The determination of permafrost thawing from long-term streamflow measurements:
The case of eastern Siberia
BRUTSAERT，Wilfried Hendrik（コーネル大学土木環境工学部教授）
- 要 旨：
- For some time now permafrost has been reported to be degrading at alarming rates over wide areas in northern regions of Eurasia and North America; the evidence has come mainly from in-situ observations in the soil profile, which have limited spatial and invariably limited temporal coverage. Herein two types of methods are proposed to relate low flows (or baseflows) in a river during the open water season with the rate of increase of the active groundwater layer thickness resulting from permafrost thawing at the scale of the upstream river basin. In the first, use is made of the rate of change of the annual lowest river flows during summer, while in the second the rate of change is considered of the characteristic groundwater drainage time scale (or groundwater storage half life) of the river basin. The methods are tested with data from four gaging stations within the Lena River basin in eastern Siberia, one in the Upper Lena basin, and three in two of its tributaries, namely the Olekma and the Aldan basins. The different results are mutually consistent and suggest that, over the 1950-2009 period the permafrost depth has been increasing at average rates of the order of 1 cm/a in the areas with discontinuous permafrost and at average rates of about half as large in colder more eastern areas with continuous permafrost. However, these rates have not been steady but have been increasing; thus it appears that in the earlier years over the period 1950-1970, some large regions may not have been undergoing permafrost thawing and perhaps even accretion, whereas from the 1990's onward large areas have experienced average thawing rates as large as 2 cm/a and some, especially those with continuous permafrost, even larger.