Variability of Thaw Depth Depending on Surface Micro-undulation andVegetation Cover in the Siberian Tundra
Kunio Watanabe1 , Tadataka Ezaki2 , Kazunari Fukumura3 , Masaru Mizoguchi4 , Hideki Kiyosawa 1
1 Faculty of Bioresources, Mie University
2 Fujita Corporation
3 Department of Environmental Engineering, Utsunomiya University
4 Department of Biological and Environmental Engineering, The University of TokyoABSTRACT
The Siberian tundra occurs in a permafrost area that is overlaid with a thin surface layer thatfreezes and thaws annually. The runoff and subsurface storage in the tundra watershed might becontrolled by spatial and temporal variation in the ground freezing and thawing processes. To
understand the spatial variation in thaw depth in a tundra watershed near Tiksi, Siberia, groundsurface level, thaw depth, and vegetation cover were investigated in the summer of 1999. The
thaw depth in this area varied between 0.3 and 1 m. The direction of runoff was determined byanalyzing micro-undulations on the ground surface. The ground thawed to a comparatively
shallow depth on long slopes, and to a greater depth at or near water-filled micro-depressions.
The depth of thaw also varied with the vegetation cover: it was shallower in moss-covered areas
and deeper near frost boils. The influences of micro-undulation and vegetation cover on thespatial variation of thaw depth are discussed. Our results imply that the spatial variation ofthaw depth in a tundra watershed can be estimated using surface information, such as
topography and vegetation.
Introduction

Observation site and method

Hydrologic processes in the Arctic tundra play animportant role in global climate (Kane, 1997). Siberiantundra occurs in a permafrost area that is overlaid with athin surface layer that freezes and thaws annually. Whenthe ground thaws, the ice in the soil melts, but cannot
drain by downward infiltration because of the low
hydraulic conductivity of the underlying permafrost(Bridges, 1990). Once water is supplied, it flows
laterally through the thawed surface layer, and the flow iscontrolled by soil horizons and the slope of the surface
layer (Hinzman et al., 1991). To characterize arctichydrologic processes, such as runoff and subsurface
storage, it is important to clarify the characteristics of thedepth of thaw in the ground as it increases through thethaw season.
Ground thawing is affected not only by the physicalproperties of soil, but also by the surface energy balance
and water movement through the soil. Soil texture,moisture content, vegetation, and evapotranspiration are
spatially variable due to surface undulations, and may
further influence the runoff and subsurface storage oftundra watersheds. Mizoguchi et al. (1999) described
the spatial variation of thaw depth in a tundra slope,caused by variations in the elevation of the groundsurface and by the vegetation cover. Watanabe et al.
(2000) reported that the depth of thaw in tundra
decreased with increasing thickness of the live-plantlayer. However, little is known about the relationships
between surface micro-undulations, vegetation cover, andthaw depth.
In this study, we report the spatial variation of thaw
depth of a tundra watershed. Then, we discuss its
relationship with the micro-undulations and vegetation
cover of the ground surface.

The survey was carried out in a typical tundra
watershed near Tiksi, Russia (latitude 71° 38'N,longitude 128° 52'E) in the summer of 1999. The
topography of this watershed consists of rolling hills,
with elevations of 200-300 m, and gently slopingvalleys; marshy wetlands form in the valley bottoms.
Tundra polygons and frost boils are found in this area;
the thickness of the permafrost is estimated to be over
500 m (Fartyshev, 1993).
The ground thaws tomaximum depth in August, then refreezes from early
September.
120˚E

140˚E

Laptev
Sea

70˚NTiksi
LenariverRiver
CALM gridObservation area

1kmFig. 1 Study areaTable 1 Characteristics of soil near the observation site.
Horizon type Depth Range Bulk density Range Hydraulic cond. Range Thermal cond. Range(cm)(g cm-3)(cm s-1) (10-4)(WmK-1)
Live-plant
5-0 0.5-20
0.53
0.23-0.74(moss)
0.21
104(sedge)
4.6 (root)
Organic soil 0-19 7-42
0.68
0.25-1.01
61
140-4
0.88
0.70-1.21
Mineral silt 19-36 9-40
1.32
0.75-1.71
0.59
1.1-0.2
1.14
1.02-1.35
Frozen36An area 100 × 100 m was selected at the southeast
end of the CALM grid (Fig. 1). Figure 2 shows air andsoil temperatures, precipitation, and groundwater level atthe observation area (10-20 August, 1999). The average
air temperature was 7.3 °C and the average soil
temperature at a depth of 0.1 m was 7.0 °C for theperiod of record; there was a precipitation event larger
than 10 mm/day. Parts of the area were covered by amat of moss, forming a 0.03-0.2 m layer of live plants.
The soil in the area is a multi-layered system thatconsists of 0-0.2 m of accumulated organic material on0.05-0.3 m of partially decomposed organic matter, overmineral silt above the permafrost (Watanabe et al.,2000). Table 1 lists the physical properties of the soil inthe observation area (Watanabe et al., 2000).
The
hydraulic conductivity of the organic soils was 10 to 100
times greater than that of the silt. The live-plant layer
had a very high hydraulic conductivity.
Thermalconductivity increased with depth from the live-plantlayer through the organic soil to the silt.
The observation area was divided into a 10 m grid.
At each intersection of the grid, the ground surface level,
thaw depth, and groundwater level were investigated.
The leveling survey was conducted from a benchmark
placed the previous year. Thaw depth was measured
using a steel rod, pushed vertically into the ground.
After measuring thaw depth, the groundwater level wasmeasured in each of the holes made by the steel rod.
The vegetation cover in each 10 m square grid wasclassified by the dominant species observed.
(a)

(˚C)

10510(˚C)(b)

5(mm/day)

Depth

5cm

10cm

20cm

(c)

100(mm)(d)

0-20
1999.8.10

date8.20

Fig. 2 Time series of the meteorological data. (a) Air
temperature at 1 m high; (b) Soil temperaturesat 3 depths; (c) Precipitation in a day; (d)Water level from ground surface.

Results
Ground surface level and thaw depth
Figure 3 shows a map of the ground surface level inthe observation area. The ground level ranged fromEL.+34.30 to EL.+35.63 m, averaging EL.+35.15 m.
The ground surface sloped gently from west to east.
Figure 4 shows a map of the groundwater level at theobservation area before the rainfall on 12 August. The
groundwater level ranged from EL. +34.31 to EL.+35.42
m and averaged EL.+35.07 m. Like the ground surface,
the water table sloped from west to east. There weresome small mounds in the northwestern part of theobservation area; at these points, water levels were lessthan 0.2 m below the ground surface. On the otherhand, pooled surface water not less than 0.05 m deep
existed at some points on the diagonal line of theobservation area (northeast-southwest) and at thesoutheast end of the area.
Figure 5 shows a map of thaw depths measured using
the steel rod on 19 August. The average thaw depth was0.61 m; the maximum and minimum were 1.02 and0.29 m, respectively. The ground thawed to a deeper
level in the northwestern part of the area. Some singularpoints were observed, at which the thaw depth was close
to 1 m, around the northwesterly midpoint of the area.
The level of the frozen ground table was obtained bysubtracting the thaw depth from the ground surface level(Fig. 6).
The frozen ground level ranged fromEL.+33.78 to EL.+35.04 m, averaging EL.+34.54 m.
Vegetation cover
The vegetation in the observation area consisted
mostly of water-tolerant plants, such as sedges andmosses accompanied by lichens. The vegetation cover
was classified into three types: sedges dominant, mosses
dominant, and mixed. Figure 7 shows a map of thevegetation, along with the thaw depth contours. The
ground was mainly covered with sedge in the westernpart of the area, where the thaw depth was greater than
0.6 m. The soil in this area consisted of a shallow
organic layer (about 0.1 m), and a silt layer that included
a lot of gravel. On the other hand, mixed vegetation ofmoss and sedge was noticeable in the eastern part of thearea, where the thaw depth was shallow. In this area, thewater level was close to the ground surface and theorganic layer was about 0.2 m thick. There were somepatches of moss-covered ground in the southeastern partof the area, where the live-plant layer was thicker than
0.1 m and the thaw depth was especially shallow. Inaddition, frost boils about 0.5 m in diameter were foundin the area. These frost boils supported no vegetation
cover and the soil consisted of silt only. The groundFig. 3 Map of the ground surface level.

Fig. 5 Map of the thaw depth (19 August).

Fig. 4 Map of the ground water level (12 August).

Fig 6. Map of the frozen ground level (19 August).

that was studded with frost boils thawed to a greater
depth than the surrounding area.
The appearance of the vegetation varied with moisture
conditions at the soil surface. Sedges were green andgrew densely in ground close to saturation; sedges wereyellowish or whitish and grew thinly in unsaturatedground; and a few sedges grew in the waterlogged
surface. No marked difference was observed in thedensity of moss due to moisture conditions, butdifferences in color similar to those in the sedges wereobserved.

Discussion
The slope direction of the ground surface and thegradient were calculated from the ground surface level
using GIS analyzing software (GRASS). The gradients
ranged from 0 to 3°. Figure 8 shows a map of the slope
direction for gradients larger than 1°, along with thethaw depth contours.
The slope directions wereseparated into four segments (north, east, south, andwest). Slopes were well scattered in the western part ofthe observation area, indicating that the ground wasrolling. Slopes predominantly faced east in the eastern

Fig. 7 Vegetation map. The solid lines are the thaw
depth contour with a 0.1 m interval.

Fig. 8 Map of the slope direction for gradients lager
then 1˚. The solid lines are thaw depthcontour with a 0.1 m interval. The circles
indicate micro-depressions.

part of the area, indicating that the ground continuously
sloped in one direction. The thaw depth of the ground
was less than 0.4 m in areas of continuous slope.
Slopes in all four segments faced each other at somepoints (circles in Fig. 8), indicating that the groundformed a micro-depression with a depth of about 0.1 m.
The ground thawed to a greater depth near the microdepressions than in the surrounding areas.
The silt layer tended to remain saturated, with little
temporal variation, due to the presence of permafrost
below it; on the other hand, the moisture content of theorganic layer fluctuated greatly over a short time. This
indicates that, the water flows into, and then through,the organic layer along the slope direction (indicated asarrows in Fig. 8). If the organic layer has driedsufficiently, the outflow is delayed by rewetting of theorganic layer. Referring to Fig. 2, the ground waterlevel fluctuated between –20 and 10 mm, indicating thatthe micro-depressions became filled with water during
the survey period. In the initial period of groundthawing, water tended to flow into the micro-depressions
and form puddles before runoff occurred. When thishappens, heat advection by the water flow will accelerate
thawing of the ground in the micro-depressions.
Meanwhile, the ground surface in this area repeatedly
freezes and thaws several times in May. The heat
capacity of the standing water in the micro-depressions
will prevent the ground from refreezing.
In the moss-covered areas, shallow thaw depths wereobserved. Since the moss covered the ground surface
densely and formed a thicker live-plant layer than otherplants, thawing was prevented in moss-covered areas. Athick organic layer was observed in the area where mixed
vegetation of moss and sedge covered the ground. The
organic layer had much less thermal conductivity thansilt and insulated the silty layer. Thus, the silt layer
remained cool during the ground thawing period. Frostboils, near which the ground thawed deeper than in thesurrounding area, consisted of silt with a high thermal
conductivity, and their surface had no vegetation and a
low albedo. Consequently, thawing proceeded fasternear the frost boils. The variation in the thaw depth inAugust may reflect the thawing processes.

Conclusion
To understand the spatial variation of thaw depth in atundra watershed and its relationship with surfaceconditions, ground surface level, thaw depth, andvegetation cover were investigated in the summer of1999 near Tiksi, Siberia.
The thaw depth varied
spatially within a range of 0.3 to 1 m throughout the100 × 100-m observation area.
Surface microundulations were observed in the area, and theirdistribution was surveyed. The differences in microundulation and vegetation cover influenced the spatial
variation of thaw depth.
The thaw depth of a tundra watershed varies spatiallyand temporally. Although this variation is important inunderstanding hydrologic interactions, it is difficult toidentify the variation in a large watershed in a shorttime. This study showed that thaw depth is closely
related to micro-undulations and surface conditions, suchas the species, color, and density of vegetation. If
surface information can be obtained from photographs

and combined with information about micro-topography,
the spatial variation of thaw depth can be geographicallypredicted. Knowledge of these characteristics is vital forhydrologic modeling of tundra watersheds.
Acknowledgment: We thank Dr. Kaoru Fukuyama, Ms.
Miyuki Nakashima, Mr. Takeshi Hiraiwa and Mr.
Hiroshi Ishikawa for preparation of figures.

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