International Journal of River Basin Management

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Evaluating climate and water regime transformation in the European part of Russia using observation and reanalysis data for the 1945–2015 period

Maria Kireeva, Natalya Frolova, Ekaterina Rets, Timofey Samsonov, Andrey Entin, Maksim Kharlamov, Ekaterina Telegina & Elena Povalishnikova

To cite this article: Maria Kireeva, Natalya Frolova, Ekaterina Rets, Timofey Samsonov, Andrey Entin, Maksim Kharlamov, Ekaterina Telegina & Elena Povalishnikova (2019): Evaluating climate and water regime transformation in the European part of Russia using observation and reanalysis data for the 1945–2015 period, International Journal of River Basin Management, DOI: 10.1080/15715124.2019.1695258 To link to this article: https://doi.org/10.1080/15715124.2019.1695258

Accepted author version posted online: 25 Nov 2019. Published online: 01 Dec 2019.

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Full Terms & Conditions of access and use can be found at https://www.tandfonline.com/action/journalInformation?journalCode=trbm20 INTL. J. RIVER BASIN MANAGEMENT https://doi.org/10.1080/15715124.2019.1695258

RESEARCH PAPER Evaluating climate and water regime transformation in the European part of Russia using observation and reanalysis data for the 1945–2015 period Maria Kireeva a, Natalya Frolovaa, Ekaterina Retsb, Timofey Samsonov c, Andrey Entinc, Maksim Kharlamova,b, Ekaterina Teleginaa and Elena Povalishnikova a aFaculty of Geography, Hydrology Department, Lomonosov Moscow State University, Moscow, Russia; bWater Problems Institute Russian Academy of Sciences, Moscow, Russia; cFaculty of Geography, Cartography and Geoinformatics Department, Lomonosov Moscow State University, Moscow, Russia

ABSTRACT ARTICLE HISTORY The river runoff regime significantly changed worldwide in the end of twentieth – the beginning of Received 27 November 2018 twenty-first century. In the European part of Russia the current changes are manifested in different Accepted 3 November 2019 ways in various regions. The main trend that is observed for most river basins is the ‘levelling’ of fl KEYWORDS annual hydrographs. The interception of melt water during occasional thaw oods has resulted in a fl ff Occasional ood; decrease in river runo during the spring months. Simultaneously an increase in the low water hydrological regime; runoff period during winter and summer occurred. Reducing in the depth of soil freezing, caused by warm winters, lead to higher groundwater replenishment. At the same time, the loss of melt water to surface retention as well as evaporation has increased. Almost in all seasons of the year the number of occasional flood peaks increases. Occasional floods are superimposed both on the rise and the decline of seasonal snowmelt flood wave, making it difficult to separate it as an independent phase of the water regime. Changes in seasonal characteristics of river runoff are the most evident in the Don and Oka basins. A significant transformation of the water regime is observed in the north-west of European Russia as well. To a lesser extent, the described patterns are typical for the northern rivers and the eastern part of the European Russia – the Kama and the Ural Basin.

1. Introduction runoff over the past 70 years. Particular attention is paid to The current changes in the water regime of rivers are now an obtaining these characteristics from the daily runoff hydro- indisputable fact, which is reflected in a number of papers of graph. One of the issues of forecasting and assessing the various scientific groups. The main trend that is observed for characteristics of occasional flood runoff is the difficulty in most rivers of the European part of Russia, is the ‘levelling’ of allocating these phases of the water regime. Typically a super- annual runoff hydrographs. This is noted in the study of river position of occasional floods on main seasonal flood wave and runoff in the Volga basin (Issledovaniya stoka i protsessov low water period is observed. The existing guidelines of formirovaniya pavodkov [Studies of runoff and flood for- Roshydromet (Russian State Hydrometeorological Service) mation processes] 1947, Yasinsky and Kashutina 2007,Dmi- do not contain quantitative criteria allowing to identify the trieva 2011, Dmitrieva 2014, Dzhamalov et al. 2015, Frolova beginning or end of seasonal and occasional floods, as well et al. 2015, Kireeva et al. 2017, Barabanov et al. 2018). Pre- as to separate individual floods and flood periods. sumably, the more often thaw floods intrusion of moist and The classical for the Russian hydrological school Zaikov warm air result in meltwater losses to interception and a con- classification is used to characterize the river water regime sequent decrease in river runoff during the spring months. and its phases in the study. The Zaikov classification (Zaikov Analysis of meteorological data over the past decades indi- 1946) is based on the intra-annual distribution of the river cates that this period was not only the warmest, but also the runoff. He distinguished ten different types of water regime wettest during instrumental observations (Dzhamalov et al. and gave a very complete description of the hydrological 2013) on the European part of Russia. An important conse- regime of the former USSR Rivers. All rivers, except artifi- quence of this is increase in the number, duration and cially or naturally highly regulated, he divided into the follow- ‘depth’ of thaws and the growth of melt water losses during ing three main groups: (1) rivers with spring high-water winter that led to higher groundwater replenishment. As a period; (2) rivers with high-water period in the warm part result, the role of flood events increases, especially in winter. of the year; (3) rivers with flood regime. A single seasonal snowmelt flood wave, which used to be The term ‘flood’ is used in the study according to the Inter- observed annually in spring, has now transformed into a national Glossary of Hydrology of the World Meteorological long multi-peak flood period during the winter. Winter Organization definition (WMO 1974): flood is a rise, usually thaw floods contribute up to 20–30% (as it was in 2007– brief, in the water level of a stream or water body to a peak 2008) to the total annual volume runoff of some rivers of from which the water level recedes at a slower rate. The reader European Russia (Figure 1). shouldn’t confuse it with inundation – overflowing by water One of the main tasks of this study is to estimate and ana- of the normal confines of a watercourse or other body of lyse the change in the characteristics of the occasional flood water (WMO 1974).

CONTACT Maria Kireeva [email protected] © 2019 International Association for Hydro-Environment Engineering and Research 2 M. KIREEVA ET AL.

Figure 1. Multi-peak (2007–2008) and single-peak (2006) hydrographs of the Don River in the Kazanskaya gauging station, separated by components using the GrWat algorithm (Kireeva et al. 2018), where: 1 – rainfall runoff;2– snowmelt runoff of spring flood; 3 – runoff of thawing floods; 4 – base runoff component.

The rivers of the first two groups of the Zaikov classifi- precipitation is higher than the water infiltration rate or the cation are characterized by a high water discharge annually soil is already saturated with moisture or covered with ice repeated in the spring or the warm part of the year. During crust and infiltration does not occur (Kuchment and Koren the rest of the year, low flow or occasional floods are 1969, Koren 1988). Mostly occasional floods are observed observed. The rivers of the third group are distinguished by when these conditions are combined with additional water sharp and usually short-term systematic floods, occuring at input from the upper subsurface layer. In any case, an any time of the year or most frequently repeated in certain increase in the number and duration of floods should lead seasons; in the interflood periods the low flow is established to an increase in the infiltration recharge of groundwater. on these rivers. The rivers with spring high water period Presumably, these processes led to decrease in peak flow sim- are most common in the former USSR. During the spring ultaneously with an increase in low flow – during the sum- high water period from 50% to 100% of the total annual mer-autumn and winter periods (Dzhamalov et al. 2013, flow passes depending on size and geographical position of Dzhamalov et al. 2015, Bolgov et al. 2016). the river basin. Depending on the high water pattern and the runoff distribution in the rest of the year, the rivers of 2. Study area this group are divided into the following five types: (1) Kazakhstan; (2) Eastern European; (3) West Siberian; (4) Hydrological characteristics were estimated based on repre- East Siberian; (5) Altai. sentative hydrological stations. Daily-averaged water dis- Occasional floods on the rivers of the European part of charges time series were used for the gauging stations of Russia have different origins and formation conditions. This principal rivers: main rivers or its first tributaries, covering is mainly determined by regional peculiarities of climatic, homogenous hydrological region and showing main ten- soil-geological, geomorphological and other natural factors, dencies in seasonal flow changes for the sub regions (Figure as well as by the individual characteristics of each water 2). In the course of the study, 20 watersheds were chosen catchment area. These aspects lead to the individuality of for the analysis in all geographical and climatic zones of the the processes of formation and development of occasional European Part of Russia (Figure 2, Table 1). Together they flood runoff and peak runoff in different climatic zones, cover almost 40% of its total area and can represent the areas and regions (Problemy pavodkov [Flood problems] water regime transformation on the whole territory of the 1959). The formation of floods directly depends on the European Part of Russia with a high level of confidence. amount of moisture (snowmelt or rain water) coming to The Northern Dvina is the largest river on the Russian the surface of the river basin during certain time period, European North and has a drainage area of 348,000 km2 and the previous state of the basin (mainly the water content), and annual discharge of 3343 m3/s in lowest gauging station which determines the runoff losses. Conceptual issues of flood Ust-Pinega (Table 2). The second one is the Pechora, located formation models and forecasting are discussed in detail in a on the north-east sector of the European Russia in the basin rather large number of papers (Kocherin 1932, Problemy of the Barents Sea, its mean annual discharge is 3611 m3/s pavodkov [Flood problems] 1959, Befani 1967, 1977a, (calculated for Ust-Tsilma station). The 1.5 times higher 1977b, Mikhailov et al. 2007). The formation of thaw floods specific discharge rate in the Pechora river basin compared depends on a number of meteorological and hydrological fac- to the Northern Dvina is connected with an increase in the tors. The first include the characteristics of the intensity of precipitation in foothills of Polar Ural Mountains. The precipitation, in the case of the winter period – their phases, Sukhona River is the main tributary of the Northern Dvina the duration of precipitation and the presence of ice crust on and is located in the western part of Russian European the surface. From the hydrological point of view, the most North, close to the border with Baltic region. It can be important prerequisite for both winter and summer floods classified as a middle zonal river, as well as the Mezen with is moisture content of watershed and discharge during pre- a drainage area of 56,000 km2 and annual discharge of 627 flood period (Oldekop 1947, Volftsun 1953, Befani 1977a). m3/s. The anthropogenic impact on these basins is very low The process of occasional flood formation in the river and their hydrological regime can be considered as absolutely implies several possible scenarios: the intensity of natural. Meanwhile the Volga basin is mostly regulated, hence INTERNATIONAL JOURNAL OF RIVER BASIN MANAGEMENT 3

Figure 2. Representative watersheds used as reference basins for estimating the flood runoff dynamics in various natural zones. The number of the watershed on the map corresponds to the number in Table 1. 4 M. KIREEVA ET AL.

Table 1. Characteristics of gauging stations used for analyses. Water catchment River Gauging station area, km2 Observation period Natural zone Russian North basins 1 Northern Dvina Ust-Pinega 348,000 1930–2014 Taiga 2 Sukhona Kalinkino 49,000 1930–2014 Taiga 3 Mezen Malonisogorsk 56,000 1930–2014 Taiga, forest tundra 4 Pechora Ust-Tsilma 248,000 1932–2014 Taiga, forest tundra Kama River basin 5 Vyatka Vyatskiye Polyany 124,000 1918–2015 Mixed forests 6 Kama Gainy 27,400 1911–2015 Taiga Volga River basin 8 Mologa Ustyuzhina 19,100 1934–2015 Mixed forests 9 Volga Staritsa 21,100 1920–2010 Mixed forests 10 Unzha Makaryev 18,500 1900–2015 Mixed forests 11 Vetluga Vetluga 22,200 1938–2015 Mixed forests 12 Samara Elshanka 22,800 1878–2015 Forest-steppe Oka River basin 13 Oka Murom 190,000 1936–2015 Forest-steppe 14 Oka Kaluga 54,900 1876–2015 Forest-steppe 15 Moksha Temnikov 15,800 193–2015 Forest-steppe Don River basin 16 Don Kazanskaya 102,000 1928–2015 Forest-steppe 17 Khoper Besplemyanovsky 44,900 1929–2015 Forest-steppe 18 Medveditsa Archedinskaya 33,700 1928–2015 Forest-steppe, steppe North Caucasus 19 Kuban Armavir 16,900 1960–2015 Altitude zonality 20 Terek Kotlyarevskaya 8920 1960–2015 Altitude zonality only the tributaries of the Volga upstream are used for the daily discharges (grad calibrated parameter): study as representative of hydrological conditions in this = | – |/ ∗ region. The same situation observed for the Kama River. grad ( Qn+1 Qn Qn) 100%, The Oka River basin has a quite natural hydrological regime th – where Qnand Qn+1 are daily river discharges on the n there are no large reservoirs here. Watersheds within the and (n+1)st day correspondingly. Don-River basin were chosen as representative for the (2) The Seasonal snowmelt flood is a single flood, largest southern part of the European Russia. The Upper Don and ff – within the year, formed by an overland runo of the its tributaries the Khoper and Medveditsa have a lot of snowmelt water small ponds and reservoir in the watershed, but as most of (3) Floods occurring during the summer low flow period are them were constricted in the first part of twentieth century, fl fl fl assumed to be rain oods, during winter low- ow period there in uence is stable during last 70 years. All the rivers – thaw floods. The winter low-flow period starts after the of the European Russia have the East-European type of fi fi fi fi rst signi cant frost event, that is de ned as a period of hydrological regime according to B.D. Zaikov classi cation specified number of days (nWin calibrated parameter) (see above). It is characterized by a spring high water during which the mean daily air temperature doesn’t period caused by snow melting which gives more than 50% rise above a certain critical value (Twin calibrated of annual runoff, low-flow summer period from June till fl parameter) October usually followed by a few occasional ood events (4) Rain floods are separated from the seasonal snowmelt in the autumn and low-flow winter period with the lowest fl ff ood by a critical sum of precipitation on the event runo . (Pcr calibrated parameter)

The GrWat tool allowed to obtain the characteristics of the 3. Materials and methods genetic runoff components for each of the selected water- An GrWat tool for the hydrograph genetic separation into sheds: 5-, 10-, 30- days minimum discharge and its timing, groundwater, melt water during seasonal snowmelt flood, the runoff volumes of each phase of the water regime, the melt water during thaw floods, rain water during rain dates of their start and end, the characteristics of the water floods developed by the authors was used in the study (Kude- discharge variability, the number of thaw and rain occasional lin 1966, Kireeva et al. 2017). The developed algorithm is flood events, characteristics of peak runoff, etc. based on graphical approaches by Kudelin (1966) to the sep- To analyse the changes in the meteorological conditions of aration of the hydrograph of the middle-size rivers. the occasional floods formation in the European part of Rus- The GrWat tool uses daily river discharges, and series of sia, the daily data of the third-generation EARA-Interim rea- mean daily temperature and daily precipitation sums as an nalysis, created by the European Center for Medium-Range input data. The mean daily temperature and daily precipi- Weather Forecasts (ECMWF) were used (Dee et al. 2011). tation sums can be averaged for the watershed on the basis Due to the lack of meteorological information for earlier of the reanalysis data. The hydrograph separation method periods, analysis of the most obvious climate trends was car- is based on the following concept: ried out by comparing the averaged characteristics over the past three decades (1978–2016) with respect to the average (1) The subsurface runoff component is drained more slowly for the recent (1958–1977) period. Data for the latter was than the overland flow. Accordingly, the groundwater is obtained out from the ERA-55 reanalyses. In addition, in separated from the surface runoff by a critical gradient of order to reflect the most striking trends in climate change INTERNATIONAL JOURNAL OF RIVER BASIN MANAGEMENT 5

Table 2. Seasonal runoff parameters for gauging stations, averaged for the period 1978–2016: maximum discharge (Qmax) and its date, annual discharge (Qannual) and minimum monthly discharge (Qmin.month) for winter and summer. 3 3 3 3 River Gauging station Qmax,m/s Date of maximum Qannual,m/s Qmin.month (summer), m /s Qmin.month (winter), m /s Russian North basins 1 Northern Dvina Ust-Pinega 21,281 9-May 3343 1796 871 2 Sukhona Kalinkino 3485 27-Apr 439 252 99 3 Mezen Malonisogorsk 5667 14-May 627 331 146 4 Pechora Ust-Tsilma 23,755 1-May 3611 2385 617 Kama River basin 5 Vyatka Vyatskiye Polyany 4848 11-May 1024 488 347 6 Kama Gainy 1713 17-May 247 113 52 Volga River basin 8 Mologa Ustyuzhina 938 19-Apr 151 59 46 9 Volga Staritsa 1254 18-Apr 181 89 67 10 Unzha Makaryev 1796 10-May 205 97 61 11 Vetluga Vetluga 1348 4-May 183 62 35 12 Samara Elshanka 661 21-Mar 51 22 18 Oka River basin 13 Oka Murom 4484 10-May 971 533 512 14 Oka Kaluga 2464 1-Apr 299 158 157 15 Moksha Temnikov 802 14-Apr 71 27 28 Don River basin 16 Don Kazanskaya 1472 20-Apr 315 163 190 17 Khoper Besplemyanovsky 615 16-Apr 117 44 55 18 Medveditsa Archedinskaya 467 18-Apr 60 21 22 North Caucasus 19 Kuban Armavir 694 19-Jun 117 62 35 20 Terek Kotlyarevskaya 443 23-Jun 136 107 73 in the last decade, a comparison was made between the periods i.e. loss of moisture due to infiltration (Problemy pavodkov 1980–2006 and 2007–2016. As a result, representative meteor- [Flood problems] 1959). If the soil freezing depth is not ological characteristics influencing the runoff formation in the enough and its water content is low, then water loss starts study area have been calculated, including: the number of days to intercept a significant part of the melt water. Resulting in with negative air temperature, the sum of negative tempera- an extremely low main seasonal flood wave. Ice crust on tures, the duration of the winter period, the average winter the surface acts as an aquiclude. air temperature, the number of thaws, the amount of solid In the central region of Russia, in the Don River basin, and liquid precipitation over the winter period, the amount noticeable trends in the minimum soil temperature at the of precipitation for the summer period. 40 cm depth have been observed over the last 30–40 years. For some meteorological stations in the Don River Basin If in the late 1970s and mid-1980s it was −4 to 6 degrees (m/s Efremov, m/s Yelets, m/s Kazanskaya, m/s Millerovo) (Figure 3(a,b)), reaching in some years −8 to 10 degrees, the soil temperature at various depths was analysed to indicate then in the 2000s (2000–2013) its average value reached 0 change in soil freezing depth. This data is available from the … −1 degrees. In about half of the cases the minimum soil AISORI database on the RIHMI-WDC website (AISORI data- temperature during the winter was positive. Thus, freezing base in the RIHMI-WDC website). Due to the inaccessibility of was not observed already at a depth of 40 cm. the soil freezing depth data, it is possible to use the minimum The minimum soil temperature is also affected by the soil temperature at a given depth (20, 40, 60, 80, 100, 120, 160, snow cover – the higher it is, the more noticeable is the 180 sm) as an indirect indicator. The surface layer is of the heat-insulating effect that prevents deep freezing. Therefore, most interest because its conditions play a decisive role in in the southern regions, for example, on the Don basin, the the flood runoff formation. In this study, soil temperature at tendency for soil temperature rise is less pronounced (Figure a depth of 40 cm was used for the analysis. 3(c,d)). Both in the 1970s–1980s, and in the 2000s, there were individual cases (for example, in 2009), when the minimum soil temperature at a depth of 40 cm dropped to −6 … −8 4. Results and discussion degrees. Probably, it is due to the sudden cold weather con- ditions during periods of no snow cover. In the north of 4.1. Changes in meteorological conditions of the the European part of Russia, similar trends are observed seasonal snowmelt flood and occasional thaw and within the northwestern regions, and an increase in the mini- rain floods formation mum soil temperature in the eastern regions, the Kama River In most models of flood runoff formation (Problemy pavod- basin, the Middle and Lower Volga, and the Ural River basin, kov [Flood problems] 1959, Befani 1967, Befani 1977a), in is less pronounced. addition to the main factors (amount and intensity of precipi- Summarizing, it can be stated that an increase in the mini- tation), considerable attention is paid to water losses, that mum soil temperature in some regions (for instance in the directly depend on the previous conditions in the water Don river basin) and, as a consequence, a reduction in the catchment area. There could be significant changes in recent soil freezing depth due to warmer winters, leads to an increase years right here. The formation of floods in winter is deter- in water losses to interception. At the same time, the losses of mined by the state of the soil surface – its humidity, freezing runoff due to surface retention and evaporation increase depth and the presence of ice crust on the surface (Problemy during the winter and especially in the spring period. pavodkov [Flood problems] 1959, Befani 1977b). Three of The change in a ratio of solid to liquid precipitation has a these major factors determine the possibility of water seepage, great role in the transformation of river water regime during 6 M. KIREEVA ET AL.

Figure 3. Change in the minimum soil temperature at a depth of 40 cm during the winter at the meteorological stations of the Don River basin: (a) Efremov, (b) Yelets, (c) Kazanskaya, (d) Millerovo. the winter period and, as a consequence, in the spring period. calculations made in the north-west, in the centre and In general, for the European part of Russia, according to the south of the European part of Russia, the amount of solid pre- report on climate change (Semenov 2012), annual precipi- cipitation falling during the winter period has decreased tation amounts increase. Nevertheless, according to the noticeably. The most significant decrease is observed in the INTERNATIONAL JOURNAL OF RIVER BASIN MANAGEMENT 7 upper reaches of the Dnieper River and its tributaries. Here dynamics of dates of the maximum discharges of both rain the value is about 20–40 mm (Figure 4(a)). For a significant and thawing floods. The ‘spread’ of dates for the maximum part of the Don basin, the Oka and the Upper Volga, the water discharges during floods tends to increase in the steppe decrease of the solid precipitation amount is 10–20 mm and forest-steppe regions – the Don River basin, tributaries of (Figure 4(a)). There are reverse trends in the trans-Volga the lower reaches of the Oka River, and the rivers of the region. For example, in the Upper Northern Dvina, the basins Middle and Lower Volga River basin (Table 3). Thus, for of the left-bank tributaries of the Upper and Middle Volga example, in the Samara River until the mid-1970s, maximum (the Unzha River, the Vetluga River), the Vyatka River rainwater floods were observed mainly in May-August Basin, an increase of the solid precipitation amount over (Figure 6(a)) and were associated with the intensive summer the winter by 10–20 mm is observed over the period 1978– showers. In the following decades, another ‘cluster’ of dates 2016 compared with the average value for 1958–1977 (Figure has been formed: winter rain floods observed in October– 4(a)). Even more noticeable is the increase of the solid pre- December. Nevertheless, it was not possible to detect a stat- cipitation amount in the Ural River basin, an average increase istically significant increase in the maximum rain flood dis- is 20–40 mm, and sometimes 40–80 mm. A very different charge (Figure 6(b)). Similar tendencies are observed for situation is typical for the liquid precipitation amount over the dates of the maximum discharges of thawing floods. If the winter period (Figure 4(b)). It increases from 10 to prior to the beginning of the 1970s the maximum discharges 40 mm over the right bank of the Volga River, in the Don of thawing floods were observed mainly in December–Janu- River basin, in the rivers of the North-West. Most clearly ary (Figure 6(c)), in the last decades the most intensive the trends are manifested in the basin of the Ladoga Lake, thaws began to be observed in March–April, just before the the rivers flowing into the Gulf of Finland, and the rivers of spring high water period. The maximum discharges of thaw- the Azov Sea basin between the Don and Kuban River basins, ing floods in the steppe and forest-steppe zones have and small tributaries of the Tsimlyansk reservoir. In recent increased significantly in the last three decades (Figure 6 decades, these trends have become even more obvious. The (d)). Similar trends exist in the forest zone, but they are liquid precipitation increases from 20% to 40% and the much less distinct. decrease in the number of days with negative temperatures Two characteristics should be considered while evaluating is from 15% to 25%. the thaw floods contribution to the total runoff increase: the The most intensive transition from snow to rain observed volume of the thaw flood runoff with and without the during February. The most intensive increase of the liquid groundwater component. The first indicator reflects the precipitation occurred in the western part of European Russia volume of water that enters the channel network relatively and amounts to 10−20 mm (Figure 5(a)). Redistribution of quickly as a result of snow melting during thaws. The second the precipitation between solid and liquid component is the shows the total amount of water in the river channel during consequences of the rise in positive temperature sum during the event. Figure 7 shows that both parameters are character- the winter. Basically, the increase of this rate is observed on ized by certain cyclicity with a pronounced increase in the late the whole territory of European Russia except North Cauca- 1970s. For a number of gauging stations located in the steppe sus (Figure 5(b)). To the West of the line connecting the cities zone (the Don and the Samara River basins) the number of Petrozavodsk-Vologda-Kostroma-Cheboksary-Penza-Sara- thaws tends to increase during the period after 1978 relative tov the rise of this characteristic is evaluated from 10 to 80 to the previous one. However, this trend is mostly not statisti- degrees. The most intensive increase is associated with the cal significant yet. southern part of the European Russia – to the south of Vol- After the mid-1970s, hydrographs of rivers clearly showed gograd and Rostov. Another region with evident increase is the transformation of the intra-annual distribution of river the North-West of the European Russia – Pskov and Velikiy runoff, which is noted by most researchers (Issledovaniya Novgorod district (Figure 5(b)). stoka i protsessov formirovaniya pavodkov [Studies of The combination of the described trends leads to losses runoff and flood formation processes] 1947, Frolova et al. increase due to infiltration, soil overmoistening in the winter 2015, Dzhamalov et al. 2015). Figure 8 shows the histograms due to frequent thaws, and the surface runoff decrease in the of the components of the annual runoff, which allow tracing spring (Yasinsky and Kashutina 2007, Barabanov et al. 2018). the contribution of various components to annual runoff As a result, the groundwater is replenished (Dzhamalov et al. volume over the past 70 years. Practically for all rivers, the 2015), and this, in turn, leads to an increase in the share of tendency of growth of a basic component is revealed. It is ground water inflow into the rivers. The increased water dis- least evident in the north of the European part of Russia charge during the low-flow periods contributes to the floods and the Kama River basin (Figure 8(a,b)). The groundwater formation. Liquid precipitation, or a strong and prolonged runoff increased during the study period by 40% in the thaw leads to a significant increase in water discharge; if the Kuban River and by 20% in the Terek River. It was about soil is already saturated with moisture by that time, the sur- 58% of the annual flow in the Terek River until 1981 and face runoff is added to the previous growth of the seepage in the modern period its share increased to 63%. In the flow of the river. It leads to the floods formation. Kuban River, 30% of the annual runoff was composed of baseflow until 1981, and recently its share increased up to almost 40%. The most obvious transformation of the annual 4.2. Observed changes in the runoff regime runoff redistribution is characteristic for the Oka and Don characteristics River basins (Figure 8(c–e)), left-bank tributaries of the The occasional floods occur almost every year on the majority Middle and Lower Volga (Figure 8(f)). For individual stream of rivers in the European part of Russia. In recent decades, gauges, an increase in the thaw floods runoff volume was floods have been observed almost in any hydrological season detected, but this trend is not observed everywhere. It can of the year. This process can be clearly seen from the be assumed that the mechanisms of thaws’ influence on 8 M. KIREEVA ET AL.

Figure 4. Difference in mean annual total sum of solid (a) and liquid (b) precipitation over the winter period (average for 1978–2016 as compared to 1958–1977).

Figure 5. Difference in liquid precipitation during February (a) and mean annual sum of positive temperatures during winter (b) (average for 1978–2016 as com- pared to 1958–1977). INTERNATIONAL JOURNAL OF RIVER BASIN MANAGEMENT 9

Table 3. Characteristics of occasional floods for gauging stations, averaged for the period 1978–2016: specific discharge for rain (Qrain) and thaw (Qthaw) events, number of rain (Nrain) and thaw (Nthaw) events, annual runoff yield from rain (Ytotal rain) and thaw (Ytotal thaw) events, changes in total runoff volume (with ground water component) for rain and thaw occasional floods. 2 2 River Gauging station Qrain, l/s·km Qthaw, l/s·km Nrain Nthaw Ytotal rain,mm Ytotal thaw, mm Delta Wrain, % Delta Wthaw,% Russian North basins 1 Northern Dvina Ust-Pinega 61 8 3 1 50 7 −21 20 2 Sukhona Kalinkino 53 9 2 1 57 12 −519 3 Mezen Malonisogorsk 31 5 2 1 52 11 −2 −9 4 Pechora Ust-Tsilma 37 5 3 1 59 9 12 −8 Kama River basin 5 Vyatka Vyatskiye Polyany 21 5 2 1 25 9 24 12 6 Kama Gainy 20 6 3 1 54 10 23 19 Volga River basin 8 Mologa Ustyuzhina 28 4 3 1 81 9 84 −19 9 Volga Staritsa 37 11 10 5 98 24 94 51 10 Unzha Makaryev 37 5 4 1 108 14 56 27 11 Vetluga Vetluga 44 3 3 0 65 6 30 −35 12 Samara Elshanka 6 1 2 1 5 5 6 31 Oka River basin 13 Oka Murom 9 4 3 1 44 14 66 26 14 Oka Kaluga 16 6 5 2 34 13 51 2 15 Moksha Temnikov 9 4 1 1 3 3 −54 40 Don River basin 16 Don Kazanskaya 4 4 4 13 12 14 62 14 17 Khoper Besplemyanovsky 5 1 2 2 3 3 −34 −5 18 Medveditsa Archedinskaya 7 3 1 2 6 7 110 −29 North Caucasus 19 Kuban Armavir 41 6 11 4 45 23 28 −12 20 Terek Kotlyarevskaya 50 11 13 4 73 67 10 27 winter runoff are more intensively reflected in the growth of rain flood is added to a base high-water wave formed by the groundwater runoff than in the growth of the flood peaks’ snow and ice melting and recession of the previous rain volumes themselves. floods. Annual maximum discharge in the Kuban River The rivers of the North Caucasus are characterized by shows a statistically insignificant positive trend, in the the floods throughout the year. For Terek and Kuban Riv- Terek River – statistically significant negative trend. For ers, this happens on average 17 times a year: from 7 to 23 both rivers, the date of the annual maximum discharge is floods during summer and from 0 to 10 in winter. The shifted to earlier periods: from the end of June – middle maximum annual discharges of both rivers occur when a of July toward the end of June for the Terek River and

Figure 6. Dates of maximum rain (a) and thawing (b) floods, as well as the maximum rain (c) and thawing (d) floods in the Samara River – Elshanka gauging station. The smooth line is the trend line and shadow bar around it characterized standard deviation. 10 M. KIREEVA ET AL.

Figure 7. Change in the number of thaws (1) and snowmelt runoff values taking into account (2) and without taking into account (3) the ground component for rivers: (a) Northern Dvina River – Ust-Pinega gauging station, (b) Pechora River – Ust-Tsilma gauging station, (c) Oka River – Murom gauging station, (d) Ugra River – Tovarkovo gauging station, (e) Don River – Kazanskaya gauging station, (f) Samara River – Elshanka gauging station. from the end of June – middle of July toward the middle of maximum flow rate and the groundwater flow and a signifi- June for the Kuban River. Occasional floods overlap sum- cant reduction in its dispersion. mer high water season and make up to 9–10% of the total runoff of the Kuban River and 4–5% of the Terek River. Winter floods do not make a significant contribution 5. Conclusions in the Terek River (up to 1%), while in the Kuban River they contribute 3–5% to the total flow. An increase by In recent years an intensive reduction of the seasonal flood almost 2 times in the dispersion of the flood volume series maximum and increase in groundwater flow is observed in occurs in the Kuban River in winter and summer. At the the European part of Russia. Before 1978 the seasonal flood Terek River, a similar dispersion increase is characteristic volumes significantly exceeded the annual groundwater of the total occasional flood volume in winter. Both rivers flow, but in the last thirty years these values became compar- are characterized by a directional increase in the dispersion able even for large basins or the percentage of the latter began of the flood rise time and the total flood duration in sum- to prevail. The main reason for the increase in minimum mer and winter. The maximum water discharges of sum- runoff is associated with an increase in flood runoff due to mer floods are characterized by homogeneity relative to more intensive and prolonged thaws. The results of the calcu- the average value and Cv for both analysed watersheds. lations confirmed a number of earlier findings on the change The winter floods in the Terek River are characterized by and transformation of the water regime on the Russian Euro- a directed increase in the maximum water discharge. This pean rivers and showed a decrease in the volumes of seasonal increase is due to the growth of groundwater flow in the flood wave, and an increase in low flow discharges as well as basin, the reduction of the difference between the an increase in the proportion of occasional flood runoff in the INTERNATIONAL JOURNAL OF RIVER BASIN MANAGEMENT 11

Figure 8. Changes in the volume of groundwater runoff (1), water runoff for high water (2), rain (3) and thawing (4) floods by the example of river basins: (a) Pechora River – Ust-Tsilma gauging station, (b) Ugra River – Tovarkovo gauging station, (c) Moksha River – Temnikov gauging station, (d) Oka River – Murom gauging station, (e) Don River – Kazanskaya gauging station, (f) Samara River – Elshanka gauging station. annual runoff over the last 30 years in the basins of Volga, Funding Oka, Vyatka and Don River. ff The study has been carried out with the support of the Russian Science A decrease in the spring snowmelt runo has occurred Foundation [Project No. 19-77-10032] in terms of new software (GrWar against the background of the minimum winter runoff rise Version 2.0) and calculation, Russian Foundation for Basic Research mainly due to winter floods, increased in volume by 20– (RFBR) [grant number 18-05-60021] in calculation for arctic rivers, Rus- 30% in most river basins under study. Also, due to thaws, sian Foundation for Basic Research (RFBR) [grant number 16-35-60080] in terms of algorithms (GrWar Version 1.0). groundwater recharge has increased. A distinctive feature of the water regime in recent decades is the floods in almost any season of the hydrological year. Thus, some rivers of ORCID the European part of Russia can hardly be referred to the riv- ers with the East European type of water regime according to Maria Kireeva http://orcid.org/0000-0002-8285-9761 ’ fi ff Timofey Samsonov http://orcid.org/0000-0001-5994-0302 B.D. Zaikov s classi cation, since the spring high water runo Elena Povalishnikova http://orcid.org/0000-0002-6065-6330 is less than 50% of the annual runoff.

Acknowledgements References The development of the GrWat software option for the mountain terri- AISORI database in the RIHMI-WDC website. Available from: http:// tories and the analysis of the regional trends of the flood runoff within aisori.meteo.ru/ClimateR [Accessed June 25, 2018]. the North Caucasus were carried out with the support of the RSF Barabanov, A.T., et al., 2018. Poverkhnostnyj stok i infil’tratsiya v (Grant No. 17-77-10169). pochvu talykh vod na pashne v lesostepnoj i stepnoj zonakh Vostochno-evropejskoj ravniny [Surface runoff and infiltration into Disclosure statement the soil of thawed waters on the plowed fields in the forest-steppe and steppe zones of the East European Plain]. Pochvovedenie [Soil No potential conflict of interest was reported by the authors. Science],1,62–69. 12 M. KIREEVA ET AL.

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