Permafrost and hydrology in the source area of the Yellow River

Abstract

Frozen ground was investigated in 2003-2006 to evaluate the present-day distribution and ongoing degradation of permafrost in the source area of the Yellow River, located at the northeastern margin of the Tibetan Plateau. The presence of permafrost was examined by seismic, electrical and/ or thermal soundings at 18 sites between 3,250 m and 4,800 m a.s.l. Then, permafrost distribution fi tted to the sounding results was mapped by means of GIS. Temporal variations in ground thermal and hydrological regimes were also investigated for two years at Madoi observatory (4,273 m a.s.l.), by automatic and manual observations of air and ground (0-8 m deep) temperatures, precipitation, snow depth, near-surface soil moisture and groundwater level. In addition, numerical simulation of the ground thermal profiles was performed to discuss the degree of permafrost degradation under the observed atmospheric warming. High P-wave velocities (>2 km/s) and relatively high DC resistivities (650-1,100 Ωm) below a thin uppermost layer show that permafrost 10-30 m in thickness occurs above 4300 m a.s.l. In contrast, low P-wave velocities (<1 km/s) throughout the sediments indicate that permafrost is absent below 4,000 m a.s.l. On widespread alluvial plains between 4,200 m and 4,300 m a.s.l., permafrost is lacking or significantly degraded. Negative values of the mean annual ground surface temperature (MAST) also indicate widespread permafrost only above 4,300 m a.s.l. under the present climatic condition. The seasonal frost penetration reachs a maximum depth of 2.6 m at the observatory. Intermittent and very shallow snow cover favor frost penetration. The ground between 4 m and 8 m in depth was kept at slightly positive temperatures (0-4 °C) throughout two years, although the presence of permafrost at this site was suggested by a few reports in the 1980s. Assuming that the inter-annual variation in MAST follows that in the mean annual air temperature, permafrost is estimated to have signifi cantly thawed on the alluvial plains at 4,200-4,300 m a.s.l. during the last half-century. The numerical simulation suggests that thin (<15 m) permafrost can signifi cantly degrade within this time scale. The resulting degradation of the permafrost is assumed to have extended 3,000 km on the alluvial plains in the source area.