2. Research area and study basin
The Suntar River basin at the Sakharyniya river mouth (basin area 7680
km2) was selected as the object of the study (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). In the central parts of the south it is
adjacent to the Ugamskiy Ridge. Together, they form triple headwaters
between the Indigirka basin in the northeast, the Aldan basin in the
west, and the rivers of the Okhotsk sea basin in the south. The slopes
of the Suntar-Khayata Ridge are very divers: high peaks (the Mus-Haya
mountain, 2959 m a.s.l.) are combined with deep river valleys.
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 (in July +6.4 and +17.5 ºС, in
January -28.0 and -39.6 ºС) 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 correspondingly. Annual average precipitation at the
Vostochnaya (1966–2012) is 280 mm and at the Suntar-Нayata (1957-1964)
is about 690 mm. Most precipitation occurs in summer (Vostochnaya,
1966–2012).
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 intrapermafrost and
suprapermafrost
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. According to
(Landscape map of the USSR, 1985) the following landscape units can be
found within the study region. They are 1) lowland plains, sometimes
swampy, with larch woodlands or larch forests in combination with
hummock and moss tundra; 2) the 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 tundra areas are located from the absolute altitudes of 1500-1550 m
(from 1400 m under the glaciers). The main feature of the overlying
high-altitude zone (1600-2100 m) is the predominance of lichens and
almost complete absence of shrubs and flowering plants.
The average altitude of the Suntar river basin is 1410 m a.s.l. from
2794 m a.s.l. to 828 m a.s.l. Therefore, the basin covers the landscapes
from upper reaches of the mountains to the lowland plains in the river
valley.
The Suntar river regime is characterized by high spring freshet and rain
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
(1957-1964). Water levels at the gauge range from 198 cm (1964) to 781
cm (1980) (Fig. 2) with the variation of river depth up to 6 m. Daily
streamflow data (1956-2015) for hydrological 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, reference
date: 01.03.2018).
About two dozen small glaciers with areas from 0.05 to 2.7
km2 and total area of 14.7 km2 are
located within the upstreams 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), 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 the
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 flow in
July and August.
In the last decades, a steady decreasing trend of the Suntar-Khayata
Ridge glacierization has been observed (Lytkin, 2016) which sums up to 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 really be accounted for
explicitly due to a lack of information.
There are numerous aufeis fields that are formed at mountain ranges, 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 amount of
aufeis fields at the Suntar river basin has increased from 45 to 53, and
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 estimated 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 atmosphere precipitation,
may represent a significant source of streamflow, however in this
respect they have barely been studied (Lytkin, 2016; Zhizhin et al.,
2012).