No abstract

The article analyses, on the example of the river Bityug - Bobrov (Verkhniy Don, Voronezh Region), the dynamics of the absolute low water minimums of the low-water run-off. It demonstrates that, in the current century, due to the increase in summer-autumn and winter water content, the dates of absolute minimums of runoff have been shifting (rounded up to a month) for later periods of summer-autumn and earlier periods of winter low water. The highest recurrence of absolute minimums of the winter low water falls on January, and of the summer-autumn low-water - on September.

The article deals with the water regime of the Upper Don`s rivers in the current climatic period, comparing characteristics of 1939-1970, 1971-2000, 2001-2014. The features of the formation of flood peaks were identified. The spring flood caused by melting snow increased to about 10 days. The shortest period of spring flood on the Don River near the town of Liski lasted 41 days in 1961, and the longest – 78 days in 2001.The timing of the beginning of the spring flood has shifted toward earlier dates of its occurrence and the ending -to the side of later dates. With increasing total duration of spring flood on 15-20 days, the late onset of the highs was observed. Due to milder winters and reduction of depths of soil freezing on the pick of the spring flood, water on the slopes of the river filtered to a greater extent in the soil, than goes on the formation of slope runoff. And only during a more intense snowmelt and water flow in the riverbed is formed the maximum flow of floods. Temporal and spatial heterogeneity of the formation of maxima in the river system of Don and of Khoper were marked. In the upper reaches they represent two independent, isolated from each other zone of runoff formation. In the basin of the Khoper highs are formed later than in the Don`s basin. The emergence of the thaw in February is almost natural now. In February of 2016, with abnormal weather conditions, the boundary between the winter and spring seasons due to numerous thaws were conditional. The frequency of occurrence the bimodal spring floods in the basin of the Khoper has frequented. In 2010, the spring flood hydrograph had atypical ridged shape. Reduction of runoff of spring flood and its share in annual volume in this century up to 38-45 % in the rivers of the Central Russian upland on the right bank of the Don leads to a change in the priority of sources of supply. The dominant source of power for the rivers of the basin of the hopper remains the snow supply.

Considered the timing of the onset (with rounding up to a month) lows of the summer-autumn low water period in the basin of the Upper Don for the periods: from the beginning of the observations in the posts, before 1970, 1971-2000, 2001-2017. Three zones have been identified that differ in generation time of the absolute lows of water flow: the Don itself and the right-bank tributaries of the Don; the left-bank tributaries of the Don; a basin Hoper. It is established that in the current century the absolute minimums of water flow are formed in the selected zones in July – August, August-September, mainly in September, respectively, which is about a month earlier than the dates observed before 1970. The main shifts in the onset dates of the investigated characteristic occurred in 1971-2000, fixed in the current century. In the Hoper basin, there is greater consistency in the onset of absolute lows of water flow than in the river system of the Upper Don. Despite the increase in the duration of high water and summer-autumn low water, as well as the tendency of increasing water content in the period of low runoff, there is a movement of formation dates of low extrema of water content to the beginning of summer-autumn low water. The identified nature of the formation of absolute minimum is apparently the result of intra-annual redistribution of the flow and a response to the positive dynamics of air temperature, which directly or indirectly infruences the hydrological processes in the river catchment area. Earlier summer depletion of rivers can contribute to increased risks of water use during the summer-autumn low-water period, adversely affect the agricultural sectors focused on the use of water for agricultural water supply, significantly change the quality of water in water bodies and create environmental tensions in the basin.

The Don river basin has high population density, robust economy, but suffers from lack of water supply in low water level periods and seasons. This fact prompts researchers to study various aspects of the water resource dynamics. The analysis of the water flow volumes is demonstrating the decrease of the maximum spring flood water flows, the increase in the low level season water volumes, the leveling of the water flow volumes throughout the year and lower variance of the yearly water resource fluctuations. The duration of the spring floods and the summer-fall low water periods is increasing, but the flood maximums occur later (by 15-20 days), and the summer-fall low water periods start 15-25 days earlier compared to the previous century. The snow-related spring floods show anomalous patterns in terms of time of onset, duration, and height and shape of the flood wave. Autumn floods are episodic. The small rivers are showing signs of degradation, some of them are disappearing altogether. This is one of the reasons of the water resources depletion and the shift towards the earlier occurrence of the yearly low water level extremums. The low water flows and volumes of the current century pose risks for resource utilization.