Ablation and snow accumulation are the basic processes influencing glacier mass variation. Their size relation allows estimating changes of glacier mass i.e. mass balance of glacier. Ablation is the process more or less responsible for ice mass decrement. This process is one of the main problems of glaciology, which has been dealt with by many authors (including W.W. Bogorodski 1968, Golub'ev 1976, D.E. Sugden 1976, B.S. John 1977, J. Jania 1993, 1994, J. Jania, J.O. Hagen 1996, J. Dowdeswell & others 1997). The problem of ablation is more and more often considered in mathematical and statistical formula, aiming to forecasting this occurrence in time (F.F. Hawkins 1985, R.J. Braithwaite 1986, V.G. Konovalov 1987, M.S. Pelto 1988a, M.S. Pelto 1988b, W, Ambach 1993, A.N. Krenke, V.M. Menshutin 1987, C. Vincent, M. Vallon 1997). Many of the issues connected with the problem have not been, however, well recognised so far.
The issue of ablation of the Spitsbergen glaciers has been discussed in many scientific papers (S. Baranowski 1977, J. Jania 1993, 1994, J. Leszkiewicz 1982, 1987, J. Jania, J.O. Hagen 1996, W. Haeberli and others 1991, 1994, 1996, S. Bartoszewski 1998). In spite of expeditions, which have been organised to Kaffioyra since 1975, glaciological research of the area rarely concerned ablation of the glaciers located there. Only since 1995 has that research been systematic being a part of ice mass balance estimation of the Waldemar Glacier (M. Grześ 1997, I. Sobota 1997, M. Grześ, I. Sobota 1997, M. Grześ, I. Sobota 1998). Water conditions of the Kaffioyra area, however, have been widely discussed in literature (among others: W. Szczepanik 1977; W. Szczepanik 1982; W. Marszelewski 1987; C. Pietrucień i in. 1987; R. Skowron 1995; I. Sobota 1997, I. Sobota 1998, D. Brykała 1998).
The research on the Waldemar Glacier's ablation was conducted in three summer seasons of 1996, 1997 and 1998. During the individual seasons the investigation period was different. In 1996 the measurements were taken between 29 July and 13 August, in 1997 between 21 July and 31 August and in 1998 between 21 July and 31 August. The research enabled to estimate the size of time variation of ablation, determine its relation to the weather conditions, mainly air temperature, as well as its diversity with the changes of the height above sea level. The estimation of the total value of summer ablation included the measurements carried out in the spring season preceding a given summer season. Figure 1 shows the network of the ablation measurements on the Waldemar Glacier. They were taken in three periods: the first one was in the spring preceding a given summer season, the next one during a given summer season and the last one in the spring of the next year (final poles' measuring). Thanks to that system it was possible to estimate the total ablation value. It is visible clearly that from ending the summer measurements ablation is small (correction 2). It is estimated that it makes only about 5% of the total ablation.
THE AREA OF INVESTIGATIONS
The investigated Waldemar Glacier of the alpine type flows down the glacier valley to the Kaffioyra Plateau. With the area of 2.66 km2 the Waldemar Glacier occupies 61% of the catchment basin locked by the ice-morainal ramparts at the water gap. Firn part of the glacier is located at a height of 380 - 490 metres above sea level, while front part at 130 m above sea level. The glacier is composed of two relatively parts separated by a rampart of medial moraine of 1,600 m (Figure 2). The foreland occupies the area of 0.44 km2 (K. R. Lankauf, Z. Preisner 1982). A small ice-dammed lake has been observed on the glacier's area since 1995, the development of which is tightly related to glacial recession causing ice melting and outflow blocking. The catchment basin of the Waldemar River occupies the area of 16.5 km2. Its surface has been shaped mainly by the activity of the Waldemar Glacier's water. Within the area of the catchment basin there are streams fed by water from ablation, rainfall as well as melting of the cores of ice-morainal ramparts. The outlet section of the river is influenced by the sea tides.
The location of the front part is also significant for glacial rivers' regime. The Waldemar Glacier has showed the signs of high intensity of recession lately. The area of the glacier has lowered at the rate of 1% annually (K. R. Lankauf 1989, 1993, 1995). That process undoubtedly influences the outflow form the catchment basin of the Waldemar Glacier to a large extent.
ABLATION OF THE WALDEMAR GLACIER
The investigations aimed to determine the course of ablation in time and its variation with height above sea level, as well as to estimate average ablation in summer season. Thanks to detailed research it was possible to compare the size of ablation with the size of outflow from a given part of the glacier.
Many agents, both morphological (morainal cover, slope, density of supraglacial streams, shielding) and meteorological (mainly air temperature) influence the ablation rate. The highest mean air temperatures were recorded in the summer season of 1998, which considerably influenced the size of ablation. The mean air temperature amounted to 4.2°C in 1997 and 6.3°C in1998 on the Kaffiöyra area (A. Araźny 1998a, 1998b). It reached 4.8°C on the Waldemar Glacier's surface, 5.5°C at its ice wall and 4.1°C at its corrie (according to the measurements of the automatic meteorological gauging station "Davis").
Time variation of ablation is tightly connected with the air temperature changeability. Such a relation was observed in the all analysed seasons (Figure 3).
The values calculated on the basis of the above formula (1) are in accordance with the measured values (figure 11, Table 2). It must be stressed that the 1998 value of ablation is not the total value (the last measurement took place on 31 August). In reality it is a bit bigger. That will improve the formula's precision. The accordance of the trends of the calculated values with the values calculated for the other glaciers of the region speaks in favour of the above formula (Figure 12). Slight changeability in the individual years is characteristic for the results of the calculations while it is much bigger for the other glaciers. Thus the suggested formula (1) was modified.
The error was calculated about 5-15 %. This formula can be used to calculate size of Waldemar Glacier ablation for long time period. If we use the suitable size of a constant it may be a theoretical method to estimate ablation for others glaciers. Average ablation of the years 1969-1998 for the Waldemar Glacier amounts to 86,3 cm w.e. according to Ao method and 93,1 cm w.e. according to A' method.
Despite its small size, the Waldemar Glacier shows considerable spatial and time variation of ablation. The size of the ablation gradient varies in dependence on weather conditions. Similarly to most of glaciers, ablation decreases significantly with the increase of the height above sea level. Such regularity, however, is not that distinct during a warm summer. During the all analysed summer seasons glacial zones were observed. They included slush, superimposed ice ice and dry snow patches. The highest total ablation, 120.5 co of w.e., was recorded in 1998. In the summer season of 1996 it amounted to 72.4 cm w.e. while in 1997 it was 86.0 cm w.e. (Table 1). The 1996 - 1998 cumulated ablation value reached about 280 cm w.e.; at the glacier's front part it was 395 cm w.e. and at its firn part it was 180 cm w.e.
On the basis of the detailed analysis of the meteorological data and their relationship to the size of ablation the Waldemar Glacier's ablation value for the1969 - 1998 period was calculated. It amounted to a mean value of 93.1 cm w.e. It was confirmed that the Waldemar Glacier's ablation values are like those of the other Svalbard glaciers of the similar area.
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