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:
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).