Estimating the spatial distribution of thaw depth in the Siberian tundra near Tiksi
from micro undulation, vegetation and image color index
Tadataka EZAKI1, Kunio WATANABE2, Masaru MIZOGUCHI3 and Hideki KIYOSAWA2
1

Department of Bioresources, Mie University
2
Faculty of Bioresources, Mie University
3
Department of Biological and Environmental Engineering, The University of TokyoINTRODUCTION
Siberian tundra occurs in a permafrost area that is overlaid with a layer that freezes andthaws annually, the so-called active layer. The depth of thaw of permafrost varies spatially andmay further influence hydrologic processes in tundra watersheds. Watanabe et al. (2000)
reported that the maximum thaw depth of tundra decreased with increasing thickness of the living
plant layer. This implies that the spatial distribution of thaw depth in the tundra can be estimatedfrom the spatial distribution of boundary conditions between the ground surface and atmosphere,i.e. vegetation and micro undulations of the ground surface. However, little is known about thisrelationship. In this study, we report the thaw depth, ground surface elevation, and vegetation intundra. Then we develop a method for estimating the spatial distribution of thaw depth in atundra watershed.
OBSERVATION SITE AND METHODSThe survey was carried out in tundra near Tiksi in the summer of 1999. A 100 × 100m area
divided into a 10m grid was located at the southeast end of the CALM grid. Thaw depth, groundsurface elevation, and vegetation were investigated at each intersection of the grid. The thaw
depth was measured using a steel rod. The ground surface elevation was surveyed from abenchmark placed the previous year. The vegetation in each 10 × 10m grid square was classified
by the typical species (Watanabe et al. 2000). To analyze the vegetation conditions anddistribution in detail, an image of the ground surface at each observation site was captured bydigital video camera.
ANALYTICAL RESULTThe slope direction of the ground surface and the gradient were calculated from the groundsurface elevation using GIS analyzing software (GRASS). Figure 1 shows a map of the slope
direction for gradients larger than 1°. The slope was separated into four directions. The solidlines are the thaw depth contour with a 10cm interval. The thaw depth ranged from 30 to100cm.
The ground near a depression, where four slopes faced each other, thawed to a greater depth (morethan 80cm) than the surrounding ground. The depressions were inundated and contained sedge.
As seen in the southeast part of the figure, the thaw depth of ground that sloped continuously in
one direction was shallower than 40cm.
Figure 2 shows a vegetation map with the thaw depth contours. The thaw depth ofground mainly covered by moss was shallow as seen in the southeast part of the figure. Sincemoss covers the ground surface more densely than sedge or other plants, it is thought that the moss
prevents the soil from thawing. Where the ground was studded with frost boils, the thaw was
deeper than in the surrounding area. Since the surface of a frost boil consists of clayey silt with alow albedo and no vegetation to prevent thawing, the thaw depth near frost boils is deep. Someareas of sedge-covered ground also had a deep thaw depth. These areas corresponded to theinundated depressions shown in Figure 1.
ESTIMATION
The analysis indicates that the thaw depth depends on the ground surface conditions. The
proportion of vegetative cover on the surface varied, even when it consisted of similar plants. If
an index of surface conditions can be determined from a quantitative estimation of the vegetative
cover, the spatial distribution of thaw depth can be estimated using the index. Measuring thenumber of plants per unit area and the species is an effective way to quantitatively estimate thevegetative cover. However, it is difficult to measure an open watershed directly in a short time.

Therefore, we first captured images of the vegetative cover with a digital camera, and thenanalyzed the color of the image, which is related to the vegetative cover, to make an index.
Images of a 75 × 100cm area around each intersection were used. The color of each
image was separated into four hues (CMYK). We used the yellow (Y) level, since it is strongly
related to the vegetative cover and expressed the main color of vegetation. The yellow level ofan image was defined as the mean value of all the pixels in the image; it has a range from 0 to 255.
When an image has many yellow components, the yellow level is low. Figure 3 shows a map
using the yellow level as an index of the vegetative cover. The yellow level was proportional tothe thaw depth, as shown in Figure 4. Ground that thawed to a depth exceeding 80cm had ayellow level higher than 160. This indicates that if the proportion of vegetative cover is low, theground thaws to a greater depth. Where moss covered the ground widely, the thaw was shallower
than 40cm and the yellow level had a middle value (about 135). It seems necessary to add factors
to the index in order to include differences in the insulating effect of each plant species.
SAMMARY
The following relationships were obtained from the analysis of micro undulation, vegetation,and thaw depth using GIS. (1) The ground thaws to a greater depth at or near meter-scale
inundated depressions. (2) The thaw is shallower in moss-covered areas. (3) Ground studded
with frost boils thawed to a greater depth than the surroundings. From these results, wedeveloped a method for estimating the spatial distribution of thaw depth. The yellow level ofdigital images can be used to estimate the distribution of thaw depth, although other factors shouldbe added. Applying the index to aerial photographs or satellite maps should prove useful formaking a hydrological model of the tundra region.
North

Moss

EastSedge

WestSedge

Southmoss

Flat

Frostboil+Fig. 1 Slope direction of ground surface
102

Fig. 2 Vegetation map
120

inundated0Fig. 3 Map using the yellow level

183

Moss-coverd
90

140
Yellow level

190

Fig. 4 Relationship between yellow
level and thaw depth

REFFERENCES
Watanabe et al. (2000) : Properties and horizons of active layer soil in tundra atTiksi, Siberia, J. Japan Soc. Hydrol. & Water Resour. Vol. 13, pp 9-16