SPATIAL DISTRIBUTION OF ACTIVE LAYER ON A HILLSLOPE IN SIBERIAN TUNDRA

Masaru Mizoguchi, Kunio Watanabe, Kazunari Fukumura and Hideki Kiyosawa
Faculty of Bioresources, Mie UniversityTsu, Mie 514-8507, Japan
Telephone 81-59-231-9574
Fax 81-59-231-9571
Email mizo@bio.mie-u.ac.jp
Abstract
Spatial distribution of active
layer thickness (thaw depth) influences on thehydrological processes. We have observed theactive layer thickness on a 100 m x 100 mhillslope grid set in tundra region related to theelevation of ground surface and the vegetation
cover during the summer of 1998. From thevariogram analysis, it was found that there were
the anisotropy and the periodicity in the active
layer thickness between horizontal andperpendicular direction to the slope. In thepresent paper, we will discuss that theanisotropy and the periodicity may come fromtundra polygon and solifraction that aregenerally observed in the tundra region.
Introduction
The active layer, i.e., theannually freezing and thawing upper ground inpermafrost areas, is of hydrological importanceas a sensor for global warming. From theobservation of plain grid in Siberian tundra lastyear, we found that the active layer thickness(thaw depth) varied spatially mainly due tovegetation cover(1998). The observationsuggests that permafrost table hard to permeable
can be bumpy and give a profound affect on thecirculation of soil water and total hydrologicalprocess in tundra region. Thus we observed thisyear the spatial distribution of active layer
thickness , elevation and vegetation in ahillslope grid set as well as a plain grid set.
Field site and Methods
Field observation
was carried out during the summer of 1998,
August 10 to 29 within the framework of a
GAME (GEWEX Asia Monsoon Experiment)
project in the tundra watershed. A 100 m x 100
m grid set with 10 m interval was chosen in ahillslope about 2 km north-northwest of a
CALM (Circumpolar Active Layer Monitoring)
grid set that was observed last year. The
hillslope faces to southeast with the gradient of
about 15 degree. We measured the spatial

distributions of active layer thickness, elevationand vegetation at the each grid point with 10 minterval for the 100 m x 100 m grid set and also
at the each point with 1 m for a 10 m x 10 m
grid set which was nested within the 100 m x
100 m grid. In addition, active layer thickness
and elevation were measured at the interval of
25 cm on two 50-m lines which were a steepest
slope
direction(y-direction)andtheperpendicular direction to the y-direction (xdirection).
Elevations of ground surface and frozen table
Figure 1 shows the elevations ofground surface and frozen table in the 100 m x
100 m grid set subtracting the average gradientof the slope. The figure includes the result
obtained by detail survey for two eroded gulleys
in the grid set. This figure indicates that theelevation of frozen table was bumpy reflecting
the ups and downs of the ground surface. Figure
2 shows contour maps of active layer thickness
and vegetation in the grid set. The field site wascovered mainly by sedge and mosses.
Spatial co-relation of active layer thickness
Figure 3 shows the active layer
thickness and the elevations of ground surface
and frozen table on a 50-m line of x-direction.
Although both the ground surface and the frozen
table had a slight inclination of about 1/50 to thex-direction, the active layer thickness, which isdefined as the difference between the groundsurface and the frozen table, fluctuatedperiodically. Figure 4 shows the semivariogramsfor the active layer thickness set on the two 50m lines of x- and y-directions to analyze theperiodicity. The semivarience of the x-direction
had local minimums with the periodicity of 7 m.
Although the active layer thickness are mainly
dependent on the vegetation and the thermalproperties of soil, the periodicity of thesemivarience for the x-direction may beattributed to tundra polygons which arecharacteristic of geographical feature in thetundra region. On the other hand, the periodicityof the semivariences was not detected for the ydirection. The reason why the periodicity
diminished in the y-direction can be explainedin terms of movement of subsurface soil socalled solifraction.
Development of water tracks in hillslope
Numerous downslpoe water tracks run
in the hillslope as shown in Photo. 1. The water
tracks meander through the ice wedges formed
with an angle around the tundra polygon andsome of the water tracks develop into gulleys.
This morphological characteristic of the water
tracks suggests that two hydrological processes
are important in the tundra hillslope hydrology:
soil water movement from the active layer to theice wedges around the tundra polygon anddevelopment of the water track from ice wedge
to gulley. For understanding such processes, weneed to get more information on the thawing
rate, the thermal erosion and the collapse ofhillslope frozen soil.
Conclusion
Ground surface elevation,active layer thickness and vegetation wereinvestigated in a 100 m x 100 m hillslope grid
set in tundra during the summer of 1998. It was
found from the investigation that there existed
the anisotropy and the periodicity of the active
layer thickness. The characteristics may comefrom tundra polygon and solifraction that are
generally observed in tundra region.
References1)
Masaru Mizoguchi, et. al: Spatial
variation of active layer thickness in Siberian
tundra, Activity Report of GAME-Siberia,
1996-1997, p.54, Japan sub-Committee forGAME-Siberia. GAME Publication No.
10(1997)

160140THAW DEPTH
SURFACEFROZEN TABLE

Fig.1 Elvations of ground surface
and frozen table

LEVEL (cm)

120
10080
60
40200

Photo.1 Numerous downslpoe water tracks
in the hillslope

SEMIVARIANCE

010

100
90
80
70
60
50
4030
201002030
DISTANCE (m)

4050

2025

X-direction
Y-direction

051015LAG (m)