The KWBS was established on 15 October 1947 and was initially known as the Itrikanskaya runoff station of the Dalstroy (Far North Construction Trust organized in 1931) Hydrometeorological Service. In 1948–1956 and 1957–1969 it was called the Kulinskaya and Kolyma runoff station respectively. The primary goal of this station was to study runoff formation processes in small river catchments in mountain permafrost landscapes, typical for the North-Eastern USSR.

In May 1948, the first runoff observations at the Kontaktovy creek and Vstrecha brook were launched, as well as regular observations at the Nizhnyaya weather station (850 m a.s.l.). A few months later, on 1 September 1948, observations at the Verkhnyaya weather station (1220 m a.s.l.) were started. In 1948, stage gauges Sredny, Nizhny and Vstrecha were equipped with automatic water level recorders, gauging footbridges and flumes.

During the period 1949–1957, at the Vstrecha brook catchment, a rain-gauge network was organized. Runoff gauges at the Severny, Dozhdemerny, and Vstrecha brooks were equipped with various hydrometric facilities. Observations of soil, water and snow evaporation, soil freezing and thawing commenced, as well as experimental observations at a runoff plot.

At the end of the 1940s and early 1950s, technical staff of the station were mainly former convicts. During the first few years, the workers of the station built houses for themselves, collected firewood and organized the household. The winter of 1955–1956 appeared to be especially severe for the staff, since due to the deep snow cover it was difficult to move around the territory of the KWBS, there was no transport connection with the Tenkinskaya highway, and delivery of firewood, needed for heating houses and service buildings, was also difficult. When it was impossible to get to the highway by car, bread and mail were delivered from the Kulu village settlement utilizing horses once every 7–10 days.

The 20 to 25 staff members were accommodated in three small huts, hardly suitable for living. That winter they mainly had to collect and prepare firewood in the afternoon; in the morning everybody had to go (despite their rank or position) in deep snow and at -50 ∘C to the nearest small river valley looking for firewood, and then they pulled it back home, where they fired furnaces. Only by the time it got dark did it became warm enough to stay in the workroom and could they start observation data processing.

The working day lasted until 22:00 or 23:00. Since there was no electricity, they used kerosene lamps filled with a mixture of petrol and salt. In summer 1956 there were only 13 people left at the station: some of them were taken to help with haymaking to prepare hay for their subsidiary holding that consisted of two cows and a horse (as recollected by the chief of the station V.G. Osipov and the hydrologist–technician A.I. Ipatieva, informational letter, 1988).

In 1957 the station was handed over to the jurisdiction of the Kolyma hydro-meteorological service administration, and in 1958 the electrification of the station started. At that time there were active steps taken toward fitting out the station with new types of devices and equipment, engaging new specialists in hydro-meteorology, and building accommodation facilities.

In 1960 runoff observations at the Yuzhny brook began. The optimization of the precipitation network was continued, meaning the establishment of rain gauges in new locations and shutting down some non-representative rain gauges. Radio rain gauges were also installed.

In 1963 two new water balance sites (nos. 2 and 3) were organized.

In 1968 runoff measurements were started at the unique research object, at the Morozova brook catchment, which has no vegetation cover and is composed of rocky talus.

In 1969 the Kolyma runoff station was renamed the KWBS. In these years there was a transition to broad experimental water balance observations of all elements and to an enhanced technical level of research.

From 1970, the KWBS carried out snowpack observations at avalanche sites of the Tenkinskaya road, as well as stratigraphy, temperature and physical and mechanical properties of snow at four sites. From 1980 additional observations of the dynamics of icing formation at the Kontaktovy creek were introduced. In 1982 observations of soil moisture were started at three agro-hydrological sites at the fields of the “Kulu” state farm.

In 1976 the station hosted a delegation of USA scientists. They highly praised the professional and personal qualities of the station's staff members and their commitment, on which extensive field studies and theoretical works were based, despite the equipment being rather simple and living conditions extreme. According to Slaughter and Bilello (1977), the data recorded at the KWBS were unique and unprecedented for world practice.

Since the beginning of the 1990s, the research programme at the station has been gradually cutting back. After 1997, water balance observations at the KWBS were ceased. One weather station and five runoff gauges functioned at the KWBS up to the middle of June 2013, when an extreme flash flood destroyed four level gauges. Nowadays only standard observations are conducted at the Nizhnyaya meteorological site and at the Kontaktovy (Nizhny) runoff gauge.

The observations of meteorological elements were carried out at three meteorological stations in different periods (Fig. 2a). The database includes daily values of air temperature, water vapour pressure, vapour pressure deficit, atmospheric pressure, wind speed, low and total cloud amount, and surface temperature (Table 1).

Meteorological station Verkhnyaya (1220 m a.s.l., 1948–1972) was located in the upper reaches of the Dozhemerny brook in the saddle between two hills. The horizon is closed by the hills from the south and north which are at 30–40 m distance from the site. The horizon is open from the east and west; strong winds are observed here. The surface at the station plot is hummocky, covered with grassy vegetation with no woody vegetation around it. The nearest building – the station house – was located 48 m away from the station plot. The depth of seasonal thaw of permafrost reaches 1.5 m.

Meteorological station Nizhnyaya (850 m a.s.l., 1948–1997) is located on the edge of a larch forest, on the terrace watershed between the Kontaktovy creek and the Ugroza brook, which has a slight slope to the south-west. The nearest trees are located 50 m away, the buildings 100 m from the station. The site is surrounded by mountains of up to 1400 m a.s.l., the nearest of them at a distance of 200–500 m. The height of the weather vane is 11.3 m. The surface of the station plot is covered by hummocks, with moss, peat and individual bilberry and blueberry bushes. The area is surrounded by a sparse larch forest from the north, east and south. The depth of permafrost seasonal thaw reaches 1.5 m.

Meteorological station Kulu (670 m a.s.l., 1981–1991) was located on the right slope of the broad valley of the Kulu River. The slope has a western exposure (4–6∘). The height of the weather vane is 10.7 m. The area is surrounded by larch trees of 6–8 m height from the west, north and east. The soils are loamy with the inclusion of small gravel. The underlying surface consists of berry, grass and sphagnum mosses, and sometimes bare soil. The depth of seasonal thawing of permafrost reaches 1.8–2 m.

In September 1992, the Kulu station (635 m a.s.l., 1992–1997) was moved to the residential building of the KWBS in the south-eastern part of the Kulu village. Residential and technical buildings were located around the meteorological plot. There was a road to the south of the station. The soil is marshy, covered with rubble and grass. This new location of the Kulu station is marked as Kulu2 station in the database.

The list of instruments used and the accuracy of observations for each meteorological element are presented in the description files of the database.

In total, precipitation was observed at 47 gauges within the KWBS territory during different periods. Continuous daily all-year-round precipitation data are available for the period 1948–1997 for gauge no. 12 at meteorological station Nizhnyaya and for gauge no. 54 at meteorological station Kulu for 1981–1997. Four gauges have the data of daily totals during the warm season for the period for more than 30 years and another 18 gauges for different shorter periods. Usually the start of daily observations at those gauges was initiated by the beginning of the snowmelt period and lasted until the end of September. Monthly sums of precipitation were measured at 30 gauges, 10- and 5-day sums at 21 and 18 gauges respectively.

In 1948, precipitation gauge stations for measuring daily precipitation were equipped with the Nipher-shielded and Tretyakov-shielded precipitation gauges (Fig. 3a). In 1948–1958 the observations were carried out with both devices in parallel, and after 1959 only Tretyakov gauges were used. Tretyakov precipitation gauges were also used for measuring precipitation totals in 5- and 10-day periods.

The other types of precipitation gauges applied at the KWBS are the Kosarev and ground rain gauge (GR-28) (Fig. 3b, c).

GR-28 consists of a rain bucket of a cylinder form and height of 400 mm, receiving area 500 cm2 and a socket for its installation several centimetres above the ground. GR-28 were usually installed on 1 June and dismantled on 1 September and used for rain measurements over the longer period, typically 1 month. Only those GR-28 which were installed at the soil evaporation plots measured precipitation every day (Kedrolivansky and Sternzat, 1953).

The Kosarev precipitation gauge is a cylindrical, tapering upward metal vessel with a reception area of 500 cm2. The lower part of the bucket ends with a cone equipped with a crane to drain accumulated precipitation. Kosarev gauges were used for monthly precipitation measurements from 1 October to 1 May.

In 1960–1963 there was an attempt to register precipitation with automatic radio-precipitation gauges, but due to improper performance of the devices those observations were stopped.

In 1988, precipitation observations at the KWBS were carried out with 36 precipitation and rain gauges, distributed relatively evenly throughout the area and altitudinal zones. Average density of the precipitation network at that time was 1.6 units per 1 km2.

For the period 1948–1968, precipitation data were published in the Observation Reports as they were (without any correction). Starting from 1969, all daily, 5-day and 10-day total data from Tretyakov rain gauges were corrected for wetting losses according to the Manual for hydrometeorological stations (1969). The correction value for precipitation events varied from 0.0 to 0.2 mm depending on the amount of observed precipitation and weather conditions. In the annual average, the value of wetting loss correction did not exceed 5 % of the total amount of precipitation, though in some years it could reach up to 9–10 %. In 1948–1983, monthly sums data obtained from the GR-28 and Kosarev gauges were published without any correction. In 1984 wetting loss correction was introduced to GR-28 observations as well. In the database the precipitation data are presented in their original form without any changes.

The analysis of water balance, climate change impact on river runoff, or hydrological modelling requires accurate and reliable precipitation data. Arctic and mountainous regions are characterized by high bias of precipitation measurements because of the significant amount of snowfall precipitation (WMO Report no. 67, 1998). Monthly estimates of this bias often vary from 5 to 40 %. Biases are larger in winter than in summer largely due to the deleterious effect of the wind on snowfall (Groisman et al., 2014).

Three main methods of winter precipitation bias correction have been suggested for the Tretyakov gauge, which was the main type of precipitation gauge at the KWBS. They are the WMO methodology (Yang and Goodison et al., 1995), the Northern European countries method (Forland et al., 1996) and the approach developed by Golubev (WMO Report no. 67, 1998). The basis of all three methods for correcting measured precipitation is the dependence of the aerodynamic coefficient on wind speed, air temperature, precipitation type and wind protection.

In the described database each precipitation gauge (if it was available) has the description of its location, altitude, slope exposure, and vegetation type. An additional characteristic is the degree of protection characterized by five types of Schwer (1976) classification (Ia, Ib – protected; IIa, IIb – half-protected; III – open; IV – shore station). The database also contains the series of daily wind speed for three meteorological stations which combined with the information on location of gauges and their characteristics can be used as a proxy for introducing bias corrections.

Snow cover observations were started in 1950 and initially conducted at two catchments, two meteorological plots and four typical squares. In 1959–1960 the number of catchments with snow surveys reached five. Up to 1971, snow surveys were conducted once per month starting in November and finishing in May at small catchments (the Severny, Yuzhny, Dogdemerny and Vstrecha) and once before spring snowmelt at Kontaktovy–Nizhny. Since 1972, the observations have been reduced to one survey per year (usually at the end of April) for all watersheds (Fig. 4). Table 2 shows the number of snow routes, their total length and number of measurement points, including their distribution among different landscapes of the catchments. Snow depth was measured every 10 m, snow density every 100 m at most of the watersheds, and at 5 and 50 m respectively at the Morozova brook watershed.

Based on the data on measured snow height and snow weight accounting for landscape and elevation distribution, the average SWE for individual watersheds and landscapes was calculated and published in the Observation Reports.

Average depth of snow cover is presented with an accuracy of 1 cm, density of 0.01 g cm-3 and SWE 1 mm.

Three types of evaporimeters were used at the KWBS.

A Rykachev evaporimeter was used for the soil evaporation measurements in the 1950s. It consists of a sealed square rectangular box with a core sample (Fig. 5a). The box was placed inside another box installed in the ground. Since the inner box was sealed, the device did not allow for water infiltration (Chebotarev, 1939).

A Gorshenin evaporimeter was actually designed for observations of evaporation from snow (Gorshenin, 1948). Its size was 25×30×10 cm with an evaporation area of 750 cm2. The bottom of the evaporimeter was double: the first was mesh and the second was solid, with a tightly closing hole for draining the water. The mesh bottom of the evaporimeter allowed for accounting of (melt) water infiltration; a sufficient height (10 cm) and a free flow of thawed water down under the grid approximate the processes of snow melting and evaporation to natural conditions.

Evaporimeter GGI-500-50 (later modified to GGI-500-30) is a standard device for soil evaporation measurements in Russia and the former USSR (Fig. 5b). It consists of two cylindrical vessels, one inside the other, and a water-collecting vessel. The bottom of the inner cylinder has openings; the core sample is placed in it. The quantity of water evaporated is determined from the difference in weight of the sample as measured over two successive observation periods.

From 1950 to 1953, five soil evaporation plots were opened at the sites with diverse underlying surfaces and different expositions and altitudes. The plots were equipped with the Rykachev and Gorshenin evaporimeters with evaporation area 1000 cm2. The observations until 1958 are considered to be approximate due to the absence of accompanying rain gauges and scales of the required accuracy.

From 1958 to 1966, the measurements were conducted at the soil evaporation plot, located near the Nizhnyaya weather station. The observations of evaporation were carried out with two evaporimeters GGI-500 and the Rykachev evaporimeter, precipitation – with a ground rain gauge (GR-28).

Three soil evaporation sites were established in different landscapes – in the Vstrecha (1967), Morozova (1971) and Yuzhny (1977) brooks basins. The measurements were carried out with standard weighing evaporimeters GGI-500-50, which, due to the physical proximity of permafrost, were changed to GGI-500-30, meaning that their height was decreased to 30 cm.

The accuracy of soil evaporation observations was 0.1 mm (Konstantinov, 1968).

Snow evaporation observations were conducted at the KWBS from 1951, but only the data for the period 1968–1992 are considered to be consistent and reliable and is published in the described database. From 1968 to 1981, the observations were conducted with standard evaporimeter GGI-500-6 at weather plot Nizhnyaya. In 1981 the snow evaporation observations were transferred to the Kulu weather plot and lasted until 1992.

This measurement accounts for snow evaporation on the ground. In the conditions of the KWBS with larch as the main tree type, intercepted snow was only a temporary phenomenon because of cyclonic activity in January and February. Wind during the cyclones blows away snow from all trees except dwarf cedar, which is under snow for the greater part of the winter.

Observations of evaporation from snow were made mainly in the fall (September, October) and in the spring (May–March). During winter months (January–February), the observations were made only until 1973, because the amount of evaporation from snow proved to be extremely insignificant for water balance. In the spring, during the intensive snowmelt, additional weighing of the evaporimeters was carried out every 3–6 h. In the database, the evaporation values for night (20–8 h) and daytime (8–20 h) intervals are presented, only those values that correspond to a full 12 h period of observations are published.

The accuracy of snow evaporation measurements is 0.01 mm.

Since 1952, the observations of permafrost seasonal thaw dynamics have been conducted at the KWBS. Danilin cryopedometers (frost tubes) were installed at permafrost observation sites (Snyder et al., 1971) which mostly were located in the approximate vicinity of Nizhnyaya and Verkhnyaya weather stations at the slopes with different aspects and landscapes.

The cryopedometer designed by A.I. Danilin consists of a rubber tube 1 cm in external diameter and calibrated to an accuracy of 1 cm. The tube is filled with distilled water, closed at both ends and lowered into a casing (an ebonite pipe) installed in a borehole in the soil. In order to measure the depth of freezing, the rubber tube is taken from the casing and the lower end of the ice column in the tube is determined (Manual of the water balance stations, 1973).

Despite the fact that permafrost observation sites were equipped with special bridges for observers to come close, eventually surface damage in the area where the device was installed began to influence thaw depth (Sushansky, 1988).

During 1952–1997 38 cryopedometers were functioning in total.

Runoff observations were carried out at 10 catchments: the Kontaktovy creek (gauges Verkhny, Sredny, and Nizhny), and brooks Morozova, Yuzhny, Vstrecha, Vstrecha (the mouth), Dozhdemerny, Severny, and Ugroza (Fig. 6). Key characteristics of the catchments are listed in Table 7.

All the water level gauges were equipped with “Valdai” water level recorders, as well as needle and hook water level gauges. In spring and autumn, when recorders did not work properly due to ice on the creeks, discharge was measured more frequently, every 4 h. To prevent the recorder floats from freezing, the wells were heated with electric bulbs.

At the micro-watersheds of the Morozova and Yuzhny brooks, runoff was measured by means of a V-notch weir, and at Severny brook, with a flow measuring flume.

In the database, mean daily values of streamflow are presented.

Originally, daily discharges were published in Observation Reports in L s-1 with accuracy of three significant figures, but not more accurately than 0.01 L s-1 for runoff gauges equipped with weir or flume. For gauges with a natural channel for discharges more than 1000 L s-1 the rounding to three significant figures was performed, for discharges less than 100 L s-1 – to two significant figures, but not more precise than 1 L s-1.

Small discharges, which are less than 0.05 L s-1 for the gauges equipped with hydrometric facilities and less than 0.5 L s-1 for larger watershed gauges, were published in Observation Reports as 0.00 and 0 respectively. The periods with no runoff because of drying and freezing were marked with special symbols.

In the database, water discharges are converted to m3 s-1; the number of significant figures was preserved, but the values 0.00 and 0, as well as special symbols for freeze and dry periods, are indicated as 0.

In 1969–1997 the data on the accuracy of discharge were published for several runoff gauges in the Observation Reports. They consist of information about (1) the number of measured discharges used to create the stage curve, (2) the percentage of the curve stage amplitude described by measured discharges, and (3) the percentage of stage curve extrapolation in both directions. These data are available for one or several runoff periods (ranging from 1–3 days to more than 2 continuous months) per year depending on how many stage curves were applied for each gauge. Figure 7 shows the boxplot of the percentage of stage curve extrapolation for seven runoff gauges for the period 1969–1997. Though the information is given not for individual discharges but rather for the periods, one may assume that the extrapolation is conducted for extreme events and analyse presented boxplots from this point of view. For most gauges the interquartile range (IQR) lies within -3 + 5 % limits; the tails (1.5 × IQR) exceed ±10 % for only two of seven gauges.

The data on measured and estimated instantaneous maximum and minimum discharges are available for the period 1983–1997. We calculated the percent difference between those values and built the boxplots for maximum and minimum discharges (Fig. 8). Mostly estimated maximum instantaneous discharges exceed measured ones by 3–20 %. Minimum instantaneous discharges (above 0) are underestimated in comparison with measured ones by a wider range (5–35 %).

The data on the accuracy of flow discharges are presented in the database and may be analysed on a single-event basis.

The climate of the study area is severely continental with harsh long winters and short but warm summers. Average annual temperature at the Nizhnyaya meteorological plot during 1949–1996 is -11.3 ∘C. Mean monthly temperature in January was -33.6 ∘C, in July +13.2 ∘C (Figs. 9, 10). The absolute minimum daily temperature of -53.0 ∘C was registered in 1982 and the absolute maximum daily temperature was +22.8 ∘C (1988). The period of negative air temperatures lasts from October to April, freeze-free period is, on average, 130 days long.

Air temperature inversions are observed at the KWBS. In December, the air temperature gradient reaches +2.0, in May it accounts for -0.5 ∘C per 100 m of elevation respectively.

The average air humidity at the Nizhnyaya station is 3.6 mb, reaching its maximum and minimum values of 9.8 and 0.4 mb in July and December respectively.

Total cloudiness at the Nizhnyaya station has an average annual value of 7.0 and does not change considerably through the year. Its minimum and maximum values are 5.9 and 8.0 in March and July. The lower cloudiness dynamic is more significant: its mean monthly value changes from 0.5 to 4.7 in March to July with an average value of 2.2 points.

Mean wind velocity is more than twice as high at Verkhnyaya station (1220 m a.s.l.) in comparison with Nizhnyaya station (850 m a.s.l.) and amounts to 3.0 and 1.3 m s-1 accordingly. The average monthly value changes from 0.83 in December to 1.70 in May at Nizhnaya, and from 2.7 in November and February to 3.4 in May. Maximum daily wind speed amounted to 16 and 36 m s-1 at Nizhnyaya and Verkhnyaya.

Precipitation at Nizhnyaya meteorological plot from 1969 (the year when wetting losses were introduced) to 1997 varied from 229 (1991) to 474 (1990)  mm per year with a mean value of 362 mm. After introducing wetting losses correction to the period 1949–1968 and computing average mean amount from 1949 to 1997, the mean annual value decreased to 351 mm. Maximum and minimum monthly amount of precipitation at Nizhnyaya station was observed in July and March and correspond to 72 and 8 mm respectively for the whole period of observations 1949–1997 (Figs. 9, 10).

Maximum daily amount of precipitation at Nizhnyaya station was observed in June 1968, reaching 48.1 mm. On average for the period of 50 years, maximum daily precipitation per year amounted to 26 mm.

On average, a stable snow cover at the KWBS is formed in the first weeks of October, and melts in the third week of May (1949–1996). The KWBS area is characterized by an increase in the thickness of the snow cover over this period due to the absence of thaws during the snow season. In the open treeless and watershed divide areas, the redistribution of snowpack due to wind blow is observed.

The averages for the watershed mean, maximum and minimum snow water equivalent (SWE) before spring freshet estimated based on snow survey results at Kontaktovy–Nizhny amount to 121, 213 (1985) and 59 (1964) mm respectively in the 1960–1997 period. In general, rocky talus and tundra bush landscapes are characterized by lower SWE due to wind blowing. Much snow is accumulated in the forest landscape. However, at the Morozova brook watershed which is fully covered by rocky talus landscape, mean SWE before snowmelt was estimated as 161 mm, with the maximum value of 298 mm observed in 1985 reaching on average 0.99 m snow height (Tables 3 and 4, Fig. 11).

The highest values of soil evaporation during the summer period were observed at the larch forest (site 9) and reached 136 mm. At a similar landscape (site 1), this value is lower, at 119 mm, which indicates the influence of local factors. The lowest values of soil evaporation are 104 mm at the plot located at dwarf cedar tree bush (site 7). In July, soil evaporation values range from 33 to 40 mm, depending on the landscape. In September, the contribution of evaporation decreases to 14–24 mm (Table 5).

Average values of annual soil evaporation were previously estimated by Semenova et al. (2013) and Lebedeva et al. (2017) based on the partial KWBS dataset as the following: 140 mm for larch and swampy sparse growth forest, 110 mm in dwarf cedar and alder shrubs of the tundra belt, and about 70 mm for rocky talus.

The average values of evaporation from snow in mm per day as determined from measurement data are as follows: January–February – -0.04 (condensation); March – +0.09; April – +0.40; May – +0.74; September – +0.20; October – +0.01. Typical values of evaporation from snow for 1976–1977 are presented in Fig. 11.

The longest observation period is 33 continuous years (cryopedometer 17.5 located at the forest with bushes, maximum thawing is 130 cm, 1964–1997). The deepest values of thawing were observed in rocky talus landscape and can reach more than 240 cm. The shallowest values of thawing range from 60 to 70 cm at swampy forest. Thawing of soils at the forest zone varies in large ranges and depends on the location of the cryopedometer at a slope (Table 6, Fig. 13).

Lebedeva et al. (2014) reviewed the patterns of soil thaw/freeze processes and their impact on hydrological processes based on the analysis and modelling of the data at the cryopedometers in the main landscapes of the KWBS: rocky talus, mountain tundra with dwarf tree brush, moss-lichen cover and sparse-growth forest or larch forest.

Flow at the KWBS begins in May, and most of it occurs in summer. At the outlet of the KWBS Kontaktovy creek at Nizhny, 33, 24 and 20 % of flow occurs in June, July and August respectively. For the summer period, rainfall floods are typical (Fig. 14).

Small brooks freeze completely in October. Surface flow stops at the channel of the Kontaktovy creek at the Nizhny gauge in November, but there is evidence that the river valley talik located lower than the Kontaktovy–Nizhny gauge experiences runoff until the beginning of snowmelt, which is evidenced by a continuous drop in levels in hydrogeological wells (Glotov, 2002).

Annual runoff of the Kontaktovy stream basin with area 21.3 km2 (average altitude 1070 m) is 281 mm for the period 1948–1997, it increases with elevation and at the Morozova catchment (mean elevation 1370 m, basin area 0.63 km2) reaches 453 mm (1969–1996). The flow from south-facing (Severny) and north-facing (Yuzhny) micro-watersheds with area of 0.38 and 0.27 km2 are 227 and 193 mm for the period 1960–1997 respectively.

Maximum daily discharge was observed in August 1979 and amounted to 7.6 m3 s-1 (daily flow 30 mm) and 0.438 m3 s-1 (60 mm) at the Kontaktovy–Nizhny and Morozova watersheds respectively.

The time series of flow characteristics and basic meteorological elements were evaluated for stationarity, in relation to the presence of monotonic trends, with Mann–Kendall and Spearman rank-correlation tests, at the significance level of p<0.05 (Mann, 1945; Kendall, 1975). If both tests proved a trend, a serial correlation coefficient was tested. With the serial correlation coefficient r<0.20, the trend was considered reliable. In the case of r≥0.20, to eliminate autocorrelation in the input series “trend-free pre-whitening” procedure (TFPW), described by Yue et al. (2002), was carried out. “Whitened” time series were repeatedly tested with a Mann–Kendall non-parametric test. The trend value was estimated with a Theil–Sen estimator (Sen, 1968).

The annual air temperature at Nizhnyaya station increased by 1.1 ∘C; positive trends are observed in March and October, accounting for the rise in temperature by 2.3 and 3.3 ∘C correspondingly. The annual sum of precipitation has grown by 74 mm (21 %). Maximum annual daily precipitation has also increased by 8 mm, or 31 %.

The analysis of monthly and annual flow (mm) for the Kontaktovy creek – Nizhny – from 1948 to 1997 has revealed the changes in hydrological regime in those 50 years of runoff observations (Fig. 15). Positive trends of monthly flow are identified in May, amounting to 29 mm, or 92 %, as well as in October (5.7 mm, 166 %) and November (0.35 mm, 252 %). The annual flow trend increased by 67 mm, or 24 %. These results confirm the general situation of increasing low flow which is observed in Siberia (Tananaev et al., 2016) and North America (Yang et al., 2015; St. Jacques and Sauchyn, 2009).