The constant shortage of fresh water for drinking and municipal use in the Republic of Kalmykia has become critically acute recently. Both natural factors, i.e. increasing climate aridity, limited reserves of fresh surface and groundwater, lack of sufficiently long sections of large rivers with constant flow, and the deterioration of hydraulic structures are the reasons. The exploitation of groundwater in existing deposits has led to their depletion and deterioration, while large deposits of fresh and slightly brackish groundwater in the southern Caspian region cannot be used because the content of pollutants in them is rather high. Recovery of old and construction of new water pipelines from the Volga River requires permanent significant financial investments at the federal level. Therefore, recently a new way to increase the supply of fresh water suitable for drinking purposes, i.e. an artificial groundwater recharge (AGR), has been actively discussed. The article provides an actual assessment of water resources of the littlestudied surface runoff of the rivers of Kalmykia, considers the possibility and feasibility of creating infiltration basins to replenish groundwater reserves, and calculates the parameters and operating mode of five planned basins. The main source of fresh surface water in Kalmykia is the rivers of the Ergeni Upland. The average long-term flow from its eastern slope is estimated at 0,120 km3/year, from the western slope at 0,105 km3/year and from the southern slope at 0,045 km3/year. Trends in modern modification of the hydrological regime of rivers under the influence of climate change have been revealed, namely a decrease in annual runoff and its intra-annual redistribution with a reduction of spring floods, a slight increase in winter runoff and a leveling of summer low-water periods. Calculation of the operating mode of planned infiltration basins allowed us to draw the following conclusions: the maximum possible volume of total infiltration will be about 220 thousand m3/year, which is 36 times less than the presentday water intake from the Troitsk and Bayarninsky fields for public water supply; sufficiently efficient operation of pools is only possible with annual cleaning of accumulated silt deposits, otherwise the soil filtration coefficient will come down each year, resulting in the reduction of infiltration supply by dozens times. Thus, the AGR can only be a temporary measure to compensate for the subsidence of groundwater level until the reconstruction and construction of new water pipelines from the Volga River to supply the city of Elista and the surrounding area with drinking water of acceptable quality

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

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

The article examines the flow features of the spring flood in the Don River basin during 1930-2017. Its large-scale changes over the past half century took place in 1980 and 2006-2010. During these periods the most significant climatic changes were observed, aggravated in the 20th century by significantly increased anthropogenic runoff regulation by numerous reservoirs. The application of the method of integral curves in combination with statistical analysis of the spring runoff by the data of 50 stream gauges made it possible to separate the contribution of climatic changes and anthropogenic impacts to the formation of modern decrease of flood runoff on the rivers of the Don River basin. According to our results over the past 40 years the flood runoff has decreased by an average of 30-40%, i.e. from 10% in the upper reaches of the Voronezh, Khoper and Medveditsa rivers to more than 50-60% in the basins of the Seversky Donets and Oskol rivers. In terms of the changes in spring runoff, the rivers of the Don River basin are divided into three groups: with the runoff decrease after 1980, after 2006-2010 and with no changes in spring runoff. According to calculations, the amount of visible evaporation from the surface of reservoirs in the Don River basin upstream the Tsimlyansk reservoir is relatively small and equals to 0.09 km3 or 0.5% of the Don River's average annual flow for the Kalach-on-Don section. However, in dry years this factor can significantly increase the deficit of water resources for most watersheds with intensive economic activity. It is estimated that the total usable volume of reservoirs in the basins of the largest Don River tributaries averages 43% of the decrease in their runoff during the flood period, thus indicating highly possible seasonal redistribution of water runoff by reservoirs with a volume of more than 0.1 km3. The potential of technogenic regulation (PTR) of reservoirs with a volume of more than 0.1 km3 was evaluated. A decrease of the spring river flow with increasing PTR of reservoirs in their basins was noted.

Among all hydrological factors, water regime characteristics have always received special attention as integral parameters of the hydrological conditions of a water body, because they can be used in various estimation and forecast procedures. This study considers the development, in both Russia and other countries, of various theories and approaches to zoning the territories by the specific features of the water regime of riv-ers. The evolution of the classification developed by the late XX century, reflects changes in the level of knowledge about river basins and the accumulation of data on their runoff formation conditions. It is shown that the development of specialized databases, containing information about the characteristics of climate and river runoff, thematic electronic GIS-projects, and maps have determined the development of modern quantitative methods. The incorporation of a large volume of hydrometeorological data, the automatization of the means for processing and interpretation of data on river runoff characteristics, software complexes, supporting databases and geoinformation technologies, and web-applications made it possible to pass from general geographic and descriptive classifications and zoning to quantitative approaches, based on rigorous calculations of runoff characteristics and the development of new criteria characterizing the specific features of river water regimes, which can be of use in various economic branches.

The article focuses on the study of flood flow changes on the rivers of the European Territory of Russia (ETR) in the last thirty years. This is an extremely important problem, as with floods, whose contribution to the structure of the annual flow of European rivers in recent decades has been increasing, the most destructive floods are associated. On the example of 55 representative hydrological gauge stations located on the ETR in regions with different conditions of runoff formation, the features of both summer and winter floods formation are considered. It has been established that over the past thirty years on the most rivers there has been an intensive reduction in the ratio of volumes and maximum flood discharges in relation to the similar characteristics of the basic runoff. Increased groundwater supply is observed, and the absence of significant freezing of the soil leads to an increase in infiltration. The volumes of flood runoff and the basic runoff become comparable or the proportion of the latter begins to prevail. The main reason for the increase in minimum water discharge is associated with an increase in flood flow under the influence of more intense and prolonged thaws. A distinctive feature of the water regime of recent decades has been the flood peaks in almost any season of the hydrological year. In the middle and southern part of the ETR – in the basins of the Volga, Oka, Vyatka, Don Rivers – there is an increase in low-water flow and in the quota of flood in annual flow. On some rivers of the ETR, the spring flood runoff currently accounts for less than 50% of the annual runoff.

Modern features of the annual flow distribution of the Volga Basin were analyzed. Statistical analysis of the annual flow data for the spring, summer-autumn, and winter seasons were carried out to study the changes of the water regime of the Volga Basin Rivers in 1946–2010, 1946–1977, and 1978–2010 observation years. Using the data of 207 stations, new maps of seasonal flow were created and changes in the natural river flow regulation were shown. In recent decades, the water regime of the rivers in European Russia and their annual flow distribution has changed greatly. A statistically significant decrease in the river flow irregularity after 1946 has been revealed for the most of the Volga Basin Rivers. It resulted in the increase in the natural regulation index φ, change of the seasonal flow, primarily in low-water period, and especially in winter. The increase in the φ index during the last 30 years is approximately 30%, compared to the similar previous period. Such a change of the φ value took place due to the reduction of the spring flood flow and the increased significance of the groundwater flow. In the second half of the 1970s, the rivers of the Volga Basin were characterized as “snowfeeding”; at the end of the XXth century, they became the rivers with “mixed feeding” or even “mixed feeding with prevailing groundwater feeding” according to B.D. Zaikov’s river classification. This has resulted in the significant increase in the natural flow regulation comparable to the effect of seasonal storage reservoirs.

Specific features of the regime of the Borovno-Razliv lake-river system are investigated, as well as the mechanism of the flow of dissolved mineral and organic substances and exchange processes between bottom waters and soil during different seasons within theterritoryofValdainational park. The actual hydroecological state of water objects is considered taking into account specific conditions of nature and water management in the region. The results of investigations made it possible to suggest the optimum terms of further observations on the lakes of the Borovno-Razliv system and other lakes of the national park.

Комплексное гидролого-гидрохимическое и гидробиологическое обследование рек и озер в верховьях бассейна р. Мульты (Горный Алтай), проведенное силами студентов и преподавателей кафедры гидрологии суши МГУ имени М.В. Ломоносова в июле 2008 г., позволило получить представление о гидроэкологическом состоянии водных объектов региона, мало подверженных антропогенному воздействию. Полученные материалы позволяют выявить особенности водного режима и механизм формирования стока растворенных веществ в труднодоступной части горно-ледниковых бассейнов и значимость отдельных факторов их формирования