Natsagdorj Sharkhuu and Sharkhuu Anarmaa
The main criteria of permafrost mapping and monitoring within the framework of ground temperature measurements in boreholes are:
- mean annual ground temperature (t) at level of zero yearly amplitude
- depth of active layer (h)
- depth of permafrost bottom (H)
- gradient of permafrost temperatures (g)
The methods of studying dynamics of cryogenic processes such as frost heaving, thermokarst and icing is based on leveling measurements with the help of a benchmark, level and staff. Measurements for studying dynamics of thermokarst, kurum and solifluction are made yearly in late September. Observations for dynamics of frost heave and spring icing are carried out in early spring (in March).
The primary goal of the Hövsgöl Project is to study the impacts of nomadic pasture use and climate change on watershed ecosystems, biodiversity and permafrost of the taiga (boreal) forest and steppe of northern Mongolia (Goulden et al. 2002). Therefore, the Hövsgöl Project includes permafrost studies in areas of the northeastern shore of Hövsgöl Lake. Permafrost studies consist of two interrelated topics, mapping and monitoring of permafrost temperature and depth of the active layer.
The Permafrost Study team carried out the following field works:
- Drilling within the Hövsgöl project areas of nineteen boreholes. In addition, three five-meter deep boreholes have been drilled for the purpose of hydrological measurements in Turag and Borsog valleys and near Hatgal village at the southern end of the Lake.
- During Autumn field surveys for 24 days in August-September, 2002 and for 20 days in September-October, 2003 ground temperatures were measured in newly drilled boreholes, located in 6 valleys of the Hövsgöl project area as well as in the long term permafrost monitoring boreholes located in areas of Ardag and Burenkhan mountains, Sharga valley and Hatgal village. In addition, some observations for cryogenic processes have been made during the field surveys.
Permafrost monitoring in the Hövsgöl project areas are still in a preliminary stage and includes monitoring of active layers and permafrost thermal state in some newly drilled boreholes. So, in order to estimate recent changes in permafrost of the areas, we need to use available data on permafrost monitoring from the Hövsgöl mountain region on the western side of Lake Hövsgöl.
The authors are conducting permafrost monitoring of Hövsgöl mountain region in Burenkhan and Hövsgöl phosphorite deposit areas, which are located about 30 km north-west from Muren and Hatgal respectively. The Burenkhan area is characterized by discontinuous and widespread island permafrost, which underlies 40 to 60% of area territory. Average thickness of permafrost with mean annual temperature of 0°C to -1°C can be determined as 5-20 m in the valleys, 20-50 m on the north facing slopes and top of mountains. Predominant permafrost in valleys has medium ice content. The Hövsgöl area that is located along the southwestern shore of Hövsgöl lake is dominated by continuous and discontinuous permafrost. Here, permafrost underlies almost in all types of landscape, but taliks (thawed ground) with temperature of 0-1oC mostly occur on southern slopes of mountains and along the Hövsgöl lakeshore. As permafrost temperature decreases from -0.5°C to -5°C, the extent and thickness of permafrost increases from 75% to 95% and from 20-40 m to 250-500 m respectively. Ice rich permafrost is observed in lacustrine and alluvial gravely sands. Average depth of seasonal thawing or active layer is 2.5-3.5 m in Burenkhan area and 2.9-3.0 m in Hövsgöl area. However, the depth does not exceed 1.5-2.0 m in water saturated loamy deposits of swampy valleys or on forested watershed and north facing slopes of high mountains and reaches 4-6 m in low moisture sandy deposits of dry valleys or on south facing slopes of mountains. Icings, frost mounds, stone polygons and hummocks among the cryogenic processes are commonly developed in the areas under consideration.
Since 1996 we have been carrying out geo-temperature measurements in 5 boreholes in Burenkhan area where we conducted geothermal measurements in period of 1983 to 1987. The boreholes are located on south and north facing slopes and in valley bottoms. Surface conditions at borehole sites almost were not disturbed during borehole drilling. From the result of the measurements conducted during the last 10 years, depth of active layers and mean annual ground temperature were increased by 6-12 cm and 0.20-0.27°C respectively. Therefore, it is proven that permafrost in the Hövsgöl mountain region has a tendency to degrade.
Since 1998 we have been carrying out geo-thermal measurements in 4 boreholes in Hövsgöl area where we conducted geothermal measurements in 1987. The boreholes are located on forested watersheds and south-facing slopes, and in valley bottoms. Surface conditions at some borehole sites were disturbed to a considerable extent due to removal of forest cover around the boreholes by 10-20 m in diameter. This factor leads to considerable increase in mean annual ground temperature and depth of active layers by 0.2-0.8°C and 15-25 cm per 10 years respectively. Therefore, it is proven that forest and moss covers create favorable thermal (cooling) conditions for formation and development of permafrost. According to measurements and calculations, mean annual ground temperature at a expense of forest and moss covers decreases by 0.4-1.2°C.
According to the comparative data (results) on permafrost monitoring in Mongolia, the permafrost in Hövsgöl mountain region is degrading more intensive than in Hentei and Hangai mountain regions. The permafrost degradation under influence of climate warming and human activities leads to some changes in natural and ecological balance of the project areas.
Distribution, thickness and temperature of permafrost and project area are very changeable even at short distance depending on various natural factors. This situation requires us to study thermal conditions of permafrost formation and development in detail.
Permafrost conditions are also being monitored by Dr. Bernd Etzelmuller and Eva Heggam of the University of Oslo. They have developed tomography profiles of permafrost conditions in several of the study valleys, and have used temperature data loggers buried in the surface soils to monitor year-round temperature conditions under different plant cover situations and on different hill slopes facing towards or away from the sun.
Dr. Vladimir Romanovsky has placed two meter-long MRC temperature probes with thermisters at 10 cm intervals, in Ih Dalbay and in Turag to study temperature changes with depth throughout the year. These data will be used to develop a model of permafrost thaw conditions.
Due to soil characteristic and vegetation cover active layer depths and ground temperatures varies between valleys. In the figure below we have compared active layer depths and soil temperatures for Dalbay (lightly grazed) and Turag (heavily grazed).