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Water regime of lake Baikal under conditions of climate change and anthropogenic influence
Quaternary International 524 (2019) 93–101
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Water regime of lake Baikal under conditions of climate change and anthropogenic influence
T
V.N. Sinyukovich, M.S. Chernyshov∗ Limnological Institute of the Siberian Branch of RAS, Ulan-Batorskaya St., 3, Irkutsk, 664033, Russia
A R T I C LE I N FO
A B S T R A C T
Keywords: River inflow Runoff Level Climate warming
Based on water level data, we have studied some specifics of the water regime in Lake Baikal associated with the warming climate, which has intensified since the early 1970s. We have also addressed the overregulation of the lake after the construction of the Irkutsk Hydroelectric Power Station (IHPS). We have estimated the influence of climate on the water regime of the lake through observing the change in the surface water inflow from the watershed basin and the characteristics of runoff from the three largest Baikal rivers: the Selenga, the Upper Angara and the Barguzin. A decrease in the runoff of these rivers during the intensification of warming reduced the surface water inflow into the lake, causing a decrease in level between 1976-1981 and 2014–2017. Notably, the spring flood on the rivers during climate warming becomes shorter and is characterized by a decrease in the maximum water discharges, contributing to the slowdown of the filling of Lake Baikal during spring and summer. Contrastingly, in the winter, river runoff and the lake inflow increase with the rise in air temperature. However, this increase depends on the runoff of rivers during the previous summer and autumn. This runoff determines the formation of groundwater reserves that serve as the main source of riverine water in the winter. We have suggested the possible decrease in runoff during the years of the air temperature rise due to an increase in evaporation. Warming also contributes to the increase in evaporation from the watershed of Lake Baikal, as well as the rate of decline in its water level, which reaches 10–15 cm per month in November and December. The backwater from the dam of the IHPS has been observed in Baikal since the autumn of 1958; it has caused an increase in the lake level by an average of 0.79 m. Within the year, there was an alignment of outflow from the lake through the Angara River, and in periods with low water level, it was kept at a minimum value of 1300 m3/s for a long lime in order to maintain the Baikal water level. Using indirect data, we have investigated the fluctuations in water level of Lake Baikal between 1729 and 2017, which indicates its potential increase at a rate of 1 cm per 10 years. The changes in the Baikal water regime determine the restructuring of various processes inside the water body and are important for the ecosystem of the lake.
1. Introduction The ratio of the elements in the water balance of Lake Baikal determines its water regime. The surface water inflow from its watershed of 540 000 km2 (Fig. 1) dominates the income of the lake balance, while the runoff through the Angara River accounts for its expenditures. Since the middle of the 20th century, the Baikal water regime has been under regulatory influence of the dam of the Irkutsk Hydroelectric Power Station (IHPS), constructed 60 km downstream from the head of the Angara River. The regulation influenced the significant intra-annual levelling of the runoff from the lake, and its level has increased by an average of 0.8 m. The increase in global warming, which has occurred
∗
since the 1970s, is particularly intensive in Siberian territory. In the area of Lake Baikal, over the past 30 years of the 20th century, the air temperature rise of 1.9°С was twice as high as the global average (Shimaraev et al., 2002; Shimaraev and Starygina, 2010). The warming of Lake Baikal and its watershed led to the restructuring of various hydrological processes (Shimaraev et al., 2002; Goldman et al., 2013; Sinyukovich et al., 2013). Since 1996, there is a long-term low water level in the watershed, which was particularly acute in 2014–2017 and caused a decrease in the level of the lake below the legislative standard values. During this period of low water, the problems of using the water resources of the lake for hydropower engineering, water supply and shipping were complicated. Moreover, there were some changes in the
Corresponding author. E-mail address: [email protected] (M.S. Chernyshov).
https://doi.org/10.1016/j.quaint.2019.05.023 Received 24 December 2018; Received in revised form 19 May 2019; Accepted 20 May 2019 Available online 21 May 2019 1040-6182/ © 2019 Elsevier Ltd and INQUA. All rights reserved.
Quaternary International 524 (2019) 93–101
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Fig. 1. Lake Baikal basin. 1 – Catchment area boundary, 2 - State boundary, 3 – Settlements.
Baikal ecosystem. Considering that approximately 23 thousand km3 of fresh water (20% of the global reserves) are concentrated in Lake Baikal, understanding its water regime and the change in the conditions of its formation under climate instability and hydropower regulation is extremely important. Notably, unlike the lake itself, the runoff of the Baikal rivers is not regulated by reservoirs and used only on a small scale for water consumption by the population, as well as in industry and agriculture. In this regard, the observed changes in the runoff of rivers are mostly due to climate.
Standard 19179-73, 1988), we regard the water regime of lakes as the regime of changes in their volumes and levels, without considering the characteristics of currents, waves and mixing. In the case of Lake Baikal, only the surface (variable) part of the lake volume is of practical importance, as it changes during both seasonal and long-term hydrological cycles. Considering the maximum increase and decrease of the lake volume in the 20th and 21st centuries, the surface waters are not more than 65 km3 or 0.3% of the total water reserves in the lake. Since the area of the Baikal water surface in calculations of water balance for the variable part of the volume is assumed constant (31500 km2), the levels and volumes in this zone are linearly related; thus, both of these indicators reflect the changes in the water regime of the lake. On this basis, we regarded information about water level, for which observations are the longest and reliable, as the source data. The hydrometric
2. Methods and source data According to the existing terminology (Chebotarev, 1978; State 94
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low runoff of rivers. Within this period, in 2014–2017 there was particularly low water level. Obviously, the decrease in the runoff of rivers in the late 20th century and early 21st century has a larger scale, as indicated by the decrease in the water level of some European lakes (Filatov et al., 2016) and other regions worldwide, including Lake Urmia in Iran (Zoljoodi and Didevarasl, 2014) and Dongting Lake in China (Yuan et al., 2015), etc.
station is located close to the head of the Angara River and near the Baikal railway station (Port Baikal). It has the longest series of instrumental observations of water level lasting for 120 years (since 1898). For a more distant retrospective, we used the water levels of Lake Baikal from Аfanasiev (1967a,b), which were reconstructed with the relation to a solar activity (the Wolf numbers) between 1729 and 1897. Due to the construction of the IHPS, the history of the water regime in Lake Baikal has two different periods when considering the formation conditions: natural and disturbed (regulated) one. To restore the natural fluctuations of the lake level that has been disturbed by regulation, we used the water balance scheme of the level reconstruction (Sinyukovich, 2005; Sinyukovich and Chernyshov, 2018) and the data from the Irkutsk Department of the Russian meteorological service on the main elements of the Baikal water balance: surface water inflow, runoff from the lake via the Angara River and precipitation and evaporation on the lake surface. Despite the fact that the runoff of rivers inflowing into Lake Baikal has been recorded since the 1930s, there is a rather reliable dataset on the inflow since 1901 that was calculated from the measurements collected since 1889 that are close to the runoff of the Angara River (since 1964, the runoff of the Angara River has been recording at IHPS). The formation of changes in the surface outflow from the Baikal watershed is estimated from the fluctuations in the runoff from three largest rivers feeding into the lake: the Selenga, the Upper Angara and the Barguzin, which provide 2/3 of the total water flow into the lake. The catchment area of the Upper Angara is 21400 km2, and the Barguzin River – 21100 km2. To characterize climate changes in the Baikal watershed, we used data on air temperature and precipitation from the certain stations of the Russian meteorological service, as well as data from the archive of the global reanalysis database of The National Centres for Environmental Prediction/The National Centre for Atmospheric Research (NCEP/NCAR).
3.2. Scheme of formation of the level and dynamics of the surface water inflow The level regime in Baikal is formed depending on the ratio of the main elements of its water balance. When simulating perennial fluctuations in the level of Lake Baikal, dynamic-stochastic models that describe the system dynamics with the lake's water balance equation appear the most appropriate (Frolov, 1985; Frolov and Vyruchalkina, 2017). Among the incoming components, the surface inflow predominates, which comprises 80–88% of the total water inflow (Sinyukovich, 2011). Precipitation on the lake surface comprises 10–12% of the inflow. The runoff from the lake through the Angara River (73–86% of the total flow) and evaporation (14–27%) from the water surface of the lake dominate the expenditures of the balance. According to the values of water-balance components, the change in the water level (ΔН) of the lake for a certain period is determined by the following equation (Sinyukovich and Chernyshov, 2018): ΔН = Х + Y1 – Y2 – E − S,
(1)
where Х is the atmospheric precipitation on the surface of the lake; Y1 is the inflow of river waters; Y2 is the surface outflow from the lake through the Angara River; Е is evaporation; and S is the discrepancy in water balance. Components of water balance such as underground inflow and condensation are not considered here due to their insignificant contribution and low accuracy of determination (Shiklomanov, 2008; Sinyukovich, 2016). Introduction of a discrepancy, which is mostly positive and in certain months reaches 2.59 km3 (8 cm of the level) reduces the calculation errors caused by the imbalance of water inflow to the lake that with a constant discrepancy of one sign, contributes to the accumulation of error. In equation (1), inflow and outflow are the main components, but the inflow is undoubtedly the principal one as the dominant element of the water supply, the primary link of the water cycle in the lake. Since the onset of active warming in 1971, the inflow was generally high until 1996, except for the low water level period of 1976–1981 (Fig. 3). After 1996, the inflow changed from slightly reduced or near average values to abnormally low values in 2014–2017. All of these changes are associated with various fluctuations in the runoff of the rivers flowing into Lake Baikal, yet most of the changes are primarily associated with the largest river (Selenga). In general, the long-term dynamics of the inflow follows the changes in the runoff of the Selenga River, the main tributary of the Baikal watershed, which is characterized by the decrease in runoff between 1976 and 1981 and since 1996. For the Upper Angara, the second most important tributary of Lake Baikal, the runoff increased slightly with air temperature rise, and it was especially high in 2007–2009, but in 2013–2017 there were minimum water discharges during all observations. The runoff of the Barguzin River was characterized by certain stability, except for the low runoff period between 1976 and 1982 and after 2010. The low runoff in all three rivers caused an abrupt reduction in inflow between 2014 and 2017. Within the year, an increase in the winter supply of the surface waters from the watershed basin reflects the transformation of the runoff during climate changes. Since 1970, the average winter inflow (November–March) has increased by 0.6 km3 or from 10.2 to 11.6% considering the annual volume of the river flow. In the runoff dynamics of the main tributaries of the lake, the
3. Results and discussion 3.1. Periods with different climatic conditions and anthropogenic influence Characterizing the water regime from water bodies formed under the influence of variable intensity factors first includes distinguishing periods with different climatic conditions and different levels of anthropogenic influence. For Lake Baikal, the main anthropogenic factor has been the regulating influence of IHPS, which has been extended to the lake level since the autumn of 1958 (Sinyukovich, 2005). Between periods with natural and regulated levels, a transitional period can be distinguished, during which there was a rise to the designed level. In addition, the 2001–2017 period can be distinguished after the rules of the use of water resources of the lake was changed. Global warming is the main factor for climate instability. In Siberia, warming events occurred in the first half of the 20th century and steadily increased in intensity since the early 1970s. At the same time, due to the insignificant increase in air temperature during this warming period (Climate Change 2001; Second Assessment Report, 2014), the start of its impact on the territory of Russia and in the Baikal region is often attributed to 1975–1976 (Loginov, 2008; Second Assessment Report, 2014). In fact, based on fluctuations in air temperature in the region, it is more likely that its beginning was in 1971. The stations with long-term observations in the watershed (Fig. 2) indicate the longterm change in air temperature occurs mainly for 1969–1970, which is associated with the cold 1967 and 1969. In this regard, we considered 1971 as the beginning of warming in the Baikal watershed. Therefore, in the assessment of changes in the hydrometeorological elements caused by warming, we primarily rely on the comparison of the indicators of their state within the period before and after this year. When we investigated the fluctuations of surface water inflow into Baikal, we paid particular attention to the period of 1996–2017 with 95
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Fig. 2. Deviations of the average annual temperature (ΔT) in Russia from the average ones for 1961–1990 (Second Assessment Report, 2014) and average annual temperatures at Baikal (the Babushkin settlement). 1 – current values; 2 – averages for all years; 3 – trend for 1976–2012.
average annual air temperature and a slight decrease in the summer. On the contrary, during the current low water level (since 1996), there was a more significant decrease in winter and spring temperatures, but the rise in the annual air temperature of the region also slowed down significantly. Despite different specifics of the temperature regime, a decrease in the intensity of the processes of zonal transfer of air masses was common for indicated periods of low water (Shimaraev and Starygina, 2010). During the first low water period, the weakening of the zonal circulation activity was accompanied by a decrease in the amount of precipitation over almost the whole Lake Baikal watershed. However, during the second low water period, the low humidity in some areas (the Selenga catchment area) was compensated by growth in other areas. As a result, the runoff of some Baikal rivers (primarily, the Upper Angara and the Barguzin) increased, and the decrease in the inflow into the lake was not as significant as in 1976–1981. According to (Biryukova, 2001; Shimaraev et al., 2002; Shimaraev and Starygina, 2010; Sinyukovich et al., 2013) and the results of observations in the Selenga catchment area, we may infer the differently directed precipitation trends in this watershed, but particularly their slight decrease after 1996. Reanalysis data confirm (Fig. 4) that the overall decline in the annual precipitation for 1996–2017 was insignificant, reaching 6 mm only in the southern part of the Selenga catchment area. Therefore, this decline in precipitation cannot explain a more significant decrease in the river runoff (23%) implying the involvement of additional causes. Of them, the main cause is that more than 80% of the Selenga catchment area belongs to very dry, dry and moderately wet areas (Biryukova, 2001), thus rendering it deficient in moisture. This deficiency will increase with a rise in air temperature due to high evaporative water loss, which will contribute to the additional decrease in river runoff. Therefore, the observed low runoff of the Selenga River is due to not only the small amount of precipitation but also increased evaporation (evaporating capacity) in its basin, which was also indicated by previous research (Sutyrina, 2018). However, there is a lack of studies focused on quantifying the increase in evaporation in different parts of the Baikal watershed, including the Mongolian territory, where 66% of the Selenga basin is located. Within the year, a decrease in the river inflow to Lake Baikal during the first winter months is accompanied by a significant increase in evaporation from the water surface of the lake. The increase in temperature contrast at the water-air interface, as well as the intensification of wind activity, increases the evaporation between October and December up to 3–4 km3 per month or an order of magnitude or greater in comparison with the summer period. This evaporation may exceed the overall inflow of rivers by a factor of two. During this period, evaporation becomes the main factor when determining the lake level, due to which the Baikal level decreases up to 10–15 cm per month.
increase in the winter water runoff is the most obvious for the Selenga River. However, after 1996 the average winter runoff became low due to the overall decrease in the runoff in the river during the warm season, when ground water, the main source of water for the Baikal rivers in winter, is replenished. Between 1971 and 1995, the winter water runoff of the Selenga River increased by an average of 15 m3/s (10%). However, since 1996, the runoff has become significantly lower due to a decrease in the water saturation of the groundwater active layer. In that case, the reserves resulted from the overall low humidity in the Selenga catchment area and the runoff during the warm season. The average winter runoff of the Upper Angara during the warming period also increased approximately by 8.5 m3/s (11%), but unlike the Selenga river, it held especially high values from 2005 to 2012. An increase in the winter runoff of the Barguzin was even more significant (20%) with warming before 2012. However, since 2013, winter water runoff has decreased almost by half, going from 50 to 60 to 30–35 m3/s. The observed increase in the winter runoff in the region after 1971 is directly associated with air temperature rise. This increase is bound to the growth in the accumulating and regulating ability of the catchment areas with increasing summer melt layer in the zone of permafrost distribution (Djamalov and Potekhina, 2010). The territory of the lake Baikal basin is fully included in the permafrost zone (Surface Water Resourses, 1973), which has a predominant insular distribution with the frozen rock thickness of 25–50 m. Thawing of ice in degraded permafrost rocks also contributes to an increase in groundwater reserves drained by rivers. In the absence of permafrost, the air temperature rise is accompanied by a high frequency of thaws and a decrease in soil freezing, leading to an increase in groundwater recharge and, in turn, an increase in river recharge in the winter (Shiklomanov, 2008; Bolgov et al., 2016). At the same time, climate warming creates increasingly favourable conditions of groundwater flow into river systems. However, water reserves in underground basins, which are formed during the previous warm season, remain the main factor for increasing winter runoff. Indeed, the runoff during the summer months explains the significantly low the winter runoff of the studied rivers since 2013–2014. Generally, based on the indicated changes in runoff of the main rivers of the Baikal watershed, an increase in the total winter inflow into the lake from 1971 to 2013 was ∼10%. However, by 2017, due to the subsequent low runoff of the most Baikal rivers, the inflow reduced and increase of it was only 8%. 3.3. Climate causes of fluctuations in surface water inflow During the initial period of warming, low surface water inflow into Lake Baikal from 1976 to 1981 was observed with a slight increase in 96
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time, the onset began five days later, resulting in the reduction of the flood duration by 22 days. On the contrary, since 1971, the Barguzin flood has begun two days later, on average and its peak is observed 14 days later than before climate warming. Moreover, the maximum water flow during the Barguzin flood also shifted to a later date by 9 days, whereas in the other two tributaries, this indicator changed insignificantly. In addition, with air temperature rise in the region, we observed a decrease in the river runoff during the flood (Sinyukovich and Chernyshov, 2017), except for the Upper Angara. The indicated changes in the dates and volume of the flood also changed the character of the river inflow to the lake. Since the onset of warming until 1996, the maximum useful inflow occurred between late June and early July, then shifting to mid-June. The inflow held significantly lower values in June–August, since the main Baikal rivers lacked high floods. 3.4. The influence of IHPS and the role of the regulatory capacity of the lake The water level of Lake Baikal under existing climate conditions and without the influence of IHPS would have varied depending on the ratio of the elements in its water balance, according to equation (1). The conditionally natural Baikal levels calculated on its basis for the years after regulation (Fig. 6) characterize the most realistic picture of a level regime in the lake, which could be observed under natural conditions, i.e. in the absence of backwater from the dam of IHPS. At the same time, within a year, the role of the surface water inflow as the main factor in the formation of the water level significantly weakens in the cold season due to the extremely irregular intra-annual distribution of the inflow, resulting from the seasonal decrease in the runoff of all Baikal rivers. During such periods, the limited carrying capacity of the head of the Angara River, as well as large dimensions of Baikal surface waters become highly important, since they determine the known inertia (overregulation) of fluctuations in the level and runoff from the lake. Therefore, with an average surface inflow into the lake of 1900 m3/s and minimum water discharges through IHPS of 1300 m3/s, a rise in the average water level of the lake by 1 cm will occur only after six days. With a higher runoff through IHPS, the rate of level rise will become even lower. In general, the regulatory capacity of Lake Baikal may be estimated by the natural overregulation coefficient of inflow and outflow calculated from the ratio of the area of annual hydrograph below the average value to its total area. In the period of the natural regime (1901–1956), this coefficient was 0.64 for the inflow and 0.89 for the runoff from the lake. Consequently, due to the smoothing effect of the lake, the overregulation coefficient of the Angara runoff increases by 0.25 or 39%. After the construction of IHPS, the overregulation coefficient of inflow in 1962–1995 did not change. However, for the low water level period of 1996–2016, its average value was 0.66, and in 2015–2016 it was 0.69. At the same time, the overregulation of runoff through IHPS reached 0.99. Due to the regulatory capacity of Lake Baikal, the water level stabilization after events of a high rise or decline could theoretically continue for up to two years (Sinyukovich, 1984), but there is the abovementioned high intra-annual irregularity of the water inflow into the lake. The level change stops with the setting of equilibrium between the inflow and runoff of water, and with the further decrease in the water flow, the water level also decreases. Moreover, the decline is more intense during the initial period, when there is the highest level of the surface water in the lake. Then, the decline gradually weakens. In any case, under the natural regime or after overregulation, the regulatory capacity of Lake Baikal remains an important factor in the formation of its level regime, smoothing the effect of the small-scale fluctuations in inflow and runoff components of water balance. The role of IHPS in changing water regime of Lake Baikal is primarily the raising of the lake water level. Under natural conditions
Fig. 3. Dynamics of long-term fluctuations of the inflow into the lake and runoff of the main rivers. 1 - current values, 2 - linear trend, 3 - polynomial trend.
In the long-term perspective, the role of evaporation also increases due to the higher warming of waters and reducing the freeze-up time. Both the beginning and intensity of the increase in the level of the lake after winter drawdown occur depending on the time of onset and the nature of the spring flood on the Baikal tributaries. Despite certain stability of these indicators, in the course of warming, there were significant shifts in the dates of onset and duration of the flood period on the main Baikal tributaries (Fig. 5). The end of the Selenga and Upper Angara rivers floods shifted to the earlier periods and its duration reduced, but the time of the maximum water flow maintained. Until 1971, the Selenga River flood normally ended on June 26, and in the subsequent period on June 16. The dates of the flood onset remained the same, but the duration reduced from 86 to 76 days. The average dates for the end of the Upper Angara floods during the warming years shifted from 6 August to 20 July. At the same 97
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Fig. 4. The difference in the amount of precipitation in different periods.
Fig. 5. Dates of the beginning (1) and end (2) of the flood period, the time of the maximum. Runoff of melt water (3). Dotted lines are linear trends.
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Fig. 6. Observed (1) and conditionally natural (2) average monthly water levels. PES – Pacific elevation system.
than for the reconstructed ones. These results also reflect the general principles of the runoff regulation, which depict a more intensive accumulation of water during the warm season and deeper drawdown during the cold season. Between 2001 and 2016, the overregulated levels were higher than natural conditions by an average of 0.74 m; their amplitude of fluctuations was higher by 0.05 m (Sinyukovich and Chernyshov, 2018). Between 2015 and 2017, both the accumulation of water in Baikal and the decrease in water runoff from the lake up to 1250–1300 m3/s were performed to the detriment of the navigation on the Angara River downstream of Irkutsk. According to The Basic Main Rules (1988), the minimum navigation outflows are 1500 m3/s in all previous years, even under conditions of moderate and decreased water level. These outflows were not observed only in 2003.
(1898–1958), the water level in Baikal averaged 455.61 m. After overregulation (1960–2017), the level already reached 456.40 m, i.e. it increased by an average of 0.79 m. Within the year, the levels of the winter months increased the most. The shift towards the delay of the onset of the highest and the lowest levels by 9–10 days is the most significant, which corresponds to the results recorded in (Abasov et al., 2017) indicating 8–9 days. Due to mostly seasonal regulation of runoff, the influence of IHPS on the interannual fluctuations of the Baikal water level is relatively small. At the same time, changes in the hydrological situation in the lake watershed under the influence of climate are accompanied by a change in the strategy for using water resources of the lake, especially when under the condition of abnormally high or low water levels. The repeated change in the amplitude of permissible fluctuations in the level of the water body since the beginning of this century clearly reflects it. Initially, the resolution of the Government of the Russian Federation of 2001 (regarding limit values) allowed the permissible amplitude of fluctuations in the level to be reduced from 1.96 to 1 m. Under the conditions of moderate and slightly decreased water level, this amplitude was observed until 2015. However, due to an anomalous decrease in the inflow, in 2014 a new resolution was adopted, and subsequently two more resolutions. In these two latter resolutions of 2016 and 2017, the permissible values of water level in the lake were significantly expanded, yet the regulation policy remained focused on minimizing the drawdown of Baikal water resources. For this reason, since the end of 2014, IHPS permanently releases outflows in the amount of minimal permissible flow rate of 1250–1300 m3/s, which determines the new conditions for the formation of the lake water level. On the contrary, until 2001, in the high water level periods, the forcing of levels (excess of normal water level) was practised. Previous research (Saveliev, 2000; Sinyukovich, 2005; Rules for the use of water resources, 2013) has conducted a comparison of the course of levels observed and reduced to natural conditions, showing that the greatest difference between them corresponds to years with increased water level during the winter months. This approach is partly the result of a regulatory strategy aimed at preventing idle discharges. From 1960 to 2000, the overregulated levels were on average 70 cm higher than natural conditions and 80 cm higher before 1958. In the absence of increased normal water level, the excess of observed levels over those expected from the beginning the 21st century to 2014 remained rather stable and averaged 77 cm, changing within certain years from 56 to 93 cm. Later, after the decrease of the actual observed levels below 456 m, it began to decrease, and in 2016 it was only 56 cm. This suggests that the observed decrease in level by 2017 under natural conditions would have been less, and, hence, the runoff from the lake would have also been smaller. In addition, a higher amplitude of fluctuations is generally typical for the overregulated levels
3.5. Long-term trends in level fluctuations The observed specifics of the formation of the level regime in Lake Baikal indicate certain short periods of change depending on the influencing factors. However, in order to discuss the longer trends, it is necessary to analyse the changes in the level for a rather large historical period with different climatic conditions and different degrees of anthropogenic influence. In this regard, we have analysed a series of average annual levels of Lake Baikal from 1729 to 2017, in which A.N. Afanasiev reconstructed the bulk of the data (1729–1897) with relation to solar activity. The remaining data were the results of the measured levels, including periods of natural (1898–1958) and overregulated (1959–2017) water regime of the lake. To eliminate the heterogeneity of the data due to the influence of IHPS, we used the abovementioned natural condition levels for 1959–2017, calculated with the water balance scheme of the reconstruction (Sinyukovich and Chernyshov, 2018). Analysis of the 289-year dynamics shows many years of natural Baikal water levels (Fig. 7), which allowed us to trace the appearance of long-term trends of different durations, including secular and intrasecular cycles. The beginning of the first (conditionally) secular cycle ranges from 1700 to 1710 (Afanasiev, 1967,1976), and its end corresponds to 1814. This cycle reached its maximum in 1751, and the minimum in 1814 appeared to be the most extreme for almost 300 years. The second secular cycle lasted until 1903, and its maximum in 1869 is absolute, which is confirmed by previous records and the levelling of the highest notch for this year made by B. Dybovsky and V. Godlevsky (1897) at Cape Shamansky. The third cycle has been longer and may continue now. Based on the natural observations of the level and inflow within this cycle, we can rather confidently distinguish three intrasecular cycles and a less obvious fourth one, beginning in 1981 and 99
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Fig. 7. Dynamics of average annual levels of Lake Baikal according to pre-instrumental data (1), observations in natural (2) and conditionally natural values (3). The straight line is the trend.
characterized by a prolonged decline. To date, this fact does not allow us to state its completion and the beginning of the next cycle. During the entire period observed, the higher levels correspond to the third secular cycle. For time periods with different cycles and a different nature from the initial data, the main parameters of the level regime appear to be close, yet they indicate insignificant gradual increase in the level with a rate of approximately 1 mm per year. The observed growth trend is rather stable and statistically reliable (5% significance level).
the main Baikal tributaries during the warming years has reduced by seven to 10 days. Along with a decrease in melt water flow rates on most rivers, the spring flood slows down the rate of the Baikal water level rise in spring. Under the low water level conditions, the limited runoff through IHPS contributes to an increase in the rate of water level rise in summer and slowdown of its decrease in the autumn-winter period. Despite the decline in the runoff of the Baikal rivers in the past 20 years, fluctuations in the water level of the lake over the past three centuries have been characterized by a tendency to increase with an average rate of 1 cm per decade.
4. Conclusion
Acknowledgements
Since the autumn of 1958, the water regime of Lake Baikal experiences the regulatory influence of IHPS, and since the early 1970s, it has been forming under conditions of increasing global warming. The regulation of the runoff from the lake raised its level by an average of 0.79 m and delayed the dates of the highest and the lowest levels by nine or 10 days. The influence of climatic conditions on lake level is mediated through the ratio of incomes and expenditures of its water balance and is mainly due to the fluctuations in the surface water inflow from the watershed of the basin. The rise in the air temperature in the region since 1971 is accompanied by the transformation of the conditions for the formation of the runoff of the Baikal rivers. It is also characterized by the formation of prolonged low runoff periods with a significant decrease in the water level of the lake. The current low water level at Baikal has been continuing since 1996. Originally, it was associated with low runoff from the main Baikal tributary, the Selenga River. The decrease in the Selenga runoff was not only due to the lack of high precipitation, but also an increase in the share of evaporated precipitation that resulted from the rise in air temperature. A particularly significant decrease in the surface water inflow into the lake was observed in 2014–2016 after the reduction of runoff of two other major rivers: the Upper Angara and the Barguzin. This decrease led to the decline in the Baikal water level below the statutory mark of 456 m. Within one year, there is an increase in river inflow during winter with warming conditions. However, during the summer and autumn months in low runoff years, the inflow decreases due to the insufficient replenishment of groundwater as the main source of water for the Baikal rivers in winter. Between November and December, the inflow of river water to the lake decreases, and evaporation from the water surface of Baikal is the primary influence on the changes in its level. During this period, there is a 10–15 cm monthly decrease in the water level of the lake due to evaporation. The duration of the spring flood on
The study was carried out within the framework of the state task of LIN SB RAS No. 345-2019-0008. References Abasov, N.V., Bolgov, M.V., Nikitin, V.M., Osipchuk, E.N., 2017. Level regime regulation in Lake Baikal. Water Res. 44 (3), 537–546. About Limit Values of Water Level in Lake Baikal at Implementation of Economic and Other Activity: the Order of the Government of the Russian Federation 26.03.2001. № 234. vol. 14. Collection of legislation of the Russian Federation, pp. 1366 (in Russian). Afanasiev, A.N., 1967. Fluctuacions of Hydrometeorological Regime in USSR Territory. Nauka, Moscow, pp. 230 (in Russian). Afanasiev, A.N., 1976. Water Resources and Water Balance of Lake Baikal Basin. Nauka, Novosibirsk, pp. 239 (in Russian). Biryukova, E.V., 2001. Landscape-Ecological Analysis of the Trasboundary Geosystems in the Baikal Region (The Selenga Basin): Author's Abstract, Cand. Sc. (Geogr.). Dissertation. Irkutsk, 18 pp. (in Russian). Bolgov, M.V., Korobkina, E.A., Filippova, I.F., 2016. Bayes forecast of minimal drainage under non-stationary conditions taking into account probable climate changes. Russ. Meteorol. Hydrol. 7, 75–84. Chebotarev, A.I., 1978. Hydrological Dictionary. Gidrometeoizdat, Leningrad, pp. 308 (in Russian). Climate Change 2001, 2001. The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the IPCC. Summary for Policymakers and Technical Summary. - WMO/UNEP. http://www.grida.no/publications/other/ipcc_tar/. Djamalov, R.G., Potekhina, E.V., 2010. The natural-climatic and anthropogenic causes for changes in Subsurface flow within the Lena basin. Georazrez 1, 1–25 (in Russian). Dybiovsky, B., Godlevsky, V., 1872. On change of Lake Baikal depth. News of Siberian Branch of Russian Geographic Society 11 (5) Irkutsk, 1872. Filatov, N.N., Viruchalkina, T.Yu, Dianskiy, N.A., Nazarova, L.E., Sinukovich, V.N., 2016. Intrasecular variability in the level of the largest lakes of Russia. Dokl. Earth Sci. 467 (2), 393–397. Frolov, A.V., 1985. Dynamic- Stochastic Models of Long-Term Level Fluctuations in Nonterminal Lakes. Nauka, Moscow, pp. 103 (in Russian). Frolov, A.V., Vyruchalkina, T. Yu, 2017. Dynamic-stochastical modeling of long-term fluctuations in Lake Baykal levels and Angara River runoff. Water Res. 44 (3), 380–389. Goldman, C.R., Kumagai, M., Robats, R.D. (Eds.), 2013. Climatic Change and Warming of Inland Waters: Impacts and Mitigation for Ecosystems and Societies. John Wiley &
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Sinyukovich, V.N., Chernyshov, M.S., 2017. Transformation of estimated characteristics of the annual and maximal runoff in the major tributaries of lake Baikal. Water Res. 44 (3), 372–379. Sinyukovich, V.N., Chernyshov, M.S., 2018. Current Problems of the Water Level Control in Lake Baikal. The Bullettin of Irkutsk State University. Series Earth Sciences, vol. 22, 99–110. https://doi.org/10.26516/2073-3402.2018.24.99. Sinyukovich, V.N., Sizova, L.N., Shimaraev, M.N., Kurbatova, N.N., 2013. Characteristics of current changes in water inflow into Lake. Geogr. Nat. Resour. 4, 350–355. State standard 19179-73, 1988,. Land Hydrology. Terms and Definitions. Moskow, pp. 36. Surface Water Resourses of the USSR. vol. 16 Gidrometeoizdat, Leningrad (in Russian) 3. Sutyrina, E.N., 2018. Changeability of climatic factors in formation of riverine drainage in Lake Baikal water catchment basin. Modern trends and perspectives of development of hydrometeorology in Russia. In: Proceedings of a Scientific-Practical Conference. Irkutsk, pp. 329–334. The Basic Rules for the Use of Water Resources in the Reservoir of the Angarsk Cascade of Hydroelectric Power Stations (Irkutsk, Bratsk and Ust-Ilimsk). Ministry of Land Reclamation and Water Resources of the RSFSR Publ., Moscow, pp. 65 (in Russian). Yuan, Y., Zeng, G., Liang, J., Huang, L., Hua, S., Li, F., He, Y., Wu, H., Liu, J., He, X., 2015. Variation of water level in Dongting Lake over a 50-year period: implications for impacts of anthropogenic and climatic factors. J. Hydrol. 525, 450–456. Zoljoodi, M., Didevarasl, A., 2014. Water-level fluctuations of Urmia lake: relationship with the long-term changes of meteorological variables (Solutions for water-crisis management in Urmia lake basin). Atmos. Clim. Sci. 4, 358–368.
Sons, Ltd. Publ., pp. 472. Loginov, V.F., 2008. Global and Regional Climate Changes: Causes and Consequences. TetraSystems, Minsk, pp. 496 (in Russian). Rules for the Use of Water Resources of the Irkutsk Reservoir and Lake Baikal. Project. Moscow University of Environmental Engineering Publ., Moscow, pp. 168 (in Russian). Saveliev, V.A., 2000. Modern Problems and the Future of Hydropower in Siberia. Novosibirsk Science Publ., pp. 200 (in Russian). Second assessment report of climate change and its impacts on the territory of the Russian Federation. M : Roshydromet 1009 p. Shiklomanov, I.А. (Ed.), 2008. Russian Water Resources and Their Use. State Hydrological Institute, Saint-Petersburg, pp. 600 (in Russian). Shimaraev, M.N., Starygina, L.N., 2010. Zonal atmospheric circulation, climate, and hydrological processes at Baikal Baikale (1968–2007). Geogr. Nat. Resour. 3, 245–250. Shimaraev, M.N., Kuimova, L.N., Sinyukovich, V.N., Zechanovcki, V.V., 2002. Climate and hydrological processes in Lake Baikal in the 20th century. Russ. Meteorol. Hydrol. 3, 52–58. Sinyukovich, V.N., 1984. Assesment of regulation ability of lake Baikal. Hydrology of Lake Baikal and of Other Water Bodies. Nauka, Novosibirsk 105–110 (in Russian). Sinyukovich, V.N., 2005. Recovery of the natural level regime of lake Baikal after construction of the Irkutsk hydroelectric power plant. Russ. Meteorol. Hydrol. 7, 47–51. Sinyukovich, V.N., 2011. Lake water balance under regulated regime. Water Manage. Russia 1, 12–22 (in Russian). Sinyukovich, V.N., 2016. The problem of regulating the level of lake Baikal under conditions of anomalous water availability. Water Manage. Russia 1, 42–51 (in Russian).
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Water regime of lake Baikal under conditions of climate change and anthropogenic influence
T
V.N. Sinyukovich, M.S. Chernyshov∗ Limnological Institute of the Siberian Branch of RAS, Ulan-Batorskaya St., 3, Irkutsk, 664033, Russia
A R T I C LE I N FO
A B S T R A C T
Keywords: River inflow Runoff Level Climate warming
Based on water level data, we have studied some specifics of the water regime in Lake Baikal associated with the warming climate, which has intensified since the early 1970s. We have also addressed the overregulation of the lake after the construction of the Irkutsk Hydroelectric Power Station (IHPS). We have estimated the influence of climate on the water regime of the lake through observing the change in the surface water inflow from the watershed basin and the characteristics of runoff from the three largest Baikal rivers: the Selenga, the Upper Angara and the Barguzin. A decrease in the runoff of these rivers during the intensification of warming reduced the surface water inflow into the lake, causing a decrease in level between 1976-1981 and 2014–2017. Notably, the spring flood on the rivers during climate warming becomes shorter and is characterized by a decrease in the maximum water discharges, contributing to the slowdown of the filling of Lake Baikal during spring and summer. Contrastingly, in the winter, river runoff and the lake inflow increase with the rise in air temperature. However, this increase depends on the runoff of rivers during the previous summer and autumn. This runoff determines the formation of groundwater reserves that serve as the main source of riverine water in the winter. We have suggested the possible decrease in runoff during the years of the air temperature rise due to an increase in evaporation. Warming also contributes to the increase in evaporation from the watershed of Lake Baikal, as well as the rate of decline in its water level, which reaches 10–15 cm per month in November and December. The backwater from the dam of the IHPS has been observed in Baikal since the autumn of 1958; it has caused an increase in the lake level by an average of 0.79 m. Within the year, there was an alignment of outflow from the lake through the Angara River, and in periods with low water level, it was kept at a minimum value of 1300 m3/s for a long lime in order to maintain the Baikal water level. Using indirect data, we have investigated the fluctuations in water level of Lake Baikal between 1729 and 2017, which indicates its potential increase at a rate of 1 cm per 10 years. The changes in the Baikal water regime determine the restructuring of various processes inside the water body and are important for the ecosystem of the lake.
1. Introduction The ratio of the elements in the water balance of Lake Baikal determines its water regime. The surface water inflow from its watershed of 540 000 km2 (Fig. 1) dominates the income of the lake balance, while the runoff through the Angara River accounts for its expenditures. Since the middle of the 20th century, the Baikal water regime has been under regulatory influence of the dam of the Irkutsk Hydroelectric Power Station (IHPS), constructed 60 km downstream from the head of the Angara River. The regulation influenced the significant intra-annual levelling of the runoff from the lake, and its level has increased by an average of 0.8 m. The increase in global warming, which has occurred
∗
since the 1970s, is particularly intensive in Siberian territory. In the area of Lake Baikal, over the past 30 years of the 20th century, the air temperature rise of 1.9°С was twice as high as the global average (Shimaraev et al., 2002; Shimaraev and Starygina, 2010). The warming of Lake Baikal and its watershed led to the restructuring of various hydrological processes (Shimaraev et al., 2002; Goldman et al., 2013; Sinyukovich et al., 2013). Since 1996, there is a long-term low water level in the watershed, which was particularly acute in 2014–2017 and caused a decrease in the level of the lake below the legislative standard values. During this period of low water, the problems of using the water resources of the lake for hydropower engineering, water supply and shipping were complicated. Moreover, there were some changes in the
Corresponding author. E-mail address: [email protected] (M.S. Chernyshov).
https://doi.org/10.1016/j.quaint.2019.05.023 Received 24 December 2018; Received in revised form 19 May 2019; Accepted 20 May 2019 Available online 21 May 2019 1040-6182/ © 2019 Elsevier Ltd and INQUA. All rights reserved.
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Fig. 1. Lake Baikal basin. 1 – Catchment area boundary, 2 - State boundary, 3 – Settlements.
Baikal ecosystem. Considering that approximately 23 thousand km3 of fresh water (20% of the global reserves) are concentrated in Lake Baikal, understanding its water regime and the change in the conditions of its formation under climate instability and hydropower regulation is extremely important. Notably, unlike the lake itself, the runoff of the Baikal rivers is not regulated by reservoirs and used only on a small scale for water consumption by the population, as well as in industry and agriculture. In this regard, the observed changes in the runoff of rivers are mostly due to climate.
Standard 19179-73, 1988), we regard the water regime of lakes as the regime of changes in their volumes and levels, without considering the characteristics of currents, waves and mixing. In the case of Lake Baikal, only the surface (variable) part of the lake volume is of practical importance, as it changes during both seasonal and long-term hydrological cycles. Considering the maximum increase and decrease of the lake volume in the 20th and 21st centuries, the surface waters are not more than 65 km3 or 0.3% of the total water reserves in the lake. Since the area of the Baikal water surface in calculations of water balance for the variable part of the volume is assumed constant (31500 km2), the levels and volumes in this zone are linearly related; thus, both of these indicators reflect the changes in the water regime of the lake. On this basis, we regarded information about water level, for which observations are the longest and reliable, as the source data. The hydrometric
2. Methods and source data According to the existing terminology (Chebotarev, 1978; State 94
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low runoff of rivers. Within this period, in 2014–2017 there was particularly low water level. Obviously, the decrease in the runoff of rivers in the late 20th century and early 21st century has a larger scale, as indicated by the decrease in the water level of some European lakes (Filatov et al., 2016) and other regions worldwide, including Lake Urmia in Iran (Zoljoodi and Didevarasl, 2014) and Dongting Lake in China (Yuan et al., 2015), etc.
station is located close to the head of the Angara River and near the Baikal railway station (Port Baikal). It has the longest series of instrumental observations of water level lasting for 120 years (since 1898). For a more distant retrospective, we used the water levels of Lake Baikal from Аfanasiev (1967a,b), which were reconstructed with the relation to a solar activity (the Wolf numbers) between 1729 and 1897. Due to the construction of the IHPS, the history of the water regime in Lake Baikal has two different periods when considering the formation conditions: natural and disturbed (regulated) one. To restore the natural fluctuations of the lake level that has been disturbed by regulation, we used the water balance scheme of the level reconstruction (Sinyukovich, 2005; Sinyukovich and Chernyshov, 2018) and the data from the Irkutsk Department of the Russian meteorological service on the main elements of the Baikal water balance: surface water inflow, runoff from the lake via the Angara River and precipitation and evaporation on the lake surface. Despite the fact that the runoff of rivers inflowing into Lake Baikal has been recorded since the 1930s, there is a rather reliable dataset on the inflow since 1901 that was calculated from the measurements collected since 1889 that are close to the runoff of the Angara River (since 1964, the runoff of the Angara River has been recording at IHPS). The formation of changes in the surface outflow from the Baikal watershed is estimated from the fluctuations in the runoff from three largest rivers feeding into the lake: the Selenga, the Upper Angara and the Barguzin, which provide 2/3 of the total water flow into the lake. The catchment area of the Upper Angara is 21400 km2, and the Barguzin River – 21100 km2. To characterize climate changes in the Baikal watershed, we used data on air temperature and precipitation from the certain stations of the Russian meteorological service, as well as data from the archive of the global reanalysis database of The National Centres for Environmental Prediction/The National Centre for Atmospheric Research (NCEP/NCAR).
3.2. Scheme of formation of the level and dynamics of the surface water inflow The level regime in Baikal is formed depending on the ratio of the main elements of its water balance. When simulating perennial fluctuations in the level of Lake Baikal, dynamic-stochastic models that describe the system dynamics with the lake's water balance equation appear the most appropriate (Frolov, 1985; Frolov and Vyruchalkina, 2017). Among the incoming components, the surface inflow predominates, which comprises 80–88% of the total water inflow (Sinyukovich, 2011). Precipitation on the lake surface comprises 10–12% of the inflow. The runoff from the lake through the Angara River (73–86% of the total flow) and evaporation (14–27%) from the water surface of the lake dominate the expenditures of the balance. According to the values of water-balance components, the change in the water level (ΔН) of the lake for a certain period is determined by the following equation (Sinyukovich and Chernyshov, 2018): ΔН = Х + Y1 – Y2 – E − S,
(1)
where Х is the atmospheric precipitation on the surface of the lake; Y1 is the inflow of river waters; Y2 is the surface outflow from the lake through the Angara River; Е is evaporation; and S is the discrepancy in water balance. Components of water balance such as underground inflow and condensation are not considered here due to their insignificant contribution and low accuracy of determination (Shiklomanov, 2008; Sinyukovich, 2016). Introduction of a discrepancy, which is mostly positive and in certain months reaches 2.59 km3 (8 cm of the level) reduces the calculation errors caused by the imbalance of water inflow to the lake that with a constant discrepancy of one sign, contributes to the accumulation of error. In equation (1), inflow and outflow are the main components, but the inflow is undoubtedly the principal one as the dominant element of the water supply, the primary link of the water cycle in the lake. Since the onset of active warming in 1971, the inflow was generally high until 1996, except for the low water level period of 1976–1981 (Fig. 3). After 1996, the inflow changed from slightly reduced or near average values to abnormally low values in 2014–2017. All of these changes are associated with various fluctuations in the runoff of the rivers flowing into Lake Baikal, yet most of the changes are primarily associated with the largest river (Selenga). In general, the long-term dynamics of the inflow follows the changes in the runoff of the Selenga River, the main tributary of the Baikal watershed, which is characterized by the decrease in runoff between 1976 and 1981 and since 1996. For the Upper Angara, the second most important tributary of Lake Baikal, the runoff increased slightly with air temperature rise, and it was especially high in 2007–2009, but in 2013–2017 there were minimum water discharges during all observations. The runoff of the Barguzin River was characterized by certain stability, except for the low runoff period between 1976 and 1982 and after 2010. The low runoff in all three rivers caused an abrupt reduction in inflow between 2014 and 2017. Within the year, an increase in the winter supply of the surface waters from the watershed basin reflects the transformation of the runoff during climate changes. Since 1970, the average winter inflow (November–March) has increased by 0.6 km3 or from 10.2 to 11.6% considering the annual volume of the river flow. In the runoff dynamics of the main tributaries of the lake, the
3. Results and discussion 3.1. Periods with different climatic conditions and anthropogenic influence Characterizing the water regime from water bodies formed under the influence of variable intensity factors first includes distinguishing periods with different climatic conditions and different levels of anthropogenic influence. For Lake Baikal, the main anthropogenic factor has been the regulating influence of IHPS, which has been extended to the lake level since the autumn of 1958 (Sinyukovich, 2005). Between periods with natural and regulated levels, a transitional period can be distinguished, during which there was a rise to the designed level. In addition, the 2001–2017 period can be distinguished after the rules of the use of water resources of the lake was changed. Global warming is the main factor for climate instability. In Siberia, warming events occurred in the first half of the 20th century and steadily increased in intensity since the early 1970s. At the same time, due to the insignificant increase in air temperature during this warming period (Climate Change 2001; Second Assessment Report, 2014), the start of its impact on the territory of Russia and in the Baikal region is often attributed to 1975–1976 (Loginov, 2008; Second Assessment Report, 2014). In fact, based on fluctuations in air temperature in the region, it is more likely that its beginning was in 1971. The stations with long-term observations in the watershed (Fig. 2) indicate the longterm change in air temperature occurs mainly for 1969–1970, which is associated with the cold 1967 and 1969. In this regard, we considered 1971 as the beginning of warming in the Baikal watershed. Therefore, in the assessment of changes in the hydrometeorological elements caused by warming, we primarily rely on the comparison of the indicators of their state within the period before and after this year. When we investigated the fluctuations of surface water inflow into Baikal, we paid particular attention to the period of 1996–2017 with 95
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Fig. 2. Deviations of the average annual temperature (ΔT) in Russia from the average ones for 1961–1990 (Second Assessment Report, 2014) and average annual temperatures at Baikal (the Babushkin settlement). 1 – current values; 2 – averages for all years; 3 – trend for 1976–2012.
average annual air temperature and a slight decrease in the summer. On the contrary, during the current low water level (since 1996), there was a more significant decrease in winter and spring temperatures, but the rise in the annual air temperature of the region also slowed down significantly. Despite different specifics of the temperature regime, a decrease in the intensity of the processes of zonal transfer of air masses was common for indicated periods of low water (Shimaraev and Starygina, 2010). During the first low water period, the weakening of the zonal circulation activity was accompanied by a decrease in the amount of precipitation over almost the whole Lake Baikal watershed. However, during the second low water period, the low humidity in some areas (the Selenga catchment area) was compensated by growth in other areas. As a result, the runoff of some Baikal rivers (primarily, the Upper Angara and the Barguzin) increased, and the decrease in the inflow into the lake was not as significant as in 1976–1981. According to (Biryukova, 2001; Shimaraev et al., 2002; Shimaraev and Starygina, 2010; Sinyukovich et al., 2013) and the results of observations in the Selenga catchment area, we may infer the differently directed precipitation trends in this watershed, but particularly their slight decrease after 1996. Reanalysis data confirm (Fig. 4) that the overall decline in the annual precipitation for 1996–2017 was insignificant, reaching 6 mm only in the southern part of the Selenga catchment area. Therefore, this decline in precipitation cannot explain a more significant decrease in the river runoff (23%) implying the involvement of additional causes. Of them, the main cause is that more than 80% of the Selenga catchment area belongs to very dry, dry and moderately wet areas (Biryukova, 2001), thus rendering it deficient in moisture. This deficiency will increase with a rise in air temperature due to high evaporative water loss, which will contribute to the additional decrease in river runoff. Therefore, the observed low runoff of the Selenga River is due to not only the small amount of precipitation but also increased evaporation (evaporating capacity) in its basin, which was also indicated by previous research (Sutyrina, 2018). However, there is a lack of studies focused on quantifying the increase in evaporation in different parts of the Baikal watershed, including the Mongolian territory, where 66% of the Selenga basin is located. Within the year, a decrease in the river inflow to Lake Baikal during the first winter months is accompanied by a significant increase in evaporation from the water surface of the lake. The increase in temperature contrast at the water-air interface, as well as the intensification of wind activity, increases the evaporation between October and December up to 3–4 km3 per month or an order of magnitude or greater in comparison with the summer period. This evaporation may exceed the overall inflow of rivers by a factor of two. During this period, evaporation becomes the main factor when determining the lake level, due to which the Baikal level decreases up to 10–15 cm per month.
increase in the winter water runoff is the most obvious for the Selenga River. However, after 1996 the average winter runoff became low due to the overall decrease in the runoff in the river during the warm season, when ground water, the main source of water for the Baikal rivers in winter, is replenished. Between 1971 and 1995, the winter water runoff of the Selenga River increased by an average of 15 m3/s (10%). However, since 1996, the runoff has become significantly lower due to a decrease in the water saturation of the groundwater active layer. In that case, the reserves resulted from the overall low humidity in the Selenga catchment area and the runoff during the warm season. The average winter runoff of the Upper Angara during the warming period also increased approximately by 8.5 m3/s (11%), but unlike the Selenga river, it held especially high values from 2005 to 2012. An increase in the winter runoff of the Barguzin was even more significant (20%) with warming before 2012. However, since 2013, winter water runoff has decreased almost by half, going from 50 to 60 to 30–35 m3/s. The observed increase in the winter runoff in the region after 1971 is directly associated with air temperature rise. This increase is bound to the growth in the accumulating and regulating ability of the catchment areas with increasing summer melt layer in the zone of permafrost distribution (Djamalov and Potekhina, 2010). The territory of the lake Baikal basin is fully included in the permafrost zone (Surface Water Resourses, 1973), which has a predominant insular distribution with the frozen rock thickness of 25–50 m. Thawing of ice in degraded permafrost rocks also contributes to an increase in groundwater reserves drained by rivers. In the absence of permafrost, the air temperature rise is accompanied by a high frequency of thaws and a decrease in soil freezing, leading to an increase in groundwater recharge and, in turn, an increase in river recharge in the winter (Shiklomanov, 2008; Bolgov et al., 2016). At the same time, climate warming creates increasingly favourable conditions of groundwater flow into river systems. However, water reserves in underground basins, which are formed during the previous warm season, remain the main factor for increasing winter runoff. Indeed, the runoff during the summer months explains the significantly low the winter runoff of the studied rivers since 2013–2014. Generally, based on the indicated changes in runoff of the main rivers of the Baikal watershed, an increase in the total winter inflow into the lake from 1971 to 2013 was ∼10%. However, by 2017, due to the subsequent low runoff of the most Baikal rivers, the inflow reduced and increase of it was only 8%. 3.3. Climate causes of fluctuations in surface water inflow During the initial period of warming, low surface water inflow into Lake Baikal from 1976 to 1981 was observed with a slight increase in 96
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time, the onset began five days later, resulting in the reduction of the flood duration by 22 days. On the contrary, since 1971, the Barguzin flood has begun two days later, on average and its peak is observed 14 days later than before climate warming. Moreover, the maximum water flow during the Barguzin flood also shifted to a later date by 9 days, whereas in the other two tributaries, this indicator changed insignificantly. In addition, with air temperature rise in the region, we observed a decrease in the river runoff during the flood (Sinyukovich and Chernyshov, 2017), except for the Upper Angara. The indicated changes in the dates and volume of the flood also changed the character of the river inflow to the lake. Since the onset of warming until 1996, the maximum useful inflow occurred between late June and early July, then shifting to mid-June. The inflow held significantly lower values in June–August, since the main Baikal rivers lacked high floods. 3.4. The influence of IHPS and the role of the regulatory capacity of the lake The water level of Lake Baikal under existing climate conditions and without the influence of IHPS would have varied depending on the ratio of the elements in its water balance, according to equation (1). The conditionally natural Baikal levels calculated on its basis for the years after regulation (Fig. 6) characterize the most realistic picture of a level regime in the lake, which could be observed under natural conditions, i.e. in the absence of backwater from the dam of IHPS. At the same time, within a year, the role of the surface water inflow as the main factor in the formation of the water level significantly weakens in the cold season due to the extremely irregular intra-annual distribution of the inflow, resulting from the seasonal decrease in the runoff of all Baikal rivers. During such periods, the limited carrying capacity of the head of the Angara River, as well as large dimensions of Baikal surface waters become highly important, since they determine the known inertia (overregulation) of fluctuations in the level and runoff from the lake. Therefore, with an average surface inflow into the lake of 1900 m3/s and minimum water discharges through IHPS of 1300 m3/s, a rise in the average water level of the lake by 1 cm will occur only after six days. With a higher runoff through IHPS, the rate of level rise will become even lower. In general, the regulatory capacity of Lake Baikal may be estimated by the natural overregulation coefficient of inflow and outflow calculated from the ratio of the area of annual hydrograph below the average value to its total area. In the period of the natural regime (1901–1956), this coefficient was 0.64 for the inflow and 0.89 for the runoff from the lake. Consequently, due to the smoothing effect of the lake, the overregulation coefficient of the Angara runoff increases by 0.25 or 39%. After the construction of IHPS, the overregulation coefficient of inflow in 1962–1995 did not change. However, for the low water level period of 1996–2016, its average value was 0.66, and in 2015–2016 it was 0.69. At the same time, the overregulation of runoff through IHPS reached 0.99. Due to the regulatory capacity of Lake Baikal, the water level stabilization after events of a high rise or decline could theoretically continue for up to two years (Sinyukovich, 1984), but there is the abovementioned high intra-annual irregularity of the water inflow into the lake. The level change stops with the setting of equilibrium between the inflow and runoff of water, and with the further decrease in the water flow, the water level also decreases. Moreover, the decline is more intense during the initial period, when there is the highest level of the surface water in the lake. Then, the decline gradually weakens. In any case, under the natural regime or after overregulation, the regulatory capacity of Lake Baikal remains an important factor in the formation of its level regime, smoothing the effect of the small-scale fluctuations in inflow and runoff components of water balance. The role of IHPS in changing water regime of Lake Baikal is primarily the raising of the lake water level. Under natural conditions
Fig. 3. Dynamics of long-term fluctuations of the inflow into the lake and runoff of the main rivers. 1 - current values, 2 - linear trend, 3 - polynomial trend.
In the long-term perspective, the role of evaporation also increases due to the higher warming of waters and reducing the freeze-up time. Both the beginning and intensity of the increase in the level of the lake after winter drawdown occur depending on the time of onset and the nature of the spring flood on the Baikal tributaries. Despite certain stability of these indicators, in the course of warming, there were significant shifts in the dates of onset and duration of the flood period on the main Baikal tributaries (Fig. 5). The end of the Selenga and Upper Angara rivers floods shifted to the earlier periods and its duration reduced, but the time of the maximum water flow maintained. Until 1971, the Selenga River flood normally ended on June 26, and in the subsequent period on June 16. The dates of the flood onset remained the same, but the duration reduced from 86 to 76 days. The average dates for the end of the Upper Angara floods during the warming years shifted from 6 August to 20 July. At the same 97
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Fig. 4. The difference in the amount of precipitation in different periods.
Fig. 5. Dates of the beginning (1) and end (2) of the flood period, the time of the maximum. Runoff of melt water (3). Dotted lines are linear trends.
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Fig. 6. Observed (1) and conditionally natural (2) average monthly water levels. PES – Pacific elevation system.
than for the reconstructed ones. These results also reflect the general principles of the runoff regulation, which depict a more intensive accumulation of water during the warm season and deeper drawdown during the cold season. Between 2001 and 2016, the overregulated levels were higher than natural conditions by an average of 0.74 m; their amplitude of fluctuations was higher by 0.05 m (Sinyukovich and Chernyshov, 2018). Between 2015 and 2017, both the accumulation of water in Baikal and the decrease in water runoff from the lake up to 1250–1300 m3/s were performed to the detriment of the navigation on the Angara River downstream of Irkutsk. According to The Basic Main Rules (1988), the minimum navigation outflows are 1500 m3/s in all previous years, even under conditions of moderate and decreased water level. These outflows were not observed only in 2003.
(1898–1958), the water level in Baikal averaged 455.61 m. After overregulation (1960–2017), the level already reached 456.40 m, i.e. it increased by an average of 0.79 m. Within the year, the levels of the winter months increased the most. The shift towards the delay of the onset of the highest and the lowest levels by 9–10 days is the most significant, which corresponds to the results recorded in (Abasov et al., 2017) indicating 8–9 days. Due to mostly seasonal regulation of runoff, the influence of IHPS on the interannual fluctuations of the Baikal water level is relatively small. At the same time, changes in the hydrological situation in the lake watershed under the influence of climate are accompanied by a change in the strategy for using water resources of the lake, especially when under the condition of abnormally high or low water levels. The repeated change in the amplitude of permissible fluctuations in the level of the water body since the beginning of this century clearly reflects it. Initially, the resolution of the Government of the Russian Federation of 2001 (regarding limit values) allowed the permissible amplitude of fluctuations in the level to be reduced from 1.96 to 1 m. Under the conditions of moderate and slightly decreased water level, this amplitude was observed until 2015. However, due to an anomalous decrease in the inflow, in 2014 a new resolution was adopted, and subsequently two more resolutions. In these two latter resolutions of 2016 and 2017, the permissible values of water level in the lake were significantly expanded, yet the regulation policy remained focused on minimizing the drawdown of Baikal water resources. For this reason, since the end of 2014, IHPS permanently releases outflows in the amount of minimal permissible flow rate of 1250–1300 m3/s, which determines the new conditions for the formation of the lake water level. On the contrary, until 2001, in the high water level periods, the forcing of levels (excess of normal water level) was practised. Previous research (Saveliev, 2000; Sinyukovich, 2005; Rules for the use of water resources, 2013) has conducted a comparison of the course of levels observed and reduced to natural conditions, showing that the greatest difference between them corresponds to years with increased water level during the winter months. This approach is partly the result of a regulatory strategy aimed at preventing idle discharges. From 1960 to 2000, the overregulated levels were on average 70 cm higher than natural conditions and 80 cm higher before 1958. In the absence of increased normal water level, the excess of observed levels over those expected from the beginning the 21st century to 2014 remained rather stable and averaged 77 cm, changing within certain years from 56 to 93 cm. Later, after the decrease of the actual observed levels below 456 m, it began to decrease, and in 2016 it was only 56 cm. This suggests that the observed decrease in level by 2017 under natural conditions would have been less, and, hence, the runoff from the lake would have also been smaller. In addition, a higher amplitude of fluctuations is generally typical for the overregulated levels
3.5. Long-term trends in level fluctuations The observed specifics of the formation of the level regime in Lake Baikal indicate certain short periods of change depending on the influencing factors. However, in order to discuss the longer trends, it is necessary to analyse the changes in the level for a rather large historical period with different climatic conditions and different degrees of anthropogenic influence. In this regard, we have analysed a series of average annual levels of Lake Baikal from 1729 to 2017, in which A.N. Afanasiev reconstructed the bulk of the data (1729–1897) with relation to solar activity. The remaining data were the results of the measured levels, including periods of natural (1898–1958) and overregulated (1959–2017) water regime of the lake. To eliminate the heterogeneity of the data due to the influence of IHPS, we used the abovementioned natural condition levels for 1959–2017, calculated with the water balance scheme of the reconstruction (Sinyukovich and Chernyshov, 2018). Analysis of the 289-year dynamics shows many years of natural Baikal water levels (Fig. 7), which allowed us to trace the appearance of long-term trends of different durations, including secular and intrasecular cycles. The beginning of the first (conditionally) secular cycle ranges from 1700 to 1710 (Afanasiev, 1967,1976), and its end corresponds to 1814. This cycle reached its maximum in 1751, and the minimum in 1814 appeared to be the most extreme for almost 300 years. The second secular cycle lasted until 1903, and its maximum in 1869 is absolute, which is confirmed by previous records and the levelling of the highest notch for this year made by B. Dybovsky and V. Godlevsky (1897) at Cape Shamansky. The third cycle has been longer and may continue now. Based on the natural observations of the level and inflow within this cycle, we can rather confidently distinguish three intrasecular cycles and a less obvious fourth one, beginning in 1981 and 99
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Fig. 7. Dynamics of average annual levels of Lake Baikal according to pre-instrumental data (1), observations in natural (2) and conditionally natural values (3). The straight line is the trend.
characterized by a prolonged decline. To date, this fact does not allow us to state its completion and the beginning of the next cycle. During the entire period observed, the higher levels correspond to the third secular cycle. For time periods with different cycles and a different nature from the initial data, the main parameters of the level regime appear to be close, yet they indicate insignificant gradual increase in the level with a rate of approximately 1 mm per year. The observed growth trend is rather stable and statistically reliable (5% significance level).
the main Baikal tributaries during the warming years has reduced by seven to 10 days. Along with a decrease in melt water flow rates on most rivers, the spring flood slows down the rate of the Baikal water level rise in spring. Under the low water level conditions, the limited runoff through IHPS contributes to an increase in the rate of water level rise in summer and slowdown of its decrease in the autumn-winter period. Despite the decline in the runoff of the Baikal rivers in the past 20 years, fluctuations in the water level of the lake over the past three centuries have been characterized by a tendency to increase with an average rate of 1 cm per decade.
4. Conclusion
Acknowledgements
Since the autumn of 1958, the water regime of Lake Baikal experiences the regulatory influence of IHPS, and since the early 1970s, it has been forming under conditions of increasing global warming. The regulation of the runoff from the lake raised its level by an average of 0.79 m and delayed the dates of the highest and the lowest levels by nine or 10 days. The influence of climatic conditions on lake level is mediated through the ratio of incomes and expenditures of its water balance and is mainly due to the fluctuations in the surface water inflow from the watershed of the basin. The rise in the air temperature in the region since 1971 is accompanied by the transformation of the conditions for the formation of the runoff of the Baikal rivers. It is also characterized by the formation of prolonged low runoff periods with a significant decrease in the water level of the lake. The current low water level at Baikal has been continuing since 1996. Originally, it was associated with low runoff from the main Baikal tributary, the Selenga River. The decrease in the Selenga runoff was not only due to the lack of high precipitation, but also an increase in the share of evaporated precipitation that resulted from the rise in air temperature. A particularly significant decrease in the surface water inflow into the lake was observed in 2014–2016 after the reduction of runoff of two other major rivers: the Upper Angara and the Barguzin. This decrease led to the decline in the Baikal water level below the statutory mark of 456 m. Within one year, there is an increase in river inflow during winter with warming conditions. However, during the summer and autumn months in low runoff years, the inflow decreases due to the insufficient replenishment of groundwater as the main source of water for the Baikal rivers in winter. Between November and December, the inflow of river water to the lake decreases, and evaporation from the water surface of Baikal is the primary influence on the changes in its level. During this period, there is a 10–15 cm monthly decrease in the water level of the lake due to evaporation. The duration of the spring flood on
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