2. Research area and study basin
The Suntar River basin at the Sakharyniya river mouth (basin area 7680
km2) was selected as the primary gauge against which
the model is to be calibrated (Fig. 1). This river belongs to the
Indigirka river basin and drains from the Suntar-Khayata Ridge which is
a continuation of the Verkhoyansk mountain system (Eastern Siberia).
The high-altitude Suntar-Khayata Station was operating in the Suntar
River basin from 1957-1959 under the program of the International
Geophysical Year (Dodds et al., 2010). Glaciological, geomorphological,
geocryological and hydrological observations were carried out (Grave et
al., 1964). The Station was located at an altitude of 2067 m in a rocky
talus (goltsy) landscape and the observations are unique for the
high-mountain areas of Eastern Siberia and the North-East of Russia.
The climate of the region is extremely continental with altitudinal
zonation and air temperature inversions in the cold season. Average
annual temperature is -13.8 and -14.1 ºС (+6.4 and +17.5 ºС in July
and-28.0 and -39.6 ºС in January) at the stations of Suntar-Нayata (2068
m a.s.l.) for the period 1957-1964 and Agayakan (776 m a.s.l.) for the
period 1957-2012. Annual average precipitation at the Vostochnaya gauge
(1966–2012) is 280 mm and at the Suntar-Нayata gauge (1957-1964) is
about 690 mm. Most precipitation (60%) occurs in summer.
The studied territory is situated in the region of continuous
permafrost. Its thickness within the mountain ranges is about 400-600 m,
and under river valleys it is 200-300 m (Geocryology of the USSR, 1989).
However, permafrost can be interrupted in fractured zones by taliks
associated with intra-permafrost and
supra-permafrost
water flow (Grave et al., 1964).
The study area belongs to the northern taiga climate zone which is
affected by altitude and aspects of mountain slopes. In (Landscape map
of the USSR, 1985) the landscapes of the study region are classified as
the following: 1) lowland plains, sometimes swampy, with larch woodlands
or larch forests in combination with hummock and moss tundra; 2)
plateaus with gentle slopes with stony-lichen and shrub tundra and larch
woodlands; 3) ridge mountains with stony and stony-lichen tundra and
areas of larch woodlands in the valleys. The average altitude of the
Suntar river basin is 1410 m a.s.l., ranging from 2794 m a.s.l. to 828 m
a.s.l. Therefore, the basin covers the landscapes from the upper reaches
of the mountains to the lowland plains in the river valley.
According to the results of the Station studies and descriptions, the
high-altitude landscapes of the Suntar River basin can be summarized as
follows:
- Goltsy (or rocky-talus, completely bare landscape) is located in the
altitude range 1900 to 2700 m a.s.l. accounting for 7% of the Suntar
river basin. Ground profile consists of argillite broken stone with
admixed loam materials, cemented together with ice and layers of clean
ice up to 2 m in depth. Vegetation is absent. Despite significant
amount of precipitation and its irregular distribution, the upper
ground layer is characterized by low moisture content with little
variation during the warm season. This is explained by the high
permeability of broken rocks, where water easily infiltrates deep down
and flows along the frozen bedrock. The unsuccessful experience of
experimental runoff site construction has shown that the bedrock has
deep splits and hollows, and even though their temperature is below
zero, they are not fully filled with ice (Grave, 1959).
- Mountain tundra is located within the altitudes of 1450-1900 m a.s.l.
and comprises 37% of the Suntar River basin. It has a tight layer of
grass and moss with bushes under which there is rock formation with
some ice with admixtures.
- Sparse larch forest (1100-1450 m a.s.l) consists of sparse growth of
larch forest at north slopes and larch forest at south slopes and
covers 42% of the basin.
- Swampy sparse larch forest is found within the river valleys and
floodplains (828-1100 m a.s.l.) and covers 14% of the basin.
The active layer depth within the study territory is very variable.
Table 1 shows data on maximum active layer depth, obtained in 1958.
In the high mountainous area (1700 m and above), the depth of thawing of
rocky talus sediments ranges from zero under glaciers and perennial
snowfields to 70-90 cm at the foot of the slopes at the alluvial cone,
folded by gravelly loam. Observed values at the Suntar-Khayata Station
reached 75 cm in 1958 and 90 cm in 1959 (Grave et al., 1964). On steep
slopes with southern exposure, the depth of penetration of positive
temperatures into the ground is expected to be greater. In similar
landscapes with the same conditions, large-scale crushed stone thaws up
to 55-60 cm during the warm season, and crushed loam thaws up to 80-85
cm. By the end of the snow cover season, large-scale sediments can be
firmly cemented by ice, which fills all the pores between the material
(Grave et al., 1964). The variation of active layer depth in the
high-altitude area is highly variable from year to year.
The depth of the seasonal thaw layer is more stable in the mid-mountain
region. The maximum depth of thawing is observed in coarse-grained rocks
in this region. In sand-gravel-pebble ground at an altitude of about
1400 m, the depth of seasonal thawing reaches 120-150 cm and in loam
soils ranges from 25 to 30 cm, depending on the moisture content (Grave
et al., 1964).
The Suntar river regime is characterized by high spring freshet and
rain-driven summer-autumn floods. In winter, the Suntar River freezes
completely. Maximum streamflow is observed in the summer months. Snow
cover is formed in September. Usually a spring freshet begins in the
third week of May. Average annual flow for the Suntar river is about 180
mm, with a maximum recorded daily discharge of 1900
m3/s. Daily average water levels at the gauge range
from 198 cm (1964) to 781 cm (1980) (Fig. 2). Daily streamflow data
(1956-2015) for the stream gauge originate from the publications of the
Hydrological Yearbooks (Hydrological Yearbooks, 1936-1980; State Water
Cadastre, 1981-2007) and are available for the period 2008-2015 on the
website of the Automated information data system for state
monitoring of water bodies (AIS SMWB) (URL:
https://gmvo.skniivh.ru, viewed
01.03.2018).
About two dozen small glaciers with areas from 0.05 to 2.7
km2 and a total area of 14.7 km2 are
located within the upstream area of the Suntar River (GLIMS and NSIDC,
2005, updated 2017) (Fig.1). This accounts for 0.2% of the Suntar River
basin area. There are no direct estimates of glacier streamflow for the
Suntar River basin, but according to Grave et al. (1964), the specific
rate of flow of all the glaciers of the Suntar-Khayata Ridge in 1957,
1958 and 1959 was about 17, 13 and 22
ls-1km2, respectively. The glaciers’
contribution to river streamflow in small catchments with higher glacier
areas can be significant. Grave et al. (1964) assessed the values for
the neighboring basin of the Agayakan river, where glaciers cover over
2.2% of the catchment. In 1957, which was average by hydrological
conditions, the glaciers contribution exceeded 3.8% of the overall
annual flow and reached 6.1% of total flow in July and August.
In the last few decades, a steady decreasing trend of the Suntar-Khayata
Ridge glacier area has been observed (Lytkin, 2016) averaging a
reduction in area of about 20% over the period 1945 to 2003
(Ananicheva, 2005). In this study we assume that the contribution of the
glaciers in the Suntar river flow is likely to be smaller than the
precipitation assessment error and cannot be accounted for explicitly
due to a lack of information.
There are numerous aufeis fields that are formed in submountain and
intermountain depressions in the study region. In the Suntar river
basin, the aufeis cover up to 0.76% of basin area (Makarieva et al.,
2018c; 2019b). In the last 70 years the number of aufeis fields in the
Suntar river basin has increased from 45 to 53, but their total area has
decreased from 75 km2 to 60 km2(Fig.1) (Makarieva et al., 2018c). The aufeis flow contribution is most
significant in May-June (Sokolov, 1975). Following the approach by
Sokolov (1975) we estimate that the share of aufeis runoff for the
Suntar river basin may reach 9.2% (17.4 mm).
Perennial snow fields and rock glaciers are widespread within the
Suntar-Khayata Ridge (USSR Surface Waters Resources, 1966). They, along
with the ice of the active layer and summer precipitation, may represent
a significant source of streamflow, however in this respect they have
barely been studied (Lytkin, 2016; Zhizhin et al., 2012).