Temperature-index modelling of runoff from a declining Alpine glacier

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