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35UDC 528.8:551.345
landscape analysis of the changes in the number o f thermokarst lakes in West Siberian permafrost based on satellite images
NA. Bryksina1, Yu^. Polishchuk2
1 Immanuel Kant Baltic Federal University (Kaliningrad, Russia) 2Institute of Petroleum Chemistry, SB RAS (Tomsk, Russia)
Changes in the number of thermokarst lakes were investigated using multitemporal satellite images. On the territory of WestSiberian permafrost, 33 test sites located in different landscape zones were selected. One hundred and thirty four (134) cloudless multitemporal images Landsat were obtained during the warmer months in the period 1973—2013, were used for remote research. Processing and interpretation of satellite images were performed using software tools of modern geographic information systems ENVI 4.7 and ArcGIS 9.3. The total number of thermokarst lakes in 33 test sites of Western Siberia exceeded 50,000. It is shown that the number of disappeared lakes during the period of research on average decreases with increasing geographical latitude, and the number of newly appeared lakes in average significantly increases with increasing latitude. A comparison of the number of disappeared and newly formed lakes showed that the tendency to form new lakes dominated in tundra, and a tendency to reduce the number of lakes was revealed in foresttundra and the northern and middle taiga. The number of the newly appeared thermokarst lakes is significantly larger than that of disappeared lakes. So we can assume that the increase in methane emissions to the atmosphere with an increasing number of small thermokarst lakes in Western Siberia will contribute to the greenhouse effect.
Keywords: Thermokarst lakes, space images, permafrost, landscape analysis, Western Siberia.
Introduction
It is now well accepted that the increase in mean annual temperature of the earth's surface observed for the last 3—4 decades results in degradation of permafrost landscapes in Northern Eurasia. Thermokarst lake landscapes are extremely sensitive to temperature changes in the permafrost territory. In connection with permafrost thawing thermokarst lakes emerge and develop in rather a short period of time — several decades, and they may disappear very quickly turning to khasyreys (ditches of drained lakes). But the lifetime of some of these lakes can be as long as hundreds or even thousands of years [1].
There were periods when thermokarst lakes appeared en masse and everywhere. Thus, during a starting period of climate optimum of the Holocene in the north of Western Siberia mass and progressive formation of embryonic lakes was started [2,3]. Today, formation of thermokarst water bodies and depressions in connection with permafrost degradation for
the last 50 years has been observed in Alaska, Canada, and Europe [3-5].Our researches [6,7] also have shown that permafrost thawing under the conditions of global warming results in speedingup thermokarst processes and changes in lakes' areas in the permafrost zone in Western Siberia. Even so, none of the papers cited above as well as other published papers considers the issues of changes in the quantity of thermokarst lakes in the permafrost zone.
Accordingly, we attempted to determine the quantity of thermokarst lakes depending on landscape peculiarities of the territory under study, based on the analysis of the data on changes in the number of extinct and emerged lakes in the permafrost zone of Western Siberia during the period of research (1973-2013). Research into the question was conducted at long distance.
Data and object of research
Research into changes in the quantity of thermokarst lakes was done using satellite images Landsat made at different times. Thirtythree (33) test sites in Western Siberia were chosen for the research. Test sites (TS) were selected in the light of specific features of landscape and zone differentiation of the area [8]. In each landscape zone (subzone) several TS were selected, which made it possible to study the dynamics of the quantity of thermokarst lakes depending on the landscape zone of the area. Figure 1 shows the scheme map of landscape zones in Western Siberia; one can see that TS are evenly distributed all around the area under study. Table 1 gives the distribution of TS around landscape zones and subzones. This shows that there are 15 sites in the taiga zone and 13 in the tundra zone.
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Fig. 1. Schememap of landscape zones location in Western Siberia with plotted boundaries of test sites
Table 1. Distribution of test sites in various landscape zones
Landscape zones (subzones) Quantity of test sites (TS) TS numbers
Arctic tundra 3 TS31 - TS33
Subarctic tundra 10 TS21 - TS30
Foresttundra 4 TS17 - TS20
Northern taiga 12 TS4 - TS16
Middle taiga 3 TS1 - TS3
For remote research 134 cloudless Landsat images of the territory under study were selected made at different times during warm months in the period 19732013. Images from Landsat collection have geographical reference to transferred projection UTM (WGS84). Table 2 contains the data about the used satellite images taken within the research period 1973—2013.
The satellite images were processed and decoded with the help of modern geo information systems (GIS) ENVI 4.7 and ArcGIS 9.3. On each TS, from several hundreds to several thousands of thermokarst lakes were detected by means of GIS. The total amount of explored lakes on 33 TS in Western Siberia exceeded 50 thousand. Decoding satellite images made at different times enabled us to form digital maps depicting the lakes' location on each TS for the year of survey.
Table 2. List of used satellite images
TS number Starting year of the research Final year of the research
TS1 Landsat4 19.07.1988 Landsat8; 30.06.2013
TS2 Landsat1 27.06.1973 Landsat8; 22.06.2013
TS3 Landsat1 24.08.1973 Landsat8; 11.06.2013
TS4 Landsat1 12.06.1973 Landsat8; 22.07.2013
TS5 Landsat1 16.06.1973 Landsat8; 18.07.2013
TS6 Landsat1 24.08.1973 Landsat8; 20.07.2013
TS7 Landsat1 16.06.1973 Landsat8; 18.07.2013
TS8 Landsat5 21.08.1987 Landsat8; 27.07.2013
TS9 Landsat1 23.08.1973 Landsat8; 08.09.2013
TS10 Landsat1 24.08.1973 Landsat8; 18.06.2013
TS11 Landsat1 08.10.1973 Landsat8; 09.07.2013
TS12 Landsat1 27.06.1973 Landsat8; 24.07.2013
TS13 Landsat1 23.08.1973 Landsat8; 20.07.2013
TS14 Landsat5 27.07.1984 Landsat8; 27.07.2013
TS15 Landsat1 17.06.1973 Landsat8; 25.07.2013
TS16 Landsat5 23.06.1987 Landsat8; 08.08.2013
TS17 Landsat1 23.08.1973 Landsat8; 17.09.2013
TS18 Landsat1 23.08.1973 Landsat8; 20.07.2013
TS19 Landsat5 27.07.1984 Landsat8; 20.07.2013
TS20 Landsat1 22.08.1973 Landsat8; 22.07.2013
TS21 Landsat5 13.09.1987 Landsat8; 27.07.2013
TS22 Landsat1 10.08.1973 Landsat8; 30.06.2013
TS23 Landsat4 26.07.1983 Landsat8; 18.07.2013
TS24 TS25 TS26 TS27 TS28 TS29 TS30 TS31 TS32 TS33
Landsat5; 07.07.1987 Landsat4; 15.07.1988 Landsat4; 10.07.1988 Landsat4; 04.08.1988 Landsat4; 07.08.1988 Landsat4; 12.07.1988 Landsat5; 28.07.1984 Landsat2; 28.07.1981 Landsat2; 28.07.1981 Landsat5; 28.07.1984
Landsat8; 23.07.2013 Landsat8; 22.07.2013 Landsat8; 21.07.2013 Landsat8; 18.07.2013 Landsat8; 19.07.2013 Landsat8; 18.07.2013 Landsat8; 21.07.2013 Landsat8; 01.08.2013 Landsat8; 18.07.2013 Landsat8; 21.07.2013
The thermokarst lakes that disappeared and formed again within the period of study were detected by means of comparison between an initial and final maps of the lakes' location on each TS. As Table 2 shows, to make initial maps the images Landsat1 (1973), Landsat2 (1981), Landsat4 (1983 and 1988), Landsat5 (1984 and 1987) were used, and final maps were made with the help of images Landsat8, made in 2013. Images from Landsat1 (1973) were used to make similar initial maps on 16 TS.The images Landsat made in 1981-1988 were used for the rest of the TS.Consequently, using images from spacecraft Landsat8 brought into service in May 2013 makes it possible to assess changes (decrease or increase) in the quantity of thermokarst lakes on different TS within the periods of 25 to 40 years.
As it follows from the foregoing comparison between an initial and final map of lakes location on each TS reveals both the thermokarst lakes that disappeared and the ones that formed during the study period. As it follows from the foregoing, both the disappeared thermokarst lakes and the formed ones during the study period were determined on each TS. Further centres of disappeared and newly formed lakes were detected with ArcGIS 9.3, which is convenient for mapping these lakes.
Research results and analysis
Figure 2 shows the map locating extinct lakes in Western Siberia. From this, one can see that the number of disappeared lakes during the study period differs in various landscape zones. Black triangles in Fig. 2 mark the centers of disappeared lakes. The largest of density of lakes is characteristic the subarctic tundra, foresttundra and the northern taiga.
The whole number of disappeared lakes in 33 TS representing the permafrost zone in Western Siberia was 390 for the study period. The total area of shrunk water surface caused by extinction of lakes on the territory under study was 14826 ha for this period. It is interesting to consider the changes in the number of disappeared lakes to depend on the latitude of their location and a landscape zone.
Figure 3 shows a graph of changes in the number of disappeared lakes depending on the latitude of their location and a landscape zone. A separate point on the graph plotted in the form of triangles, squares or diamonds in accordance with the landscape zone shows the number of disappeared lakes on each TS. As the graph shows (Fig. 3) the number of disappeared lakes in the arctic tundra is noticeably smaller than in landscape subzones located southward. It is of interest to analyse changes of mean values of the number of disappeared lakes calculated for different landscape zones and subzones given in Table 3.
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Landscape analysis of changes in the number of thermokarst
lakes in West-Siberian permafrost based on satellite images
Table 3. The average number of disappeared and newly formed lakes over landscape zones (subzones)
Landscape zones (subzones) Average number of disappeared lakes Average number of formed lakes
Arctic tundra 3.5 369
Subarctic tundra 11.2 372
Foresttundra 23.5 188
North taiga 19.8 156.2
Middle taiga 47
The analysis of Table 3 shows that in the subzone of arctic tundra on average the number of disappeared lakes is several times smaller than in other landscape subzones. In the subarctic tundra the number of disappeared lakes is noticeably smaller as compared to the foresttundra and the north taiga located southward. Consequently, on the territory under study in the permafrost of Western Siberia the number of disappeared lakes decreases on average with increasing latitude during the period under study.
A similar analysis was done on the data concerning changes in the number of newly appeared lakes. It was stated that the number of newly formed lakes for the period under study in WestSiberian permafrost exceeded 7000 with a total water surface area of 13649 ha. Comparison of the data on the total number of disappeared and newly formed lakes and their total areas shows that in spite of practically similar total areas the number of newly formed lakes exceeds the number of disappeared lakes considerably (almost 18fold). Consequently, newly appeared thermokarst lakes are much larger than disappeared lakes. So we can assume that the observed speedup of thermokarst processes resulting from global warming will be accompanied by an increase in a number of small thermo-karst lakes in the West Siberian permafrost landscapes.
Figure 4 shows a graph of dependence of the number of newly appeared lakes for the period under study on the latitude of TS location. As in Fig. 3, each point in the graph depicts a number of appeared lakes on a separate TS. As can be seen from the graph, the number of newly appeared lakes rises on average, together with the increase in latitude. Table 3 give the mean values of the number of appeared lakes in each landscape zone (subzone).
NA. Bryksina1, Yu^. Polishchuk2
The analysis of the data from Table 3 shows that the mean number of appeared lakes in tundra subzones exceeds by several times a number of newly formed lakes in landscape zones (subzones) located to the south. Consequently, thermokarst processes in the arctic and subarctic tundra cause more intensive forming of new lakes there than in the forest-tundra and taiga zone in Western Siberia.
Comparing the data in Table 3 on the number of disappeared and newly formed lakes makes it possible to conclude that in the north of Western Siberia in the tundra zone the trend of forming new lakes prevails while the number of lakes in the foresttundra and northern and the middle taiga tends to decrease.
Conclusion
The results of our investigation show that in the West Siberian permafrost zone because of the thermokarst spurt under the influence of global warming two contrasting processes are taking place: thermokarst lakes' disappearing because of drainage and forming new thermokarst lakes, the latter process prevailing and accompanied by a relative increase in small lakes. As experiments in Western Siberia have shown [9—11], little thermokarst lakes can be viewed as natural sources of greenhouse gases, methane in particular. Therefore, it is possible to assume that rising emission of methane into the atmosphere caused by an increase in the number of small thermokarst lakes in Western Siberia will promote growth of the greenhouse effect.
The research is supported by a Russian Federal Programme grant 14.B25.310001 (BIOGEOCLIM) and the Russian Foundation for Basic Research grant (Project 130590742 mol_rf_nr).
References
landscape analysis of changes in the number of thermokarst lakes in westsiberian permafrost based on
satellite images N.A. Bryksina1, Yu.M. Polishchuk2 References
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About
Bryksina Natalia А. — Senior researcher, Immanuel Kant Baltic Federal University, Kaliningrad, Russia. E-mail: NBryksina@kantiana.ru
Polishchuk Yuri М. - Dr, Institute of Petroleum Chemistry, Siberian Branch of Russian Academy of Sciences, Tomsk, Russia. E-mail: yupolishchuk@gmail.com
