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PERMAFROST - Seventh International Conference (Proceedings),
Yellowknife (Canada), Collection Nordicana No 55, 1998
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WETNESS VARIABILITY AND DYNAMICS OF THERMOKARST PROCESSES
IN CENTRAL YAKUTIA
N.P. Bosikov
Melnikov Permafrost Institute, SB RAS, Yakutsk 677010, Russia
e-mail: [email protected]
Abstract
This report reviews the relationship between the development of thermokarst processes and general wetness
of an area. Alas lakes are mainly fed by precipitation. Ground ice feeds thermokarst lakes only during the initial
stage of their formation. Therefore, lakes of closed alas depressions can indicate the degree of general wetness
during different periods of time.
A plot of a tentative wetness coefficient (total annual precipitation divided by mean summer temperature)
shows several cycles (1891-1995) that largely coincide with the available information on fluctuations of lake
levels. This means that fluctuations in the alas lake level or the rate of development of a thermokarst process
can be deduced from a tentative wetness coefficient of an area.
Development of thermokarst process has a cyclic character. There is a multi-secular cycle, due to variability of
general wetness in a region, with secular and intrasecular fluctuations within it.
Introduction
A monitoring study of the development of
thermokarst lakes was carried out in the Lena-Amga
interfluve in the vicinity of the Yukechi alas (Central
Yakutia, Siberia) in order to establish the causes of the
initiation and growth of thermokarst under present-day
climatic conditions.
Some authors argue that thermokarst lakes barely
develop at present due to the dry climate (moisture
shortage) of Central Yakutia (e.g., Velmina, 1970; Grave
and Sukhodrovsky, 1978). However, our study indicates
that thermokarst lakes do develop at present in forest
clearings and in depressed terrain between the alasses
(Bosikov, 1977).
Study area and methods
Central Yakutia has a sharply continental climate:
winters are long and cold (- 60¡C), with little snow;
summers are short and hot (+32¡C). The mean annual
precipitation is 250-300 mm (Anonymous, 1982).
In the Yukechi area, the terrain between the alasses is
a plain, dipping slightly northwest, with absolute elevations of 200-220 m. The study area lies in the zone of
continuous permafrost, with thicknesses of 250-300 m.
Mean depth to ground ice is 2.2 m; the active layer
varies in thickness from 1.3 m in the forest to 2.2 m in
clearings. The thickness of the ground ice is 23-24 m. In
forest clearings, the depth of occurrence of ground ice
often coincides with the base of the active layer.
Systematic monitoring of the development of
thermokarst topography in the Yukechi area has been
carried out by us since 1975. Also, oral communications
with local people and aerial photographs dating from
1952 were used in our work.
In the study area, there are dozens of thermokarst
depressions in both undisturbed and disturbed
(ploughed) terrain, most of them developed up to the
stage of a thermokarst lake (dyuedya).
Water accumulation in depressions on the dry surface
creates favourable conditions for thawing of the ground
ice, i.e., for the development of thermokarst lakes. The
growth of a thermokarst lake is possible only when its
hydrologic balance is positive; with a negative balance
its growth ceases.
Thawing proceeds 1.5 to 2 times faster below shallow
water basins compared to dry polygonal formations
(Vassiliev, 1982). In the summer of 1993, measurements
of water temperature at the bottoms of inter-polygonal
depressions were made. Water depth changed from
1.0 m to 0.8 m during that time. Measurements were
taken three times a day: at 9 a.m., 1 p.m. and 9 p.m.
Also, the air temperature in grass was measured.
N.P. Bosikov
71
Table 1. Rate of expansion of lakes as revealed by on-site observations (in m)
It was found that the sum of the three measurements
a day at the bottom of a shallow water depression was 5
degrees higher in sunny days and 10.2 degrees lower in
cloudy days compared with the air temperature in the
grass. Thus, the cumulative thermal effect of a shallow
water basin on the underlying soils is much greater
than that of the air.
Results
An effect of this phenomenon is well demonstrated by
the maximum thaw depth. In the summer of 1993, the
maximum thaw depth in a dry glade was 245 cm,
whereas beneath a shallow basin it was 330 cm (below
its bottom). This means that conditions for strong thawing of ground ice are created below the bottoms of shallow basins because the depth of thaw exceeds that to
the ground ice.
The results of monitoring the growth of thermokarst
lakes are given in Table 1.
Graphical representation of the growth of a
thermokarst lake illustrates the rate of subsidence of
frozen ground and talik growth under the effect of surface waters (Figure 1).
Discussion
The above-mentioned data indicate that water accumulation in depressions between the alasses starts a
thermokarst process. Under conditions of dry soils, an
increase in thaw depth to the ground ice forms a so-
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called "protection layer" (Shur, 1988) that prevents further thawing of the ground ice. As is shown by our
monitoring results, a "protection layer" ensures stable
persistence of ground ice in dry, well-drained areas
because no conditions exist for stagnant water appearance.
Fluctuations in the water levels of the lakes depend
on total atmospheric precipitation of the area, but summer precipitation is found to have a definite influence
on such fluctuations.
Gavrilova (1973) established that the amount of lake
water evaporation is two to four times greater than that
of precipitation (in their basins) in Central Yakutia.
During some dry years, evaporation exceeds precipitation by six to seven times and in some summers, by
even more than ten times. From this it is clear that air
temperature during summer definitely affects the level
of water in the lakes.
An effort was made to find a relationship between
lake level fluctuations and general wetness of the study
area. This relationship is expressed as a tentative coefficient which presents a ratio of total mean annual precipitation to mean summer temperature (May-August)
for the same period (total annual precipitation is determined based on the hydrological year) (Shnitnikov,
1957). It is assumed that temperatures for May-August
are linked to mean conditions of annual evaporation
(aridity) (Figure 2).
Analysis of our collected data indicates that fluctuations of lake water levels have a cyclical character, with
The 7th International Permafrost Conference
Figure 1. Graph of growth of a thermokarst lake in the Yukechi study area (lake F).
1: water in inter-polygonal troughs; 2: change of lake water level with time; 3: depth of subsidence with time; 4: depth of talik beneath a lake.
the period from 1891 to 1995 showing the following
cycles: 1891-1901 - low level; 1902-1917 - high level;
1918-1930 - low level; 1931-1934 - high level; 1935-1950
- low level; 1951-1973 - high level; 1974-1979 - low level;
1980-1984 - high level; 1985-1995 - low level.
The highest level of the lakes was observed at the
beginning of the 20th century. Subsequent high level
cycles had lower absolute values, leading to drying-out
of the alas lakes.
In the 1952-1957 period, a decrease in general wetness
of the Lena-Amga interfluve occurred (see Figure 2),
whereas earlier years were characterized by greater
wetness. A close relationship between the initiation and
growth of thermokarst lakes and general wetness of the
area (Bosikov, 1977) suggests that the lakes which can
be seen in aerial photographs originated and grew
slowly during the earlier period of greater wetness. The
subsequent period of decreased wetness (1952-1957)
appears to have caused reduction in the size of lakes
because thermokarst processes had not yet reached the
stage of sustained development at the expense of
intense thawing of the ground ice.
In dry years, melt water evaporates quickly and initiation and growth of thermokarst lakes ceases. Many
lakes become smaller. This period might have coincided
with a reduction of those lakes which were situated in
the forest.
Our on-site observations of the development of
thermokarst processes in the Yukechi location (Table 1
and Figure 1) show that initiation and growth of
thermokarst lakes proceeds much faster in ploughed
terrain compared with undisturbed terrain between the
alasses: no new lakes appeared in undisturbed terrain
and seven new lakes developed in ploughed terrain
during the observation period. Our observations indicate that the degree of basin incision is almost the same
in both forested and ploughed terrains. The greater
depths of lakes A and X are explained by their longer
existence. Average expansion, which characterizes
growth of the lake area, is several times greater in
ploughed terrain than in undisturbed areas. A relatively
insignificant expansion of lakes K (9.6 m) is accounted
for by the limited size of the ploughed area. Where the
ploughed area is larger and drainage is lacking, the
degree of expansion is very high (e.g., 48.0 m for lake
B).
There was a dramatic increase in general wetness of
Central Yakutia in 1980 (see Figure 2). As a result, lake
Figure 2. Fluctuation of general wetness in Central Yakutia, Yakutsk station.
Compiled using A.V.Shnitnikov's method (1957).
N.P. Bosikov
73
water levels increased and intense development of
thermokarst processes took place in the Yukechi site
(Table 1, Figure 1). Also, intense thawing of the ground
ice beneath the lakes began. When it thaws completely,
feeding of the lakes will be restricted to atmospheric
precipitation, i.e., to the state of general wetness of the
area.
Conclusion
1. The probability of development of thermokarst is
three to four times greater in forest clearings between
the alasses.
surface. Slight depressions sometimes develop to form
a hillocky topography. Meltwater, accumulating in such
depressions during periods of greater wetness, causes
deeper thawing of the permafrost. As a result, with suitable cryolithological conditions, the surface is
depressed further to form a thermokarst lake.
3. Development of thermokarst process in Central
Yakutia has a cyclic character. There is a multi-secular
cycle, due to variability of general wetness of the area,
as well as secular and intrasecular cycles. 150- to 180year cycles are the most prominent.
2. Where inter-alas terrain is made up of ice-rich soils,
the depth of thaw increases during some hot summers,
but causes no significant subsidence of the dry ground
References
Anonymous (1982). The climate of Yakutsk. Gidrometeoizdat,
Leningrad (246 pp.) (In Russian).
Bosikov, N.P. (1977). Level dynamics and development of alas
lakes in central Yakutia (in Russian). Izv. VGO, 109, 357362.
Shnitnikov, A.V. (1957). Wetness variability of the continents in
the Northern Hemisphere. Zap. GO USSR, nov. ser., v. 16 (347
pp.) (In Russian).
Shur, Yu.L. (1988). The upper horizon of frozen strata and thermokarsts. Nauka, Novosibirsk (212 pp.) (In Russian).
Gavrilova, M.K. (1973). The climate of central Yakutia (in
Russian). Yakutskoye Knizhnoye Izd-vo (120 pp.).
Vassiliev, I.S. (1982). Rules governing seasonal thawing of soil in
eastern Yakutia. Nauka, Novosibirsk (133 pp.) (In Russian).
Grave, N.A., and Sukhodrovsky, V.L. (1978). Relief-forming
processes in the permafrost zone and principles of their
avoidance and limitation in man-developed areas. In
Proceedings of the Third International Conference on
Permafrost, Ottawa, v. 1, pp. 467-472.
Velmina, N.A. (1970). Hydrogeological characteristics of the
lithospheric frozen zone. In Cryohydrogeology. Nedra,
Moscow (326 pp.) (In Russian).
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The 7th International Permafrost Conference