Irek Sobota
Department of Cryology and Polar Research,
Institute of Geography,
Nicolas Copernicus University,
Fredry 6/8, 97-100 Toruń, POLAND, irso@geo.uni.torun.pl

Irek Sobota


"Ablation of Waldemar Glacier in summer seasons of 1996, 1997 and 1998"

INTRODUCTION


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 measurements were carried out in 5 days' intervals at 30 points of the glacier (Figure 2). Aluminium poles were installed at the depth of 1.5 metres. Such a dense network of the poles compared to a small area of the glacier enables to estimate precisely the size of ablation at the given height above seal level as well as the influence of the local conditions on its value.

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 time ablation rate during the individual seasons varied (Figure 4). In 1996 and 1997 ablation on the front part of the glacier was considerably bigger than on its firn part. Time oscillations of ablation were not big. In 1996 mean five-day's surface ablation amounted to 9.6 cm of water equivalent (w.e.) while in 1997 it was 8.2 cm w.e. In summer 1997 the highest rate of ablation took place in the early part of the analysed period. At some periods at the height above 320 m above sea level ablation did not take place; sometimes snow accumulation occurred. It was connected mainly with the weather conditions in different parts of the glacier.

In the 1998 summer, the five-days' average size of ablation for the entire glacier amounted to 14.7 cm w.e. Contrary to the previous years' summer seasons, ablation started nearly at the same time on the surface of the whole glacier and lasted for nearly the same period of time. This is the result of the similar tendency in temperature changes at individual parts of the glacier. It is significant that, unlike during previous summer seasons, the rate of ablation was relatively even. The highest rate of ablation was noted at the beginning of the analysed period. Next, the decrease of ablation followed by a slight increase at the end of the investigated period of time was observed. Big changeability of the ablation rate with time is characteristic for the Waldemar Glacier. Small area of the glacier, however, determines time variation of ablation at different levels significantly, mainly in summers of mean temperature higher than the long-term mean value.
Fig. 5 Ablation of Waldemar Glacier

Both the Waldemar Glacier's area and the height difference between the firn part and the front part (365 m) are relatively small. Nevertheless spatial variation of ablation is observed. Besides the gradient of the height that variation was caused by local conditions such as exposition, selective melting, slope as well as density and course of supraglacial streams I. Sobota 1998). In 1998 the highest ablation values were noted at the front part of the glacier up to the height of 250 m above sea level. The maximum value of total ablation was 160-180 cm w.e. at the height of 200 m above sea level, whereas the lowest value of total ablation was 106 cm w.e. at the height of 350 m above sea level.
During the two previous seasons the values of ablation were much lower, but the spatial differentiation was high. The highest ablation values were noted in the front part of the glacier as well as its medial moraine. In 1997 the highest ablation values were observed in the front part of the glacier up to the height of about 250 m above sea level. The maximum size of ablation amounted to 119 cm w.e. at the height of 150 m above sea level while the lowest total ablation, 19 cm w.e., was recorded at the height of 430 m above sea level. Ablation gradient at the individual measurement terms ranged from 0.3 cm to 3.6 cm w.e. per every 100 m of height (I. Sobota 1998). There is certain regularity observed in the spatial ablation differentiation of the Waldemar Glacier. The highest values of ablation are observed in its front part, at the ice-dammed lake within the dying part of the glacier, and along the medial moraine (Figure 5). Local conditions on the glacier influence such a distribution of ablation size to a high degree. While discussing the phenomenon of ablation the analysis of glacial zones of the glacier should be taken into account. C.S. Benson (1961) suggested the classification of glacial zones; F. Müller (1962) and J. Jania (1993) later completed it. The changes in glacial zones' location depend on local ice melting conditions and spatial diversion of winter snow accumulation. In summer 1998 the zones of slush or dry snow disappeared quickly due to high temperature. At the end of the ablational season only small patches of dry snow were observed mountain slope-foot (Figure 6). During the two previous seasons the share of the given glacial zones in the glacier' area was much bigger. Every summer season slush was observed, mainly in the places showing low inclination. Throughout the whole ablation period a big patch of slush existed in the middle part of the glacier at the heights from 240 to 300 m above sea level (Figure 6). That type of glacial zone is permanent for the firn part of the Waldemar Glacier. Additionally, slush avalanches were observed.

Ablation value getting lower with the increase of height above sea level is a characteristic feature of most glaciers. In summer 1996 and 1997 decrease of ablation with the change of the height was much bigger than in 1998 (Figure 7). In 1996 the ablation gradient amounted to 20 cm w.e. per every 100 m of height while in 1997 it was 22 cm w.e. In summer 1998 it reached half of that value, i.e. 10 cm w.e.. The attention must be paid to the fact that the lowest size of total ablation was not noted at the highest point of the firn part above sea level but a bit lower. It was the result of a warm summer. Ablation on the firn part of the glacier was observed from the beginning of the ablation season. The process was so intensive that a thick network of supraglacial channels together with a few ice crevasses developed, influencing variation of ablation to a great extent. It may be concluded that in the summer season of 1998 the glacier's height above sea level did not influence the spatial variation of ablation to a great extent. Besides its changeability with the height above sea level, ablation also varies at the same level. Sometimes these differences are significant as e.g. in summer 1998 when it was as big as 58 cm w.e. at the altitude of 200 m above sea level. In accordance with the general tendency towards ablation getting lower with the altitude, the values of ablation diversity grow. This situation is tightly connected with the local conditions of the Waldemar Glacier. Its lower part, showing biggest values of ablation, stretches up to the altitude of about 300 m above sea level. It is generally covered with surface moraine. The fact that at different altitudes areas of similar features exist decreases the role of altitude gradient in shaping the size of ablation. Local conditions at different places at the same altitude play very often a greater role. Such a relation finds its confirmation in the maps of Figure 8, where the percentage of total ablation from a given part of the glacier within the mean ablation from the whole glacier during the three summer seasons is presented.

The biggest value of total ablation, which amounted to 120.5 cm w.e. (lats measurement was taken 31.08.), was recorded in 1998. In the summer season of 1996 it amounted to 72.4 cm w.e., and in 1997 it reached 86.0 cm w.e. (Table 1). In the years 1996 - 1998 the cumulated ablation of the Waldemar Glacier amounted to about 280 cm w.e.; at the glacier's front part it was 395 cm w.e., and at its firn part - 180 cm w.e. (Figure 9).

The indicators most often used for estimating the size of ablation are the meteorological parameters, mainly air temperature and precipitation (including V. Schyt 1964, A. N. Krenke, W.G. Chodakow 1966, R.J. Braithwaite 1995, B.T. Rabus, K.A. Echelmeyer 1998). A distinct linear relationship between ablation and air temperature exists. A strong relation between the 5-days' mean air temperature and the 5-days' ablation is also observed for the Waldemar Glacier (the correlation coefficient was 0.80 in 1997, and 0.79 in 1996). To estimate the size of ablation, A.N. Krenke (1975) suggests taking the mean temperature of the three summer months (June - August) into account. Similar assumptions were accepted while estimating the Waldemar Glacier's ablation.

In the presented analysis meteorological data of the 1969 - 1998 period from the Ny Alesund station, located 35 kilometres form the glacier, were taken into account. Their great representativity for the Waldemar Glacier, which is confirmed by the values of correlation coefficient between the mean diurnal air temperature of the glacier and Ny Alesund area, is in favour of including them in the analysis. In summer 1997 the above correlation amounted to 0.86 while in 1998 - 0.77 (Figure 10). The relationship between the mean air temperature on the Kaffoyra Plain and Ny Alesund in the period from 21 July to 31 August in the years 1975 - 1998 was analysed. A high regressive value 0.90 was calculated. The correlation values between the two meteorological stations allow for treating the air temperature values in Ny Alesund as representative for the Kaffioyra area.
Using the method of the smallest squares and the statistical analysis of the data referring to the mean air temperature of the 1969 - 1998 period the ablation size for the Waldemar glacier was estimated. The formula of A.N. Krenke (1975) was used for the size of ablation estimation. The following regional form of the formula was established:

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 values calculated on the basis of the Ao formula need introducing a regional empirical correction a. Thanks to that the calculated values were similar to the measured ones. Thus the following version of the formula is final:

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.


The Waldemar glacier reaches similar values of ablation in summer season as the other Svalbard glaciers of similar area (Figure 12). The Midre Loven Glacier shows great likeliness to the Waldemar Glacier as far as time variation of the summer ablation size is concerned. Mean yearly ablation of the years 1969 - 1995 for the Midre Loven Glacier amounts to 111 cm w.e., thus it is similar to that of the Waldemar Glacier.

CONCLUSIONS

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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