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).