Bradley, JD
2016,
Temperature-index modelling of runoff from a
declining Alpine glacier
, MSc by research thesis, UNIVERSITY OF SALFORD.
Abstract
The Gornera River, in the Pennine Alps, Switzerland, drains meltwater from
Gornergletscher and its tributary glaciers, which contribute a large proportion of the
runoff from this highly-glacierised basin. As the mass of ice has declined, recession
of many smaller tributary glaciers has resulted in their separation from the trunk
Gornergletscher. Declining glacier surface area might be expected to have reduced the
ice melt contribution to runoff, and since the Little Ice Age Maximum extent,
Gornergletscher has revealed a strong link between climatic change and ice-cover.
Glaciers in the Swiss Alps have revealed a total ice volume loss since the 1870s of
about 13 km3. Approximately 8.7 km3 of ice loss occurred since the 1920s, and a
further 3.5 km3 of ice mass were lost between 1980 and the present day.
This study aims to address three aspects of how changing temperatures and reducing
ice areas influence meltwater runoff.
1. Modelling runoff response from scenarios of modified air temperatures and ice
areas (e.g. a +1°C scenario with 20% reduced ice area), with the aim of finding the
extent to which the modified conditions influence ice melt. In order to model runoff
response, a temperature index model called RRM (Runoff Response Model) was set
up. RRM uses formulae to calculate runoff by inputting the following forcing
variables to the model: positive air temperature values, and ice area measurements by
elevation band, together with a degree day factor (DDF), and an air temperature lapse
rate.
2. By using the same method, the effects of climate variations on ice area were
investigated by generating modelled runoff quantities from each elevation band. The
purpose of doing this was to indicate how various areas of the glacier contribute
differing quantities of melt to runoff during the ablation season, and the potential
impact of loss of areas of ice. Using the model to show the highest contributing areas
was applied to Gornergletscher using the hypsometry of the basin – the second largest
glacier in the Alps. Gornergletscher differs from other Alpine glaciers as a result of
its wide and relatively flat trunk.
3. The study aims to calculate whether there is a linear or non-linear trend in ice area
change with elevation following model tuning. It is generally thought with
Gornergletscher that greatest ice areas are distributed at mid-elevations around the
trunk, where the tributaries join or joined the main ice body. Ice area in theory should
be most liable to melt at low and mid-elevations where both positive degree days and
exposed ice areas exceed those of higher elevations. The influence of basin
hypsometry on ice area change was studies by modifying the model to respond to
differing scenarios of energy availability and ice area available for melt.
The investigation aimed to calculate whether there is a linear/non-linear relationship
between ice area change and elevation in a modified climate scenario. It is considered
for Gornergletscher, that surface ice area distribution is greatest at mid-elevations,
where tributaries connect to the main glacier body. Ice melt in theory should be
produced most at mid-elevations where both - more ice area is exposed than that of
lower elevations and positive degree days exceed those of higher elevations.
Basin hypsometry is of interest for ice area changes because of the irregular ice area
distribution with elevation in the basin. The relationship between the wide and flat
hypsometry of the Gornera Basin and ice melt at elevation was considered by
modifying the model input values to reflect different scenarios of energy availability
and ice area available for melt.
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