeability. It is also usually much deeper
(of the order of 1–5 m; Shulmeister and others, 2009) than
normal supraglacial debris sourced from meltout of en-
glacial debris. Our experiment had an unrealistically small
Fig. 5. Ice-surface lowering of bare ice and ice under debris cover of 10, 50, 90 and 130 mm under diurnal-cycle conditions; note the
duration of the experiments and different slopes of the ice-surface lowering lines in comparison with steady-state conditions (Fig. 3). In both
sets of experiments the initial ice volumes were equal; however, the diurnal-cycle experiments were not all run to the end of melting.
Fig. 6. Examples of heat fluxes and temperature profiles through a 90 mm debris layer (at debris-cover surface, at depths of 30 and 60 mm
and at ice–debris interface) under diurnal-cycle conditions. ‘h’ indicates the heating part of the cycle with radiation exposure, and
‘c’ indicates the cooling part of the cycle. Heat-flux profiles show the delayed response of the deeper layer of the debris to radiation exposure
during the ablation period of the cycles, where it takes >6 hours (half the period) to start heat conduction through the whole layer. Note the
same trend in the temperature profiles, where as a result the temperature variation decreases towards the ice–debris interface.
Reznichenko and others: Effects of debris on ice-surface melting rates388
depth of rock-avalanche material, so the effects we noted
would be increased by the usually much greater depth.
During the experiments with rain on rock-avalanche
material, cyclicity played a crucial role. The rate of heat
advection by rain through the rock-avalanche debris was
very slow because saturation of the initially dry debris layer
required longer than the 12 hour ablation period, so the
debris was unable to transmit heat by water to the debris–ice
interface. Furthermore, during the cooling period of the
cycle, the saturated layer of debris froze, and consequently,
during the next cycle, normal heat percolation through the
debris layer was delayed because extra energy was needed
to melt the interstitial water in the debris. In this case, the
start of ice ablation under the rock-avalanche debris cover
was significantly delayed; in fact, in our study, ablation did
not initiate (i.e. the ice-surface temperature never rose to
08C; Fig. 10). This contrasts with the acceleration of ablation
with rainfall under a 90 mm sand cover (Fig. 9). It is
concluded that debris permeability is a crucial parameter
that affects ablation during rainfall, and the direction and
intensity of the effect will depend on the amplitude of the
diurnal cycles, the duration of the part of the cycle that is
below 08C, and debris-cover thickness at a particular glacier.
In particular, the presence of rock-avalanche debris can
significantly reduce rainfall-generated ice-surface ablation.
DISCUSSION
The experimental data indicate clearly (1) the importance of
the diurnal radiation cycle, and (2) the importance of debris
permeability, on the rate of ice-surface ablation beneath a
debris cover due to radiation and rainfall, supporting our
hypotheses. The direct application of the laboratory data to
any specific glacier, however, would be unrealistic due to
Fig. 7. Examples of one cycle (24 hours) of temperature profiles and heat fluxes through debris-cover thicknesses of 50mm (a), 90 mm (b) and
130 mm (c), where the first 12 hours of the cycle is a cooling part, in comparison with the steady-state temperature profiles and heat fluxes
through 50 mm debris-cover thickness. Note the constant heat transmission through the layer under steady radiation in comparison with
changes under diurnal cycles.
Fig. 8. Average coefficients of ice-surface melting ratio (k) for bare
ice and ice under 10, 50, 90 and 130 mm of sand debris cover.
Black points: steady-state experiments after the heat flux through
the debris layer stabilized; grey points: cyclic experiments.
Fig. 9. Examples of the ice-surface level lowering beneath 90 mm of
sand and with bare ice under cyclic conditions; and also with bare
ice and with ice covered with 90mm of sand, with 10 mm of water
sprayed onto the surfaces daily.
Reznichenko and others: Effects of debris on ice-surface melting rates 389
the idealized laboratory conditions required for testing
specific hypotheses. On a glacier, a wide range of additional
effects occur that will potentially affect ice-surface ablation,
including variations in intensity, wavelength and angle of
incidence of radiation; variations of temperature and
humidity different from those in the laboratory; sensible
heat and upwelling longwave radiation fluxes; wind; spatial
variations in debris thickness and composition; varying
rainfall; and ice motion. Evidently, realistic reproduction of
specific field conditions in the laboratory is extremely
difficult, so we have concentrated on investigating some
basic processes. Nevertheless, as demonstrated below,
laboratory data indicate that diurnal cyclicity is crucial to
insulation under debris, and suggest that the global variation
of diurnal cyclicity may be a significant parameter in the
global variation of glacier-surface insulation by debris.
Diurnal radiation cycles and debris properties
The steady-state experiments showed that melting of the
underlying ice begins with different rates for different debris
thicknesses, whereas, after the system achieves a steady state
or stabilizes (which takes longer with greater debris depths),
melting rates are similar under different debris depths. The
main conclusion is that the debris cover delayed the onset of
steady melting but did not significantly affect overall ice-
melting rates after the stabilization of the heat conduction
profile through the debris layer. By contrast, under diurnal
cycles, when the system is repeatedly exposed to changing
environmental conditions, these delays in response, re-
peated daily, result in the insulating effect of the debris cover
on the glacier ice.
This suggests that the ablation rates of ice under debris will
vary with the relative amplitudes and durations of the heating
and cooling parts of the radiation cycle, because these
determine the degree of insulation effect of the debris cover.
In other words, a given depth of given debris has a particular
thermal inertia; the duration and intensity of the radiation
cycles, relative to this inertia, determine the delivery of
energy to the underlying ice and thus the melt rate.
The diurnal cycles also affected melting rates under debris
cover with rainfall. The extra energy input to a glacier due to
rainfall above 08C was pointed out by Paterson (1994) in
calculating the effect of rainfall on ablation rates of clean
ice, but he considered these effects insignificant in mass-
balance changes of the glacier. It is to be expected, however,
that when water can percolate through sun-warmed debris
to the ice surface, melt will be enhanced, as we found. An
unexpected effect was that the low-permeability rock-
avalanche debris not only reduced the enhanced melting
effect of the rainfall; as a result of the freezing part of the
cycle, it reversed that effect and, in our laboratory
conditions, led to the cessation of ablation under the debris.
Evidently, freezing may not be a universally occurring effect,
depending on debris depth, permeability and diurnal cycle
characteristics. However, the fact that it may occur indicates
that radiation intensity and duration (cyclicity) and debris
permeability need to be included in the estimation of
ablation rates under debris cover, rather than using only
debris depth and daily average values of radiation (e.g.
Kayastha and others, 2000; Mihalcea and others, 2006).
Critical thickness and diurnal cyclicity
The critical thickness (defined as the thickness at which the
underlying ice ablation equals that of adjacent bare ice, and
with increase of which the debris has an insulation effect on
the underlying ice) has been reported at a number of glaciers
round the world (Table 1). Critical thickness is determined
by the same parameters that determine total ice-surface
ablation, so consistent trends in critical thickness indicate
consistent trends in debris insulation of underlying ice.
Despite the presence of significant local effects, the global
distribution trends of critical thicknesses are quite distinct
(Fig. 11), and indicate the general trend of the effect of
debris cover on glacier-surface ablation; this must reflect the
effect of radiation cycle variability, since noise from local
effects will be more-or-less randomly distributed. Numerous
field experiments on snowmelt under dust show that critical
thickness increases linearly with increasing average solar
energy input rate (Adhikary and others, 1997).
Our experiments demonstrated the crucial role of diurnal
cyclicity in generating the insulating ability of debris cover.
Therefore we expect the variability of global radiation
Fig. 10. Examples of the heat-flux and temperature-profile changes through 90 mm of rock-avalanche debris layer under diurnal-cycle
conditions with rainfall of 10 mm d
–1
. Temperature at the ice–debris interface (grey solid curve) during the experiment remains negative and
does not reach melting point due to very slow heat conduction through saturated frozen rock-avalanche debris. ‘h’ indicates the heating part
of the cycle with radiation exposure, ‘c’ indicates the cooling part of the cycle and arrows indicate the occurrence of rainfall during the cycles.
Reznichenko and others: Effects of debris on ice-surface melting rates390
cyclicity to be reflected in debris-cover insulation, and
Figure 11 shows that global solar-energy input cyclicity
(varying with latitude and elevation) indeed correlates well
with critical thickness. Although most of the field obser-
vations in Table 1 were conducted during the maximum-
ablation seasons of the glaciers, local factors (e.g. meteoro-
logical conditions, slope orientation and shading, measure-
ment technique) will cause significant differences in
ablation rates. However, the general trends of Figure 11
are clear and must be controlled by the radiation cycles; the
critical thickness decreases with increasing latitude and
increasing elevation.
The decrease of the critical thickness with increasing
latitude is explained by the decreasing amplitude of the
diurnal cycle with increasing latitude, and the correspond-
ing decrease of average temperatures (Barry, 2008). With
increasing altitude the diurnal amplitude increases, due to
enhanced outward radiation (Oerlemans, 2001) and lower
Table 1. Measured critical thicknesses on glaciers; critical thickness is thickness at which sub-debris ablation rate equals ablation of adjacent
bare ice
Glacier Country Latitude Elevation Critical thickness Source
8N m a.s.l. mm
Khumbu Nepal 27.57 5400 50 Kayastha and others (2000)
Lirung Nepal 28.13 4400 80 Tangborn and Rana (2000)
Rakhiot Pakistan 35.21 3400 30 Mattson and Gardner (1989)
Barpu Pakistan 36.11 4000 25 Khan (1989)*
Kul’dgilga Kyrgyzstan 39.30 5000 115 Demchenko and Sokolov (1982)
Djankuat Russia 42.12 2700 70 Popovnin and Rozova (2002)
Eliot USA 45.23 2200 40 Lundstrom and others (1993)
Peyto Canada 51.41 2600 15 Nakawo and Young (1981)
Dome Canada 52.12 2200 20 Mattson (2000)
Athabasca Canada 52.12 2200 20 Mattson (2000)
Bilchenok Russia 56.10 700 40 Yamaguchi and others (2000,
2007)
Kaskawulsh Canada 60.46 1000 35 Loomis (1970)*
Isfallsglacia
¨ren Sweden 67.54 1200 30 Østrem (1965)
Fig. 11. Critical thicknesses (mm) are varying with altitude (m a.s.l.) and latitude (Northern Hemisphere) from the available observed data
(Table 1). The critical thickness of the equivalent latitudes (dashed lines) decreases with increasing elevation. Kh: Khumbu Glacier, Nepal
(Kayastha and others, 2000); Ku: Kul’dgilga glacier, Kyrgyzstan (Demchenko and Sokolov, 1982); L: Lirung Glacier, Nepal (Tangborn and
Rana, 2000); Ba: Barpu glacier, Pakistan (Khan, 1989, in Kirkbride and Dugmore, 2003); R: Rakhiot glacier, Pakistan (Mattson and Gardner,
1989); D: Djankuat glacier, Russia (Popovin and Rozova, 2002); E: Eliot Glacier, Oregon, USA (Lundstrom and others, 1993); P: Peyto
Glacier, Canada (Nakawo and Young, 1981); D: Dome and Athabasca Glaciers, Canada (Mattson, 2000); I: Isfallsglacia
¨ren, Sweden
(Østrem, 1965); Ka: Kaskawulsh Glacier, Canada (Loomis, 1970, in Kirkbride and Dugmore, 2003); Bi: Bilchenok glacier, Russia (Yamaguchi
and others, 2000, 2007).
Reznichenko and others: Effects of debris on ice-surface melting rates 391
temperatures at night. Absorption of shortwave radiation by
thin low-albedo debris cover during the day exceeds
absorption by bare ice (Slaymaker and Kelly, 2007);
however, the lower night-time temperatures enhance the
insulation effect of thicker debris covers, leading to in-
creasing requirement for heat percolation through the debris
during the next day to restart ablation. Figure 11 suggests
that at a given latitude there tends to be a slight decrease of
the critical thickness with increasing elevation; this indicates
that factors such as reduced atmospheric and vapour
pressure, humidity, and the frequency of below-freezing
temperatures outweigh the effect of enhanced daytime
radiation. This corresponds to the decrease of annual mean
ablation rate for bare ice with increasing latitude and
elevation found by Budd and Allison (1975, fig. 1).
Another factor that influences the intensity of the diurnal
cycles is the degree of continentality of the climate.
Continental climates are characterized by less cloudiness
and consequently greater shortwave radiation in the glacier-
surface heat budget than maritime climates (Oerlemans,
2001). Under continental conditions the low-albedo debris
cover absorbs more shortwave radiation and the debris-
cover insulation properties are reduced, which may explain
high critical thickness in the Pamir mountains at Kul’dgilga
glacier where very little cloud occurs and annual precipi-
tation is low.
There is a general correlation between radiation cycle
amplitude and intensity, and global distribution of critical
thickness that is logical in terms of the processes considered.
Other factors and processes (e.g. meteorological conditions,
debris-cover lithology and continuity, slope orientation and
shading, roughness, measurement technique, etc.) may
affect the distribution, but Figure 11, like the experimental
data, supports our hypothesis that radiation cyclicity is
important in determining ice-surface ablation under debris.
Application: debris cover and glacier mass balance
Our experimental results suggest that debris cover should
have a greater effect on ablation, and thus on mass balance
and glacier behaviour, in areas with strong diurnal cycles
such as the Karakoram Himalaya, compared with areas of
lower diurnal variability at higher latitudes, all other things
being equal. In a similar analysis of data from four glaciers
Mattson and others (1993) concluded that the location of the
glaciers will affect the intensity of the ablation (or the
quantity of melted ice) with differences in debris-cover
thickness. They point out that glaciers located at low
latitudes or high elevations will receive more energy per
unit time; however, when the debris cover becomes
relatively thick, the location becomes less important due
to the overwhelming influence of the debris cover.
If the debris cover significantly reduces ablation rates, the
behaviour of a glacier will to some extent become less
dependent on climate (Hewitt and others, 2008). The
reduced surface melt in the ablation zone of a rock-
avalanche-covered glacier causes ice to thicken due to
slower ablation (McSaveney, 1975). Thus, a large area of
thick ice persisted on Bualtar glacier, Karakoram (Hewitt,
2009), where rock-avalanche debris cover reduced ablation,
and had an impact on mass balance equivalent to a 20%
increase in annual accumulation. The relatively thin rock
avalanche deposited on Sherman Glacier by the Alaska
earthquake of 1964 (1.5 m deep on average; McSaveney,
1975) covered 50% of the ablation zone and altered the
mass balance of the glacier from negative to slightly positive
(Post, 1968; McSaveney, 1975), and the glacier is still
Fig. 12. The rock avalanche caused by the 1964 Great Alaska Earthquake covered part of the ablation zone of Sherman Glacier. (a) The rock-
avalanche cover after its emplacement in 1967. (b) The rock avalanche reached the terminus of the glacier in 2008 (pictures from
McSaveney).
Reznichenko and others: Effects of debris on ice-surface melting rates392
advancing some 45 years after the event (Fig. 12). Shul-
meister and others (2009) point out that such landslide-
driven advances (and their subsequent retreats) can generate
terminal moraines which can be misidentified as climate-
driven, and spurious palaeoclimatic inferences could be
drawn; given the crucial role of glacial palaeoclimatology in
understanding the likely effects of anthropogenic climate
change, improved understanding of the effect of supraglacial
debris on ablation, and hence mass balance, is important.
CONCLUSIONS
We have carried out the first laboratory experiments
comparing ablation from bare ice with that from debris-
covered ice, investigated the effects of debris permeability
and of rainfall, and suggested on this basis that variations in
diurnal cyclicity contribute significantly to the known global
distribution of critical thickness (a surrogate for the insulat-
ing effect of debris).
We conclude that:
1. The strength of the diurnal cycle plays a significant role
in controlling the rate of melting beneath debris:
in the absence of diurnal variability of radiation, the
primary role of debris cover is to delay the onset of
steady ice surface melting; once melting rates stabilize,
the debris has no further significant effect;
under cyclic diurnal radiation, a long-term reduction in
ablation rate occurs, and the degree of reduction is
controlled by the debris-cover thickness.
2. The effect of rainfall on ice ablation under debris
depends both on diurnal cyclicity and on the permea-
bility of the debris cover:
high-permeability thin supraglacial debris cover accel-
erates ablation from the ice surface to a similar degree to
that from clean ice under rainfall;
rock-avalanche debris is relatively impermeable and
under particular conditions can dramatically reduce
ablation rates with daily rainfall.
3. Ice-surface ablation rates are conditioned by debris cover
due to variations in diurnal cycle intensity and thus
geographical location because:
debris cover retards ice-surface ablation when its
thickness exceeds the critical thickness for a given
glacier;
the critical thickness generally decreases with increasing
latitude and with increasing elevation.
4. The effect of debris cover on the mass balance of a
glacier depends on the permeability and dimensions
(thickness and proportion of ablation zone covered) of
the deposit together with the diurnal-cycle amplitude at
the particular location.
5. Understanding the role of supraglacial debris on mass
balance is crucial in distinguishing terminal moraines
which reflect climatic change from those caused by
debris-cover induced advance.
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