Sustainable Mountain Development No. 56, ICIMOD, Winter 2009
and extrapolations from the well studied lower elevation the terminus is retreating. Data that report glacier retreat
glaciers to the higher elevations. describe only the conditions at the lowermost elevation
of the glacier where the current climate does not support
Glacier monitoring: terminus and mass balance
the extension, or even stability of the glacier. Thus,
terminus data alone cannot comprehensively represent
Terminus location: Recording the annual changes
those conditions controlling the changes in volume and
in the location of a glacier terminus is the simplest
mass across the entire elevation range of a glacier
measurement that indicates the status of a glacier
system. And to put the Himalayan region in a global
with respect to climate. Abundant terminus histories
perspective, the elevation range of the glacier systems in
are available in some parts of the world, Europe in
this region is the greatest in the world.
particular, while in regions such as the Himalaya, these
data are more limited (WGMS 2008). An example of Mass balance: Glacier mass balance studies in the
a summary of terminus data available for the Himalayan Himalayan region have been rare and often sporadic
region can be found in Eriksson et al. (2009). In over recent decades, with measurements on only
summary, these publications report that Himalayan about a dozen glaciers, and with only a very few of
glaciers are retreating at rates of 10 to 60 m per year those studies having a duration of more than a few
and many small glaciers (<0.2 sq.km) have already years. The conventional methods for measurement of
disappeared. mass balance, relatively common in Europe and North
America, have simply not been practical across the
It should be understood that the monitoring of the
remote and rugged terrain of these ranges. As a result,
terminus location of a glacier is neither a complete nor
very few data are available to assess the comprehensive
a comprehensive assessment of total glacier condition
‘health’ of Himalayan glaciers. In addition, the limited
or health. For example, if a glacier is noted to be
results that are available may only be representative
retreating, this simply means that the ice volume at
of those specifi c glaciers where the measurements
the terminus is melting faster than the rate at which
have been made, typically the more accessible sites at
ice is being supplied to that location by the dynamic
lower elevations. Therefore, it is important to develop
movement of ice from further upslope in the system. On
more spatially comprehensive methods to provide a
an annual basis, it is possible that a glacier could be
truly regional assessment of glacier health across the
gaining in total mass due to increasing amounts of snow
Himalaya.
arriving in the accumulation zone by precipitation, wind
deposition and avalanching, while, at the same time,
A simple methodology to compute glacier
ice melt
The annual melt from a glacier tends to increase with
Methodology and Data Sources decreasing altitude and can be represented by an
ablation gradient, the inverse relationship between
Our methodology is based on previous studies which involve concepts
glacier ice melt and altitude. This gradient is purported
variously referred to as the ‘ablation gradient’ (Haefeli 1962), the
‘mass balance gradient’ (Konz et al. 2006 ), and the ‘vertical budget
to remain constant even as the varying temperature
gradient, VBG’ (Kaser and Osmanston 2002). For any glacier, it is
and precipitation patterns from year to year may cause
assumed that the slope of this gradient, defi ned as melt/metre, (m/100
changes ranging from extreme positive to extreme
m) is relatively constant and is determined by the response of the glacier
negative net mass balance years. While complete
to regional climate variations (Armstrong 1989). The fi rst step in this
energy exchange models exist to compute melt at a
methodology is to determine the surface area over which annual melt
specifi c point on a single glacier, such a methodology
is to be calculated. We introduce the concept of an ‘E0
max
’, which we
defi ne as the highest annual altitude reached by the monthly mean zero
is not appropriate for a regional assessment, primarily
degree isotherm. By extrapolation from lower elevation station data
due to the lack of the required input data. Therefore, we
and from upper air temperature data, we estimate this average altitude
proposed an alternative method that we consider to be
to be approximately 5,400 m for the mountains of Nepal. We then
an optimal regional scale approach to determine the
compute melt extending down-glacier from the elevation of the E0
max
to
contribution of glacier ice melt to river runoff based on
the elevation of the glacier terminus. Melt over this area of the glacier
the data that are available (Alford et al. 2009).
is assumed to represent a net annual loss of mass to the glacier, i.e. it
does not include the loss of seasonal snow. For the Himalayas, ablation
For this study, it was necessary that the basins chosen be
gradients may range from 0.69 m/100 m for the Chhota Shigri glacier
in the Western Himalayas (Wagnon et al. 2007) to 0.81-1.3m/100
glacierised, contain stream gauges, and be covered by
m for Yala glacier in the Nepal Himalayas (Konz et al. 2006). In this
quality SRTM (NASA Shuttle Radar Topography Mission)
preliminary study we chose to use a mass balance gradient of 1.4 data in order to derive measurements of the elevation
m/100 in order to estimate possible maximum runoff.
15
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