Permafrost

Theme 5. Monitoring, mapping and modeling
of mountain and high-elevation permafrost
Sounding Permafrost in the Source Area of the Yellow River
(Northeastern Tibet): Degrading or Already Disappeared?
Atsushi IKEDA 1 , Tetsuo SUEYOSHI 2 , Norikazu MATSUOKA 3 and Takemasa ISHI 4
(1. National Institute of Polar Research; 2.Hokkaido University;
3. University of Tsukuba; 4. Geological Survey of Japan)
Abstract: The present status of frozen ground in the source area of the Yellow River, located at
the northeastern margin of the Tibetan Plateau, was investigated to evaluate permafrost
degradation and its impacts on groundwater hydrology. The fieldwork involved monitoring of
temporal variations in ground thermal and hydrological regimes at an observatory and sounding
of seismic and electrical stratigraphies.
The observatory was placed at the Madoi meteorological station (98°13'E, 34°55'N, 4273
m ASL) in mid-August 2004. The site lies at an elevation representative of the surrounding
plateau area (4200-4300 m ASL). A data logger has automatically recorded air temperature;
ground temperature at 0.03, 0.3, 1.3, 2.3, 4.3, 6.3 and 7.8 m depth; snow depth; soil moisture at
0.3, 0.6 and 0.9 m depth; and thermal properties (conductivity, diffusivity and heat capacity) at
0.1 m depth. Daily precipitation and weekly groundwater level have also been observed
manually. The first year’s data showed a large seasonal range with a mean annual temperature
close to 0°C at the ground surface, which resulted in deep seasonal frost despite the low thermal
conductivity of the topsoil. The seasonal frost penetration reached a maximum depth of 2.6 m in
late April. Lack of precipitation in winter (15 mm from November to February) resulted in
intermittent and very shallow snow cover, which favored frost penetration. The ground between
4 m and 8 m in depth was kept at slightly positive temperatures (0–4°C) throughout the year.
Extrapolating isotherms suggest the absence of permafrost within the upper 10 m of the surface,
although previous studies have reported the presence of permafrost at least until 1980s. Thus,
permafrost at Madoi may have significantly degraded during the last few decades.
Distribution of permafrost was examined by refraction seismic soundings and/or electrical
DC resistivity soundings at twelve sites between 3800 m and 4600 m ASL. All sites are located
on alluvial plains or terraces underlain by thick fluvial sediments. High P-wave velocities (>2
km s −1 ) and relatively high DC resistivities (650–1100 Ωm) below the thin uppermost layer
(2–4 m thick) at three sites show that stable permafrost occurs above 4300 m ASL. In contrast,
low P-wave velocities (