Les variations des glaciers suisses en 1974-1975 et quelques indications sur les résultats récoltés pendant la Décennie Hydrologique Internationale de 1964-65 à 1973-74

Glacier variations in the Swiss Alps in 1974/75 and data obtained during the International Hydrological Decade from 1964/65 to 1973/74

Laboratoires de Recherches hydrauliques, hydrologiques et glaciologiques de l'école Polytechnique Fédérale à Zurich

Abstract

In the Swiss Alps the year 1974/75 brought early snow on the glaciers and a winter of high precipitation, with a very long duration of the snow cover as a result of late melting and a normal summer, so that it favoured the growth of the glaciers. The mean annual mass balances measured for four glaciers were all distinctly positive (cf. Table 2b). In the observation network of 115 glaciers changes in length were observed at the snouts of 107 glaciers (cf. Table 2a). Of these, 56 advanced, It remained stationary and 40 retreated. About two-thirds of the length changes were between -10 and +10 metres (cf. Figure 2). The relationship between the sign of the length change and various morphological parameters was investigated for a random sample of 96 glaciers from the observation year 1974/75. A significant relation to the sign was found for the size and the mean gradient of the glaciers, advances predominating among small and steep glaciers and retreats among the large and less steep ones. The relationship between the sign and the altitude of the glacier snout was significant only for a random sample limited to the 48 smallest glaciers. Another question investigated was whether the +/- ratio of the number of advancing to the number of retreating glaciers depended on their exposure. There was found to be a clear dependence, with a maximum of 1.72 in the NE sector, where the advances predominate, and a minimum in the SW sector, where the retreats are more numerous. In the International Hydrological Decade (IHD) of 964 (65 to 1973/74 the sum of the mean annual mass balances for the Aletsch Glacier (+ 156 g cm^-2) and the Silvretta (+ 169 g cm^-2) was positive, while it was negative for the Gries (- 91 g cm^-2) and Limmern (- 134 g cm^-2) Glaciers. On the average the overall mass balance for all glaciers in the Swiss Alps over the period of the IHD seems to be about even. The annual mass balances plotted in Figure 7 show that the long recession period ended with the extreme shrinkage of the year 1963/64 and that with the growth year 1964/65 a slight recovery of the glaciers began. This is also reflected in the increase of the number of advancing glaciers. During the period of retreat the discharges of the glacier streams were large and led to marked mass losses. The glaciers losing a good deal of their ice area particularly in the ablation zone. The deterioration of climatic conditions in the IHD led to a lowering of the equilibrium line and thus to an increase in the proportional area of the accumulation to the ablation zone. The result was an equalization of the mass balances and a reduced discharge of the glacier streams which was also aggravated by the decrease in melt water due to the foregoing loss of surface area. Consequently the power generation figures of the hydroelectric stations in catchment areas with high glaciation have also diminished in the last few years.