Large Arctic river basins experience substantial variability in climatic,
landscape, and permafrost conditions. However, the processes behind the
observed changes at the scale of these basins are relatively poorly
understood. While most studies have been focused on the “Big 6” Arctic
rivers – the Ob', Yenisey, Lena, Mackenzie, Yukon, and Kolyma – few or no
assessments exist for small and medium-sized river basins, such as the
Yana and Indigirka River basins. Here, we provide a detailed analysis of
streamflow data from 22 hydrological gauges in the Yana and Indigirka River
basins with a period of observation ranging from 35 to 79 years up to 2015.
These river basins are fully located in the zone of continuous permafrost.
Our analysis reveals statistically significant (
Numerous studies have shown that river streamflow in northern Eurasia and North America is increasing (Holland et al., 2007; Shiklomanov and Lammers, 2013; Rawlins et al., 2010). Most of them are focused exclusively on the “Big 6” Arctic rivers – the Ob', Yenisey, Lena, Mackenzie, Yukon, and Kolyma (Peterson et al., 2002; Holmes et al., 2013; Rood et al., 2017). Large Arctic river basins are characterized by a great variety of climatic, landscape, and permafrost conditions, and reported streamflow changes are not homogeneous in terms of different runoff characteristics and timescales. The mechanisms of the observed changes can hardly be understood on the large scale of the Big 6.
Compared to large rivers, the changes in flow in small and medium rivers in cold regions have been studied much less. Tananaev et al. (2016) found that 30 small and medium-sized rivers out of 100 in the Lena River basin showed trends in mean annual flow. Significant changes have been recently reported for small and middle-sized rivers in northwestern Canada (Spence et al., 2011), Alaska (Stuefer et al., 2017), and the Canadian High Arctic (Lamoureux and Lafrenière, 2018) and attributed to climate change and permafrost disturbances. Further analysis of small river basins could reveal the mechanisms behind ongoing changes, as at larger scales attributing changes can be problematic.
River runoff change estimates in northeastern Siberia are limited and
contradictory. Magritsky et al. (2013) reported an increase in the total
runoff of Yana and Indigirka during the period 1976–2006 by
1.5 %–3 % compared to the period before 1976 and noted that the
runoff of these rivers increased in summer and autumn by 20 %–25 %
and did not change in winter. According to Georgievsky (2016), on the
contrary, there is an increase in Yana and Indigirka River winter runoff by
40 % over the period 1978–2012 compared with the period 1946–1977 and
an increase in the spring flood. Majhi and Yang (2011) concluded that the Yana River monthly flow
increases at the Jubileynaya gauge (224 000 km
While some studies have examined flows at the outlet of the Yana and Indigirka River basins, estimations of streamflow changes in small and medium-sized rivers in the Yana and Indigirka River basins do not exist. The objective of this research is therefore a quantitative assessment of current changes in hydrometeorological regime within two large Arctic river basins, with both basins completely located within the continuous permafrost zone, which have long-term runoff observations along the main rivers and their tributaries at smaller scales.
In Sect. 2 of this paper, we will present the study area. In Sect. 3, we will describe the data to be examined and the methods that we will use. Results of the analysis will be presented in Sect. 4, while these results will be put into context and potential causal factors influencing the results described in Sect. 5. Finally, some conclusions will be drawn in Sect. 6.
The Yana and Indigirka rivers are two of the few large Arctic rivers whose
basins are completely located in the zone of continuous permafrost (Fig. 1).
The terrain is mainly mountainous. There is high elevation in the Verkhoyansk
(Orulgan, 2389 m), Chersky (Pobeda, 3003 m), and Suntar–Khayata (Mus-Khaya,
2959 m) ranges, as well as wide river valleys. The average altitude of the
research basins ranges from 320 to 1410 m and the elevations of the outlet
gauges range from
Meteorological stations and hydrological gauges within the study basins.
The study territory is the region where the Northern Hemisphere's “pole of
cold” is located. The absolute minimum there has reached record levels for
the Northern Hemisphere: down as far as
The research basins are situated in the transitional zone between
forest–tundra and coniferous taiga (Fig. 2). For high-altitude mountain areas
above 1900 m a.s.l., goltsy (bald mountain) and small glaciers with minimum
and maximum areas from 0.024 to 5.76 km
Landscape distribution within the study basins according to
the Permafrost–Landscape Map of the Republic of Sakha (Yakutia) on a Scale
In the studied river basins, permafrost thickness can reach over 450 m at
watershed divides and up to 180 m in river valleys and depressions. There
are highly dynamic cryogenic features such as retrogressive thaw slumps found
in the Yana River basin that indicate ongoing permafrost degradation
processes (Günther et al., 2015). Permafrost temperature at a depth of
zero annual amplitude in the studied region typically varies from
The hydrological regime is characterized by spring freshet, high summer–autumn rainfall floods, and low winter flow. In winter, small and medium-sized rivers freeze thoroughly. Spring freshet starts in May–June (on average 20 May) and lasts approximately for a month and a half. In summer, glacier runoff, melting aufeis, and snowfields contribute to streamflow.
The amount of precipitation at meteorological stations varies from
176 mm yr
Since the Yana and Indigirka River basins are located in continuous
permafrost, groundwater can be found in a seasonally developed active layer,
underneath permafrost (supra-permafrost water), and in taliks (Shepelev,
2011). Depending on texture, infiltration, and other soil properties, the
active layer stores and transmits water to rivers in summer and early autumn.
Water in the active layer could sustain recession river flow in autumn only
until the freezing front reaches the permafrost table. Taliks with
thicknesses of several meters typically exist under small and middle-sized
rivers even if the rivers freeze in winter (Mikhailov, 2013). Through-taliks
are typically found along the river channels with water depth exceeding
3–5 m. Rivers in continuous permafrost often lose water to channel taliks
during summer and gain in winter (Arzhakova, 2001). Through-taliks are also
associated with fractured deposits, with depth exceeding the permafrost
thickness (Glotov et al., 2011). Rivers with lengths
Aufeis (naleds), which forms at mountain foothills, as well as in submountain
and intermountain depressions, is another distinguishing feature of the
region. According to Simakov and Shilnikovskaya (1958), the research area had
1281 aufeis fields covering an area of about 2082 km
Few glaciers are found in the research area (GLIMS and NSIDC, 2005). The
total area of glaciers is about 2.2 km
Daily discharge series for 22 hydrological gauge stations of the Russian
Hydrometeorological Network in the Yana and Indigirka River basins were
analyzed. Most of the stations have hydrological data until 2014–2015, but
two of them ceased operation in 2007 and one (river Indigirka – Vorontsovo,
ID 3871) in 1996. The median length of the observation series is 62 years with
minimum and maximum values reaching 36 and 80 years, respectively (Table 1,
Fig. 1). A total of 10 out of the 22 catchments examined here have areas less than
10 000 km
Characteristics of runoff gauge stations.
We used the values of monthly and annual flow (mm), and spring freshet start dates (counting days from the beginning of the year) used in the analysis were estimated based on daily discharge data for the entire observation period from 1936 up to and including 2014–2015. There are different ways to determine a freshet start date. For example, Lesack et al. (2013) defined the initiation of freshet discharge based on a threshold value of 3 % increase in discharge per day. In this study, wherein nonfreezing and freezing rivers were investigated, a different approach was considered to be more appropriate. A day was defined as a freshet flood start date if its discharge reached or exceeded 20 % of the average discharge value in the studied year. All the streamflow series were visually checked for the adequacy of this assumption.
The uncertainties associated with discharge determination change significantly from year to year and strongly depend on the computational methods used and the frequency of discharge measurements. Possible errors in flow values shown in the database as reliable are as follows for all gauges: average annual flow errors do not exceed 10 %, and monthly flows errors are 10 %–15 % for the open channel period and 20 %–25 % for winter months. The errors of “approximate” flow, placed in the database in parentheses, can exceed the values indicated above by 2–3 times (State water cadastre, 1979). The errors for streamflow in large rivers may be slightly lower: monthly flow errors are 4 %–12 % for the open channel period and 17 % for winter months in the Lena River basin (Shiklomanov et al., 2006). Daily air temperature and precipitation data series, observed at 13 weather stations (the elevation range varies from 20 to 1288 m) located in the studied basins over different periods from 1935 (but not later than 1966) to 2015 (some stations to 2012), were reduced to average monthly value series (Table 2). Monthly soil temperature under natural cover at different depths down to 320 cm from three weather stations over a period of 1966–2015 was also analyzed. A detailed description of soil temperature data sets and their quality control methods may be found in Sherstyukov and Sherstyukov (2015). Active layer thickness (ALT), used for the analysis, was estimated with polynomial interpolation of soil temperatures at depth (Streletskiy et al., 2015). The sources of the data are described in Makarieva et al. (2018b).
Characteristics of meteorological stations.
In this study, we applied a combination of widely used statistical methods
for trend detection and assessment of their values in hydrometeorological
data. General guidance with a detailed description of different types of tests
and their adequate application can be found in Kundzewicz and Robson (2004).
Time series of runoff characteristics (monthly flow, estimated flood starting
dates, and maximum daily flows) and meteorological elements (monthly and
annual values of air temperature and precipitation; soil temperature and ALT)
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
The results of the trend analysis are presented in Tables 3–8. Multidimensional tables with colorful designations showing not only the trend values but also presenting the distribution of change points across the basins can be found in the Supplement (Tables S1–S5). We used the same colors as in Tables S1–S5 in Fig. 3, showing the percentage change in monthly and annual streamflow at different change point periods, and Figs. 4 and S12–S17, which present the spatial patterns of the change points in different months.
Change in monthly streamflow represented as a percent, along with the period in which that change occurred. Data are for both Yana and Indigirka River basins and are sorted in order of basin area.
The periods of changes in streamflow in August, September, October,
November, December, and annually. Red and black indicate the presence
and absence of the trends, respectively. The triangles and circles indicate
the basin area of less and more than 1000 km
The increase in annual air temperature is statistically significant at all 13
studied stations (Tables 3, S1, Fig. S1) with an average cumulative value of
about
Changes in monthly and annual air temperature (
Most of the values have statistical significance of
During the period April to July, there is a significant air temperature
trend at most of the weather stations of the region; average trend values in
these months account for
Monthly precipitation analysis for 13 meteorological stations in the region, located at elevations from 20 to 1288 m (1966–2015, some stations until 2012), has shown no evidence of a systematic positive trend (Tables 4, S2, Fig. S2).
Changes in monthly, seasonal, and annual precipitation (millimeters and percent), 1966–2015.
Only the columns corresponding to the months in which the trends
of precipitation were detected are shown in the table. Most of the values
have statistical significance of
In contrast, at six weather stations out of nine in the Indigirka basin, a
statistically significant precipitation decrease is observed in January and
at three stations in February. In the Yana basin, two stations have shown a decrease
in precipitation in December. The average statistically significant decrease in
winter months ranges from
The state and timing of precipitation in May and September (which are the
months when the 0
Characteristics of rain and snow share in precipitation regime in May and September, 1966–2012 (the table contains only data for stations with changes).
There are three meteorological stations with soil temperature data available
for the entire historical period, including the beginning of the 21st century,
with a total area of 529 000 km
Changes in soil temperature at 80 cm of depth and maximum active layer thickness (ALT).
Due to the temperature decrease in summer, estimated ALT (mean
Positive statistically significant trends (
In May, a statistically significant increase in streamflow is observed at 12
of the 21 studied gauges. These 12 may be divided into two groups. The first
group (Fig. S4, Group A) contains seven gauges with basin areas from 8290 to
89 600 km
In July, no statistically significant changes in streamflow occur in the
Indigirka River basin. In the Yana River basin, two nested gauges show
opposite tendencies (Tables 7–8, S3–S4). The streamflow at gauge ID 3474
(8290 km
Changes in monthly and annual streamflow (millimeters and percent) and freshet onset dates. The Yana River basin.
Only the columns corresponding to the months in which the trends
of flow were detected are shown in the table. Most of the values have
statistical significance of
In September, positive trends were identified at 17 out of 22 gauging
stations (average value is 58 % or 9.8 mm); three of those rivers are
small ones (catchment areas are less than 100 km
Changes in monthly and annual streamflow (millimeters and percent) and freshet onset dates for the Indigirka River basin.
All designations are the same as in Table 7.
In October, streamflow increases at 15 out of 22 gauges (on average by
61 % or 2.0 mm) and in November at 11 out of 17 nonfrozen gauges (on
average by 54 % or 0.4 mm) (Tables 7–8, S3–S4, Figs. S7–S8). Most of
the changes are abrupt. In October, most of the step trends occur in
1987–1993 with several exceptions in 1982 and 2001–2002; in November, they occur in
1981–1999, mainly around 1994. In December, positive trends are found at 6
out of 15 nonfrozen gauges (Tables 7–8, S3–S4, Figs. S8, 2). The average
positive trend magnitude is 95 % or 0.15 mm. Change points occur from
1981 to 1994. A negative monotonic trend in December is found at gauge 3483
with the magnitude
In the Yana River basin, statistically significant changes in annual
streamflow are found at the Adycha River tributary. Maximum percentage and
net flow changes accounting for
An increasing annual trend is also found at gauges (ID 3478,
22.6 km
In the mountainous part of the Indigirka River basin, positive step trends in
annual streamflow are found starting from 1993 at two gauges in the upstream
of the Indigirka River (IDs 3488, 3489) and its tributary, the Elgi River
(ID 3507), with an average magnitude of 27 % or 49 mm. An increase in
annual streamflow is also observed at the smallest basin from the analyzed
set (ID 3516, 16.6 km
An analysis of maximum daily streamflow was carried out for the warm period from May to September. In general, the patterns of changes in maximum daily discharges replicate the change in monthly streamflow (Table S1). The main changes are observed in May with break points around 1966, when negative trends change into an insignificant positive one. The percentage change in May for eight gauges averages 69 % over the whole period of observations. In September, the break points in terms of maximum discharge occur around 1993 in the Indigirka and in 1976–1981 in the Yana River basin. The average increase in maximum discharge in September reaches up to 55 % for 15 gauges.
In the rivers with a basin area less than 2000 km
Global land-surface air temperature has increased over the period 1979–2012
by 0.25–0.27
Although precipitation is projected to increase over the pan-Arctic basin,
this is not always supported by ground meteorological data (Rawlins et
al., 2010). Small and insignificant positive trends from 0.63 to
5.82 mm yr
The presented analysis for 1966–2015 (2012 at some stations) has shown no evidence of a systematic positive trend in annual precipitation and even displays a negative trend of solid precipitation for several stations, including Verkhoyansk (ID 24266). Savelieva et al. (2000) reference the decrease in winter precipitation over the territory of eastern Siberia to changes in the location and intensity of the Siberian High and Aleutian Low before and after the 1970s. The complex topography of the Arctic region may cause a considerable underestimation of precipitation as meteorological stations tend to be located in low-elevation areas (Serreze et al., 2003). Analyzed precipitation data did not undergo wind correction, which varies from 10 % in summer to 80 %–120 % in winter due to the effect of wind on gauge undercatch of snowfall (Yang et al., 2005). This study therefore confirms the high uncertainty of the spatial pattern of precipitation trends in cold regions (Hinzman et al., 2013).
Permafrost temperatures have risen in many areas of the Arctic (AMAP, 2017).
Long-term data on permafrost temperature from boreholes for the Yana and
Indigirka basins are extremely scarce. Romanovsky et al. (2010) reported an
increase in mean annual ground temperature (MAGT) in the eastern part of
northern Yakutia (including the Yana and Indigirka River basins) of up to
1.5
Identified trends of air, soil temperature, and precipitation at the Verkhoyansk (ID 24266) and Ust'-Moma (ID 24382) stations agree with each other. At Verkhoyansk, a soil temperature increase in May–September follows an air temperature upward tendency in April–August with a 1-month delay. The soil temperature drop in winter may be caused by a decrease in snow depth (Sherstukov, 2008) due to a statistically significant decrease in precipitation in the cold season from October to April (Table 4). At Ust'-Moma, the soil temperature increase from May to November could be explained by a statistically significant air temperature rise for 9 months out of 12.
Detected trends from
Streamflow significantly (
Analysis of the runoff data for the Yana and Indigirka River basins has shown statistically significant positive discharge trends in May and the autumn–winter period for the last few decades (accompanied by significant warming). Statistically significant increases are seen in 12 out of 22 gauges in May, 17 out of 22 in September, 15 out of 22 in October, 9 out of 19 in November, 6 out of 17 in December, 4 out of 12 in January, 3 out of 8 in February, and 3 out of 7 in March. Note that the total number of gauges decreases below 22 in the period from November to April because the rivers freeze.
September shows the most considerable change in hydrological regime. The increase in streamflow happened at basins of all sizes in 17 of the 22 studied gauges. In September, air temperature increased only at 2, and precipitation increased only at 1 meteorological station out of 13.
In the Indigirka River basin, the increase in streamflow can be attributed to
the shift of precipitation type in September. Investigating the correlation
between monthly streamflow and precipitation in September for four small
watersheds (area
Correlation coefficient of monthly streamflow and total and liquid precipitation in September at small watersheds.
The shift of precipitation from solid to liquid occurs starting from 1993 and manifests in a considerable increase in rain fraction in comparison with snow at the stations located in the mountains. The fact that the changes in streamflow are observed regardless of basin size and in general match the break points of precipitation ratio shift indicates that it is climate rather than other possible factors (permafrost thaw, increased groundwater connectivity, etc.) that drives those changes. The average increase in streamflow reaches up to 9.8 mm, which is comparable with the mean absolute increase in rain precipitation of 12.2 mm. In the Yana River basin, one would expect a similar linkage between precipitation and streamflow in autumn, but this cannot be confirmed due to the lack of meteorological stations at higher elevations. Spence et al. (2011) have shown that the trend towards more autumn rainfall in the northwestern sub-Arctic Canadian Shield, with no increase in total precipitation, has been sufficient to cause late season peaks in discharge and higher winter low flows. Recessional curves of early autumn flood events extend later into the autumn and winter seasons. The streamflow changes in October generally repeat the spatial pattern of changes in September but with lower magnitude. The hypothesis of Berghuijs et al. (2014), stating that a shift from a snow-dominated towards a rain-dominated regime would likely reduce streamflow, is contradicted by the results of this study for the rivers in the continuous permafrost zone.
Upward trends in low flow are observed at Mackenzie River (Yang et al., 2015), at the Lena River and its tributaries (Tananaev et al., 2016), and in most of the Arctic (Rennermalm et al., 2010). The widely accepted hypothesis is that increased low flow indicates permafrost degradation, aquifer activation, and better connectivity between surface and subsurface water. Increasing active layer thickness, enlarged infiltration, and subsurface water contribution to winter discharge by deeper and longer flow pathways sustain increasing winter streamflow in permafrost environments (Karlsson et al., 2015; St. Jacques and Sauchyn, 2009; Tananaev et al., 2016).
Change points for low flow were detected in the 1980s, 1990s, and 2000s. Similar change points are observed at the Lena River basin (Tananaev et al., 2016), where the major shifts in all nonstationary time series occur in the 1990s, but when the data from adjacent decades are combined, the major changes in minimum discharge occur between 1985 and 1995 and in mean annual daily flow between 1995 and 2005.
The changes in November–December tend towards basins larger than
17 000 km
Gurevich (2009) and Dzamalov and Potehina (2010) have substantiated the hypothesis that river ice cover regulates groundwater feeding into rivers in cold regions. The hypothesis is that in colder winters, with a significant thickness of ice, the total water discharge decreases in small river basins. In less severe winters, when the thickness of ice is lower, the underground contribution to streamflow in the river is higher by the end of winter.
Shiklomanov and Lammers (2014) report significant negative linear trends for
the outlet gauges of the Lena, Yenisey, and Yana rivers where decreases in
maximum ice thickness over 1955–2012 reached up to 73, 46, and 33 cm,
respectively. Average values of maximum ice thickness for the same rivers
were about 180, 105, and 153 cm, respectively, for the period 1955–1992.
Several 10 d series of river ice depth for the Lena River at Tabaga for the
period 1955–2015 were analyzed based on a Mann–Kendall test at the
significance level of
The increases in monthly streamflow in May occur in an abrupt manner. At
7 gauges (area
No significant trends were detected for the period after the change point (1967–2015). Changes in spring freshet start date were identified at 10 gauges with streamflow changes in May. Trend value varies from 4 to 10 d earlier. This agrees with Savelieva et al. (2000), who stated that the final frost in spring has been 12–15 d earlier than the mean value in Yakutia. Smith (2000) estimated the time shift of the start date of spring ice cover events for the outlet of the Indigirka River at Vorontsovo (ID 3871) as 8.1 d for the period of observations (1937–1992).
Another five basins have shown the shift of flow in May during the period from 1980 to 1999. In four cases out of five, that shift can be attributed to an earlier freshet. Yang et al. (2015) documented increases in May flows for the Mackenzie and Yukon rivers, related to earlier melting of snow. An earlier start of freshet agrees with identified significant air temperature increases in May at 9 out of the 13 meteorological stations in the region. However, in contrast to the Lena River basin, where strong warming in spring led to an advance of the snowmelt season into late May and resulted in a lower daily maximum discharge in June (Yang et al., 2002; Tan et al., 2001), at the study gauges, streamflow has not been reduced in June (except ID 3510).
According to recent projections, the annual maximum river discharge could
almost double by the mid-21st century at the outlets of major Siberian rivers
(Shkolnik et al., 2017). Existing assessments of maximum flow changes across
cold regions do not fully confirm future projections. Shiklomanov et
al. (2007) found no widespread significant change in spring maximum
discharge, except for the Lena River, among the 139 gauging stations in the
Russian Arctic. Tananaev et al. (2016) reported trends in maximum discharge
only in 9 time series (negative at three and positive at six gauges) out
of the 105 in the Lena River basin. The Ob' and Lena rivers showed
uncorrelated changes in spring peak with those aggregated from small natural
watersheds located within these larger river basins (Shiklomanov et
al., 2007). There are significant negative trends (
Positive discharge trends in the Arctic have been established in recent years (McClelland et al., 2006; Peterson et al. 2002, 2006; Shiklomanov and Lammers, 2013). Increasing precipitation has been suggested as the one of the drivers of increasing streamflow (Dyurgerov and Carter, 2004; McClelland et al., 2004). Bring et al. (2016) project long-term streamflow increases over the Yana and Indigirka rivers by 50 % during the 21st century by combining information from global climate model runs of multiple scenarios. The findings of this study largely contradict this, with an increase in streamflow reported at most of the gauges despite no substantial change in the absolute amount of precipitation. We estimate that 10 mm of snow is lost in September as rain draining directly into streamflow. Adding the decrease in precipitation in February–March, the decrease in snow water equivalent during winter may reach up to 15–20 mm in total. Curiously, though, no evident decrease in streamflow is observed during freshet or summer months.
Inconsistent trends in discharge and precipitation have also been reported for other Arctic rivers. In some cases there is an opposite sign in the direction of runoff changes from that in precipitation (Karlsson et al., 2015). Berezovskaya et al. (2004) have shown that the patterns in increasing trends of runoff from Siberian rivers cannot be resolved from the apparent lack of consistent positive trends in the considered precipitation datasets.
Milliman et al. (2008) suggest that a fluctuating climate in the Yana River watershed precluded delineating a statistically significant change in either precipitation or discharge, but concluded that the Indigirka River basin had a statistically significant decrease in precipitation and nonsignificant increase in runoff.
The issue of increasing streamflow in the absence of increasing precipitation may be explained by decreased evapotranspiration. By increasing the active layer depth and thus potentially lowering the water table, evapotranspiration might decrease, leading to increases in runoff (McClelland et al., 2004). Milliman et al. (2008) proposed that decreased evapotranspiration due to earlier snowmelt and changes in water storage within the drainage basins could be a potential source of excess streamflow. Based on modeling results, Stieglitz et al. (2003) showed that as warming occurs mostly during winter months it does not lead to an increase in evapotranspiration.
On the other hand, Rawlins et al. (2010) argue that evapotranspiration is
increasing. This is supported by reanalysis data for the pan-Arctic domain.
According to model simulations, evapotranspiration has a significant trend of
0.11 mm yr
Permafrost temperature in Russia has been increasing and the active layer has been deepening for the last 20–30 years (Romanovsky et al., 2010; Sherstyukov and Sherstyukov, 2015). Subsurface ice melting along with air temperature increase, widespread over the studied area (Brown et al., 1997; Yang et al., 2002), can contribute to the streamflow increase (Frey and Mclelland, 2009). Though McClelland et al. (2004) argue that if water released from thawing permafrost was making a significant contribution to the observed increase in annual discharge from Eurasian Arctic rivers, one might expect that watersheds with the most permafrost cover would show the largest increase in runoff, but no such pattern is apparent.
An analysis using satellite-based gravimetry at the Lena basin suggests that the storage of water in these areas has increased over the past decades (Velicogna et al., 2012; Muskett and Romanovsky, 2009), but the causes of this phenomenon are not clear (Velicogna et al., 2012). One reason may be permafrost degradation because warming in the active layer will cause greater connectivity between surface and subsurface water (Walvoord and Kurylyk, 2016) and talik development (Yoshikawa and Hinzman, 2003; Jepsen et al., 2013). Most groundwater discharge occurs through areas overlying open taliks, so they play the important hydrogeological role of accommodating preferential pathways acting to either recharge the deeper regional aquifer from the supra-permafrost system or facilitating discharge from the deeper aquifer (Bense et al., 2011; Walvoord et al., 2012; Lamontagne-Hallé et al., 2018). An easier flow of groundwater may increase the discharge of groundwater into the rivers (Ge et al., 2011; Walvoord et al., 2012; Lamontagne-Hallé, 2018). However, this phenomenon only holds if sufficient water is available to replenish the increased discharge. Otherwise, there will be an overall lowering of the water table in the recharge portion of the catchment (Ge et al., 2011). Michelle et al. (2016) suggest that permafrost loss is more likely to contribute to baseflow increases in discontinuous permafrost than in continuous permafrost, with permafrost tending to be cold and thick. The results of this study have shown that baseflow is increasing in the zone of continuous permafrost as well.
Another possible contribution to streamflow increase could be the meltwater from glaciers, at least in the Indigirka River basin where glaciers take up to 0.12 % of the total basin area. Dyurgerov and Carter (2004) studied glacier retreat and their input into increased streamflow in Arctic rivers. Bliss et al. (2014) assessed the global-scale response of glacier runoff to climate change based on scenario modeling and have proposed that the Canadian and Russian Arctic region exhibits steady increases in glacier runoff consequently changing the hydrological regime.
Under the current climate, the glaciers of the region melt more intensively
in July and August, so their input to streamflow would be expected during
those months. In the headwaters of the Indigirka River (ID 3488,
51 100 km
Liljedahl et al. (2017) have found that in Jarvis Creek (634 km
The melt of rock glaciers and aufeis, which is widespread in the study area,
may have a similar effect of additional water input. According to Lytkin and
Galanin (2016), 540 rock glaciers are identified within the Suntar–Khayata
Range with a distribution density of 8.4 objects per 100 km
The assessment of aufeis area conducted for the Indigirka River basin based on recent Landsat imagery analysis has shown that the number of aufeis fields increased by 35 %, but the total area decreased by 1.6 times (Makarieva et al., 2019a) in comparison with the data by Simakov and Shilnikovskaya (1958). The current aufeis area share for the studied watersheds in the Indigirka River basin ranges from 0.26 % to 0.80 % (Makarieva et al., 2019a), though historical estimations are higher. For example, Tolstikhin (1974) estimates that relative aufeis area averages 0.4 % to 1.3 %, reaching 4 % in the basins of some rivers. Due to changed conditions of aufeis functioning in the current warmer climate, they may contribute to the observed increase in streamflow in May and June at several studied gauges due to more active melting.
A significant positive trend of annual streamflow was identified only for four gauges. Arzhakova (2001) emphasizes that the rivers Elgi (ID 3507, the Indigirka River basin) and Adycha (IDs 3443 and 3445, the Yana River basin) cross tectonic dislocation zones. The area is characterized by modern seismotectonic activities (Imaeva et al., 2017). Taliks, which form within faults and excessive jointing zones, supply rivers even in extreme winter conditions (Romanovsky, 1983). Glotov et al. (2011) proposed that in particular years during sublittoral seismic activity periods, winter streamflow fluctuations in the Kolyma River may depend on geotectonic extensions and compressions that occur in the region via riverbed through-taliks. Savelieva et al. (2000) pointed to redistribution of the water budget between the upper (thaw layer) water and groundwater beneath the permafrost. According to Savelieva et al. (2000), the increases in temperature and thickness of the permafrost active layer may enhance the water inputs from the seasonal thaw layer into the groundwaters beneath the permafrost through tectonic trenches and the lake's talik zone.
This study has identified increases in runoff in rivers discharging to the
Arctic Ocean. Increasing volumes of freshwater flows to the Arctic Ocean
could lead to a significant weakening of the thermohaline circulation,
changing sea stratification, and sea ice formation (Arnell, 2005; Lique et
al., 2016, Weatherly and Walsh, 1996). The total volume of water being
discharged from the Yana and Indigirka rivers is around 84 km
Approximately 85 % of the total terrestrial runoff to the Arctic Ocean is supplied by rivers draining from Russia (Aagaard and Carmack, 1989). Moreover, Makarieva et al. (2019b) have shown that most runoff of the Indigirka River is generated in mountainous areas and not from the coastal plains. Therefore, the analysis of this database, including the mountainous upstreams of the Yana and Indigirka rivers, is particularly timely.
Forman and Johnson (1996) have previously identified the assessment of volume, timing, and natural variability of Russian river discharge as a major priority in Arctic science. In particular, the Indigirka is one of the two largest rivers flowing into the East Siberian Sea (Anderson et al., 2011), and thus changes in flows from the Indigirka River have the potential to impact acidity, nutrients, and carbon cycling in this sea (Semiletov et al., 2016).
In terms of ecological impacts, increases in discharge, velocity, temperature, and concentration of suspended materials exert important effects on primary production and food-web dynamics in rivers and estuaries (Scrimgeour et al., 1994). Flooding associated with increased discharge is a primary control on the exchange of sediment and organic carbon between rivers and Arctic floodplains (Smith and Alsdorf, 1998).
Analysis of data from the Yana and Indigirka River basins has
shown increases in annual air temperature for the region of around
2.0
Despite the slight decrease in overall precipitation, analysis of daily
discharge data for 22 gauging stations in the Yana and Indigirka River basins
(1936–2015) has revealed, at most of the stations, statistically significant
(
Most of the increases in streamflow are seen via break points, rather than showing a monotonic increase over the entire period of record. The structure of the timing of these changes is very complex; however, in general, the changes occur in the Yana River basin 10 years before the Indigirka River basin. Changes in winter streamflow are seen in the larger river basins (lower-elevation gauges) before they are seen in the smaller river basins. Analysis of monthly data is required in order to identify these changes, as in many cases, changes in annual flow are not significant. Additionally, analysis of other datasets such as monthly precipitation and temperature is required in order to develop hypotheses regarding the potential causes of these changes in streamflow.
Increases in streamflow in the autumn–winter period are likely mainly caused by a shift in precipitation from snow to rain in September, with consequent increases in streamflow in that month, continuing into October and later months. Decreased depth of river ice and a better connection of the surface and groundwater due to the deepening of the active layer and prolonging the freeze-free period may also be causing higher winter flow.
There are no changes seen in spring discharge in the last 50 years (from around 1966) except at several upstream, mountainous gauges in both studied river basins. Curiously, snow losses and the earlier timing of the spring freshet in May do not lead to a decrease in streamflow in June. This additional input of water is likely related to the regional warming trend impacting more intensive melting of the cryosphere elements such as permafrost, glaciers, and aufeis.
It is difficult to directly attribute changes in streamflow to definite causes, and it is almost certainly a consequence of a complicated interplay between different changing water storages and pathways in warming permafrost. The possibility also exists of additional water input via precipitation higher in the mountains, but this hypothesis cannot be verified with current observational data. The observed changes agree with reported positive trends in Arctic river discharge, albeit with low confidence (IPCC, 2014), and the importance of local factors in streamflow response to climate change over Siberia (AMAP, 2017). Increased streamflow may also cause changes to the Arctic Ocean through an increased freshwater flux and decreased thermohaline circulation.
Combined monthly hydrometeorological data used in this
study are publicly available and can be downloaded from
The supplement related to this article is available online at:
All the authors discussed the results presented in this paper. OM, NN, and LL performed the analysis of hydrological and meteorological data. AS provided and analyzed soil data. The paper was written by OM, NN, and DAP with relevant comments from all the coauthors. The figures were designed by NN in close collaboration with OM and DAP.
The authors declare that they have no conflict of interest.
The authors are grateful to two anonymous reviewers and the handling editor, Valentina Radic, whose comments and suggestions made the paper considerably better.
The study was partially funded by RFBR under research project no. 18-45-140065.
This paper was edited by Valentina Radic and reviewed by two anonymous referees.