Droughts are among the major challenges facing Europe and pose a significant threat to food security on a continental scale. The shifting of climatic zones is forcing societies to adopt measures to cope with climate extremes. However, the pace of adaptation is much slower than the rate of change observed over the past decade. The most developed economies in Europe are already progressing toward implementing strategies aligned with the Sustainable Development Goals. One of the key scientific objectives is to support this adaptation by providing data-driven decision-making tools at both regional and national levels. At the same time, in some European countries, the understanding of drought remains uncertain. Even basic hazard assessments lack coherence, and methodologically sound and systematic vulnerability assessments are often entirely absent. This paper aims to uncover the drought research landscape over the past ten years for nine countries in Southeast Europe. Using a structured query in the SCOPUS database, we attempted to systematize scientific knowledge on drought exposure in the region and identify “white spots” and gaps in knowledge at both country and regional levels. Our findings show a significant increase in the number of research papers focused on various aspects of drought in these nine countries over the last decade. However, for Montenegro, Albania, Slovenia, and North Macedonia, only one or two papers were found. On the other hand, due to the complexity of drought phenomena – including the wide range of indicators, seasonality, methodologies, and aspects studied – it is extremely difficult to form a comprehensive picture for well-represented countries like Romania and Serbia. To enhance understanding of drought trends in the region over the past ten years, our review incorporates the Combined Drought Indicator (CDI) assessment. The CDI v4 dataset, provided by the Copernicus Drought Observatory, serves as the main unified tool for drought monitoring across Europe. The analysis revealed similar temporal patterns across the region, with some differences in outliers, such as historical droughts. As additional context, we included drought impact data gathered from the newly published European Drought Impact Database (EDID) database. This supplementary information helps us understand the “inheritance” of drought impacts along major rivers and their variability.

The study is devoted to the analysis of daily time series of river runoff in the Arctic zone of Eurasia. Unique data on daily water discharges in the closing gauges of Arctic rivers were collected and processed in the package grwat (https://cloud.r-project.org/web/packages/grwat/index.html ), which identifies genetic components of runoff. As a result, 53 runoff characteristics were obtained for each of the 25 rivers flowing into the Arctic Ocean and the contribution of snowmelt, rainfall, and groundwater components to the total runoff was analyzed. Particular attention was paid to extreme characteristics - maximum water discharges of spring freshet, rain events and minimum 1, 5, 10-averaged discharges during summer and winter.The study of maximum water discharges has shown that, in general there are trends of decreasing annual maximums for both large and medium-sized Arctic rivers. This trend, however, is not yet statistically significant everywhere. The most intensive decrease in maximums localized in the Northern Dvina, Ob, and Yenisei rivers, for which flow regulation by reservoirs has a significant impact. For the Kolyma, Yana and Indigirka rivers, there are periods of increase in maximums and their decrease lasting 5-7 years, with a general tendency to increase during 1960-2001 up to 15-20%.In contrast, the minimum discharges with different averaging intervals increases by 25-56 % everywhere; this trend is presumably related to the general climate warming, increased infiltration and the role of groundwater flow, and for the rivers in the eastern part of the Arctic zone - to the degradation of permafrost.The study also included analysis of the runoff signature transformation in Arctic zone by every year, as well as on average for the modern and historical period. The typing methodology consisted in classifying hydrographs according to two main features: a) exceedance of maximum discharge relative to the average annual discharge b) the share of flood runoff volume in the total annual runoff. The analysis showed a noticeable increase in the frequency of occurrence of smoothed hydrographs on the rivers of the Arctic zone of the Asia-Pacific region, for some basins the number of such years increased by 1.5-2 times (Polui, Turukhan, Ob rivers).

The article provides an overview of publications devoted to assessing changes in the water regime of Russian rivers under the conditions of current and projected climate changes. The most recent summary of the relevant publications is contained in the national assessment reports of Roshydromet. Since the publication of these fundamental works, a large number of studies have been published, clarifying the conclusions of the national reports. The purpose of this review is to summarize the modern ideas about the impact of climate change on the territory of the Russian Federation on the mean annual and maximum river flow, primarily based on the publications in recent years. The review is divided into two parts. The first part presents the results of the diagnosis of changes in the long-term norms of the annual and maximum flow of Russian rivers that occurred during the period of instrumental observations in the XX–early XXI centuries. Due to the geographical differences in the direction and magnitude of climate changes and associated changes in the water regime of rivers, the review is given separately for the rivers of the European and Asian territories of Russia. It is shown that the annual runoff over the territory of European Russia in recent decades has a tendency to increase, associated with a general rise in the humidity of the territory. However, for most of the analyzed river basins, the changes are statistically insignificant. The annual runoff of rivers from the territory of Siberia and the Far East into the Arctic seas of Russia has also slightly increased on average. The changes in the maximum runoff are more pronounced and differently directed. The second part of the article provides an overview of publications that present projections of changes in the water regime of Russian rivers until the end of the XXI century. The projections were obtained in ensemble experiments with climate models or with regional hydrological models. The conclusions made in the Second Assessment Report of Roshydromet regarding the insignificant positive anomalies of the annual runoff rate for most of the territory of Russia under moderate anthropogenic warming scenarios in the XXI century have been confirmed. The most pronounced positive anomalies of the snowmelt and rainfall runoff in the XXI century are possible on large rivers of Siberia in the case of implementation of the RCP8.5 scenario of anthropogenic radiation impact.

As the adverse impacts of hydrological extremes increase in many regions of the world, a better understanding of the drivers of changes in risk and impacts is essential for effective flood and drought risk management and climate adaptation. However, there is currently a lack of comprehensive, empirical data about the processes, interactions, and feedbacks in complex human-water systems leading to flood and drought impacts. Here we present a benchmark dataset containing socio-hydrological data of paired events, i.e. two floods or two droughts that occurred in the same area. The 45 paired events occurred in 42 different study areas and cover a wide range of socio-economic and hydro-climatic conditions. The dataset is unique in covering both floods and droughts, in the number of cases assessed and in the quantity of socio-hydrological data. The benchmark dataset comprises (1) detailed review-style reports about the events and key processes between the two events of a pair; (2) the key data table containing variables that assess the indicators which characterize management shortcomings, hazard, exposure, vulnerability, and impacts of all events; and (3) a table of the indicators of change that indicate the differences between the first and second event of a pair. The advantages of the dataset are that it enables comparative analyses across all the paired events based on the indicators of change and allows for detailed context- and location-specific assessments based on the extensive data and reports of the individual study areas. The dataset can be used by the scientific community for exploratory data analyses, e.g. focused on causal links between risk management; changes in hazard, exposure and vulnerability; and flood or drought impacts. The data can also be used for the development, calibration, and validation of socio-hydrological models. The dataset is available to the public through the GFZ Data Services (Kreibich et al., 2023,https://doi.org/10.5880/GFZ.4.4.2023.001).

Despite the significant role that the Lower Don plays in Russia’s fishing industry, the region’s fisheries remain in a depressed state. It is well understood that the Tsimlyansk reservoir impacts the region’s fisheries, but the potential influence of climate change and associated reductions in snowmelt water flow remains under debate. The present study provides the first investigation of the effects of reservoir management and reduction in snowmelt water flow due to global warming on fish spawning independently. Using estimates of water discharge in the lower reaches of the Don, in the absence of the Tsimlyansk reservoir, the results showed that climate-related factors, although still significant, had a smaller effect on water discharge, compared with reservoir management regimes. Hydrological characteristics affecting fish reproduction (i.e., water discharge, water level, area and duration of floodplain inundation, and fingerling abundance) are decreasing under the reservoir influence by a factor of 1.5–3.2, while the declines associated with climate change are a factor of 1.4–2.1. Artificial fish reproduction and improvements in management of the Tsimlyansk reservoir could lead to restoration of fish stocks in the Lower Don.

Megafloods that far exceed previously observed records often take citizens and experts by surprise, resulting in extremely severe damage and loss of life. Existing methods based on local and regional information rarely go beyond national borders and cannot predict these floods well because of limited data on megafloods, and because flood generation processes of extremes differ from those of smaller, more frequently observed events. Here we analyse river discharge observations from over 8,000 gauging stations across Europe and show that recent megafloods could have been anticipated from those previously observed in other places in Europe. Almost all observed megafloods (95.5%) fall within the envelope values estimated from previous floods in other similar places on the continent, implying that local surprises are not surprising at the continental scale. This holds also for older events, indicating that megafloods have not changed much in time relative to their spatial variability. The underlying concept of the study is that catchments with similar flood generation processes produce similar outliers. It is thus essential to transcend national boundaries and learn from other places across the continent to avoid surprises and save lives.

The territories of river basins historically have a high potential and act as a catalyst for the development of regional systems through access to natural resources, favorable transport conditions and recreation. The Yellow River connects the eastern and western regions of China with its basin, forming a single economic vector for the adjacent territories. Determining the effectiveness of programs and projects for the integrated development of water basin territories should be based on a system of evaluation criteria. The article presents a methodological apparatus that allows in the future to determine the most effective directions for the development of basin territories, based on the proposed system of evaluation indices.

The article presents the results of study of the application of machine learning methods to the problem of classification and identification of different river water regimes in a large region – the European territory of Russia. An accumulation of hydrological observation data for the 60 – 80 years makes it possible to create an information basis for such studies. The article uses information on the average monthly runoff at 351 hydrological gauges during the period from 1945 to 2018. The most widely used data clustering approaches were used as analysis methods – K-means, EM-method, agglomerative hierarchical clustering, DBSCAN algorithms and the application of gradient boosting methods (CATBUST). Clustering and classification algorithms were given eight parameters as a basis for prediction. It was found that the most distinct and stable clusters are formed with three parameters, and the highest silhouette coefficient (SS = 0,3-0,5) is obtained using the numbers for months of the maximum and minimum runoff and the ratio of the maximum to the minimum water flow. The best result gives DBSCAN (SS = 0,6 – 0,7). Supervised classification models also show high correspondence with the reference classification, with an accuracy of 87%. Both clustering methods and classification methods showed a shift of clusters representing southern water regimes. In the central region these regimes expanded by a 1000 km to the north. Furthermore, results demonstrate that currently available data already makes it possible to apply machine learning methods to the analysis of hydrological data. Clusters corresponding to different types of water regime can be obtained by utilizing contemporary clustering algorithms. The study shows that over the past 40 years, the southern types of water regimes have noticeably shifted to the north.

River hydrograph separation is one of the most important operations applied to the streamflow data. Numerous separation techniques and and their software implementations have been developed so far. In operational practice of Russian hydrological organizations and research institutes an event-based approach is commonly used for the hydrograph separation. Different meteorological events such as temperature transition through zero and rains are recognized in meteorological data, and then the corresponding changes in river hydrograph are identified, which eventually helps to attribute each peak in hydrograph with corresponding genetic component. The base flow component is traditionally defined according to Kudelin’s approach, taking into consideration different schemes of surface-ground water runoff interaction. In contrast, the most widespread separation approach in Western school is filtering-based. Lyne-Hollick, Maxwell, Boughton, Jakeman, Chapman and some more sophisiticated filters can be applied to separate the flow into quick and base. Results of two approaches are quite different, especially in terms of the baseflow component. In current study we present the updated open-source grwat R package, which puts both worlds together. It contains both the genetic event based and filtering-based hydrograph separation approaches with the ability to mix them together. In particular, applying the filtering-based separation inside the detected genetic events provides curve of the baseflow well corresponding to tracer-based studies. The second novelty of the package is the intellectual procedure for determination of the second-order events that complicate the freshet (seasonal) flood, such as rain floods. Finally, the updated package contains the internal spatial database of hydrograph separation parameters which is obtained over the European territory of Russia through experimental work. This database allows automated selection of the optimal separation parameters based on the location of the river gauge supplied by package user. The database can be extended to other regions of the world through collaborative work of package users. The study was supported by the Russian Science Foundation grant No. 19-77-10032

The analysis of the formation of spring runoff on the rivers of the Russian Plain is presented. A brief review of the studies dealing with the dynamics of individual climate characteristics and river floods on the Russian Plain in the past 40 years is given. It was shown that as a result of climate change and anthropogenic impact on the formation of snowmelt runoff, not incoming but outgoing factors causing its loss have become decisive. The scheme of the spring flood formation factors was developed and verified for the rivers of the Don basin, which is the most complex one in terms of the spring flood formation. According to the verification for 11 catchments situated in the Don basin, three most significant factors of the spring flood formation were identified: the absence of the significant correlation between the spring flood runoff and snow water equivalent; an increasing role of soil moisture and freezing depth; a great role of the snowmelt rate during the period just before the spring flood. The results allow making a conclusion on a possibility of applying this research scheme in other regions. Keywords: spring flood runoff, Don basin rivers, dynamic, quasi-permanent, and anthropogenic factors

Risk management has reduced vulnerability to floods and droughts globally 1 , 2 , yet their impacts are still increasing 3 . An improved understanding of the causes of changing impacts is therefore needed, but has been hampered by a lack of empirical data 4 , 5 . On the basis of a global dataset of 45 pairs of events that occurred within the same area, we show that risk management generally reduces the impacts of floods and droughts but faces difficulties in reducing the impacts of unprecedented events of a magnitude not previously experienced. If the second event was much more hazardous than the first, its impact was almost always higher. This is because management was not designed to deal with such extreme events: for example, they exceeded the design levels of levees and reservoirs. In two success stories, the impact of the second, more hazardous, event was lower, as a result of improved risk management governance and high investment in integrated management. The observed difficulty of managing unprecedented events is alarming, given that more extreme hydrological events are projected owing to climate change 3 .

Abstract. Empirical study of the isotopic features of river runoff were conducted at three hydrological posts in three different river basins: the Zakza river in the center of East European Plane (southwest of Moscow), the Dubna river (north of Moscow) and the Sosna Bystraya river in the south of central region. Samples of river water, groundwater, and precipitation for the October 2019–October 2021 were collected at weekly intervals. At total 332 samples of river water, 275 samples of groundwater and 194 samples of precipitation were collected. Precipitation was collected as an integral sample of all precipitation fallen during the week before sampling date. For each precipitation samples, the total amount of precipitation and air temperature, weighted by precipitation amount, are given according to weather station in river basin. During the observation period, there were two completely different conditions in terms of runoff formation. First, from October 2019 to October 2020, there was an unusually low spring freshet followed by a big rain flood in July. From October 2020–October 2021, there was a normal intra-annual flow pattern with high spring freshet. A significant supply of melted snow during spring freshet is the key factor influencing water regimes in these three river basins; varying degrees of anthropogenic flow regulation are also present. The new height frequency and complete data of stable isotope signature of river runoff component can help to study the response of a river runoff to climate change.

Publications on changes in river water regime in Russia under the conditions of current climate changes are reviewed. Most recent generalizations of such publications are presented in Roshydromet evaluation reports. The publication of these basic studies followed by many studies improving the conclusions of national reports. The objective of this review is to generalize the current concepts regarding the effect of climate changes in Russian Federation territory on the mean annual and maximal river runoff, primarily, based on most recent publications and the authors' own studies. New maps, developed by the authors, are given to characterize variations of the annual and maximal runoff, including the dates of the disturbance of the stationary character of the series based on data up to 2019. Considerable attention is paid to statistical analysis of the revealed changes. The annual runoff on the average for the Russian territory is shown to tend to increase in the recent decades because of an increase in the moistening of the territory. However, the changes in the majority of the analyzed basins are statistically insignificant. The average annual runoff of rivers into the Arctic seas from the territory of Siberia and Far East has also slightly increased. Changes in the maximal runoff are more pronounced and differently directed.

The review of the papers dealing with the river runoff formation is presented. The factors of the spring flood formation are typified. The groups of direct and indirect, dynamic and quasi-permanent, as well as anthropogenic factors are distinguished. The authors constructed a summary scheme of the spring flood formation factors, taking into account the peculiarities of their influence on water runoff, analyzed the role of each significant factor in the formation of spring floods, and presented their quantitative estimates. It was hypothesized that the concept of the dominant role of incoming components in the spring flood runoff formation has currently lost its relevance. Modern approaches to the calculation of the main factors of the spring flood runoff formation are considered. Keywords: spring flood, climate change, snowpack, water equivalent, snow cover, soil moisture, freezing depth

An algorithm for automated graph-analytical separation of hydrograph, underlying grwat software package is described in detail and analyzed. This system is designed for separation of runoff hydrograph into base runoff, spring f lood, rain and thaw runoff events by the method proposed by B.I. Kudelin. The starts and ends of water regime phases are identified by algorithms for distinguishing breaks on the hydrograph and analyzing their meeting some criteria for passage of water regime phases, based on the physics of runoff formation in river basins. The input data are daily series of water discharges, air temperatures, and rainfall values. Weather data are used as an indicator and means to refer hydrograph peaks to groups of events, i.e., thaws or rain (mixed) f loods, and to determine the start of winter low-water season. A series of calibrated parameters are used to specify criteria for phase separation. The program calculates 52 annual runoff characteristics, as well as more than 30 characteristics for each individual f lood: time characteristics (the dates of start, end, and maximum; the duration, and the time of rise), discharge characteristics (maximal discharge, f lood volume, water discharge before f lood start, the excess of maximal discharge over basic f lood level), various weather characteristics (the regimes of air temperature and precipitation before the f lood and during it), and others. The values of calibrated parameters of grwat were found to be stable for the majority of rivers throughout the calculation period, and their values were similar for rivers with the same type of water regime and size. Algorithm gwart also showed high tolerance to changes in the values of calibration parameters. Recommendations are given for specifying their values. The main causes of errors in determination of water regime phases and the development perspectives of the algorithm are discussed.

An increase in the average annual air temperature in the European territory of Russia against the background of climate change leads to a decrease in water reserves in the flood-forming snow cover. At the same time, an increase in the number of transitions of air temperature through 0 °С in the area of positive values in winter leads not only to an additional reduction in snow storage by the beginning, but also ensures an increase in the number and size of thaw floods. Changes in the intra-annual distribution of the range and intensity of precipitation entail an increase in the number and magnitude of rainfall floods. Observed transformations in the types of feeding and water regime of the rivers of the basin.

Over the past 20 years, the climate on the East European plain tends to be significantly warmer and drier. Winters became shorter and spring freshet’s conditions have been changed significantly. Maximum snow depth was the most important factor of spring freshet formation 30 years ago, but nowadays it has no significance at all and main factor today is melt water losses on infiltration and evaporation. We registered a decrease in the period of stable snow accumulation (on average by 20% in the southern and southwestern parts of the East European Plain) because of the increase in winter temperatures. More often during first part of winter snow cover disappeared totally. The number of thaws and their duration at the end of the winter also increase and this leads to earlier and more prolonged melting of the snow pack. In these conditions, an extremely low spring freshet is formed. Our studies show that with the condition of an equal maximum snow depth the slow snowmelt forms the spring freshet up to 4 times less in volume than the fast melting. Soil moisture also plays an important role in the melt water losses. The most part of the East European Plain is characterized by a decrease in soil moisture in late autumn, which indicates increased losses during snow melting period. Still, the most significant changes in the structure of the factors of spring freshet formation are common to the southern and southwestern parts of the East European Plain. In the northern part, conservative factors still dominate, although this area is characterized by the significant increase in winter temperatures. The study was supported by Russian Science Foundation Proj. №19-77-10032

In the past two decades we see many signs of changing behaviour in hydrological regimes of Russian Plain rivers. River regimes classification was done in the early 1990s and it's possible that some rivers (especially in Don and Oka river basins) have already changed their behaviour. We believe that's the first time this was done by objective analysis and without reliance on experts opinion. In this work we make an attempt at automatic and objective classification of water regime types for 220 rivers of Russian Plain and propose a method for automatic assesment of changes in hydrological behaviour of local rivers. We use monthly data and k-means clustering algorithm to classify each river water regime for every year with available data. Unlike most of other approaches we do not divide data by year but create clusters from all datapoints simultaniously. This allows us to use more datapoints and establish a more robust result. Next, when we have annual clusters for every datapoint we can assess the stability of water regime for each catchment over several decades and identify catchments with unstable and changing behaviour. By using this method we're able to automatically identify 5 distinct water regimes for the rivers of Russian Plain: three with dominant peaks caused by spring freshets in March, April and Februaty with most discharge happening over the course of a single month and two types of water regimes with maximal discharges in April and June, but lacking a pronounced peak in these months. Unlike previous calssifications we can identify the closest water regime for every year and therefore make an attempt at quantifying stability of these regimes and changes over time. By using a very naive approach and calculating a standard deviation over a moving window of 10 years it's possible to detect unstable regions and therefore select periods of stability and shifts for each subregion of Russian Plain. We're able to identify Don and Oka basins as regions with the most changes in water regimes and it corresponds with research data. In addition rivers in Kola peninsula and Ural regions peninsula demonstrate a slight shift in stability. In terms of hydrological behaviour we see siginificant changes in Don and Oka river basins that shifted from spring freshet peak in April into water regime type with a peak in March or a more southern water regime with less pronounced April peak having precedenig winter thaws. We believe that this simple approach at identifying water regimes and changes in them can be successfuly used for other regions than Russian Plane. The study was supported by the Russian Science Foundation (grant No.19-77-10032) in methods and Russian Foundation for Basic Research (grant No.18-05-60021) for analyses in Arctic region