## 1 Introduction  
Since the beginning of the 21 st century, the attention of many researchers has been increasingly attracted to the problems of climate change and the associated variability of water resources. Understanding the relationship between climate change and the water content of a particular basin helps to plan the use of water resources more rationally. One of the methods of studying possible climate changes and related consequences is the analysis of river runoff in order to detect long-term patterns and trends caused by these changes. It is assumed that the trends in runoff changes identified as a result of the analysis can be somehow related to climate changes in the corresponding region. Since the authors do not have much confidence in the results of climate change forecasts for various scenarios such as SSP585 (global models, MPI-ESM1-2-LR, HadGEM3-GC31-LL, CNRM-CM6-1, etc. ), it was decided to analyze the long-term series of observed inflows (not a forecast) to determine the year when a possible change occurred, substantiate this event with statistical methods, and determine the quantitative indicators of the changes that have occurred and how they affected the water management situation in the river basin. Such an approach to research is given in [1; 2; 3; 4; 5]. An important task in this approach is the adequate identification of these trends in long-term hydrological series. There are quite a lot of works devoted to the study of these problems in the basins of various rivers, in which a noticeable change in water content and its intra-annual redistribution have been observed in recent decades [6; 7; 8; 9; 10; 11; 12]. The Heihe River basin runoff analysis from 1958 to 2017 was carried out in [6]. The authors identified three intervals for this period--1958-1980, 1981-2003, and 2004-2017. In the first interval, the annual runoff showed a downward trend; in the second interval, fluctuations were observed in the annual runoff relative to a certain value; in the third interval, a clear upward trend was observed. The authors attributed these changes mainly to climate change. The Weihe River basin runoff analysis from 1954 to 2014 was carried out in [7]. As in [11], the authors also identified three intervals--1954-1971, 1972-1993, and 1994-2014. However, in contrast to the Heihe River, for the Weikhe River, the annual runoff tended to increase in the first interval and decrease in the third. Fluctuations in the annual runoff relative to a certain value were also observed in the second interval. Changes in the runoff of the Volga and the rivers of its basin were studied in [8; 9; 10; 11]. It was found in [8; 9] that long-term fluctuations in the Volga runoff contain two periods, which differ significantly in long-term average values. This difference is mainly due to changes in low-water runoff. The authors of [10] directly link the long-term variability of the Volga runoff with climate change. In [11], the main trends in the annual and low-water runoff changes of the Volga basin rivers and the nature of the intra-annual distribution of the runoff were studied. The authors emphasize that the \"synchronism\" of changes in low-water runoff and the scale of these changes are extraordinary and have no analogs in the 20 th century. The increase in low-water runoff has led to an increase in water resources over the past 30-35 years, even in river basins where there has been a decrease in spring flood runoff. The runoff of another large river in the European part of Russia was studied in [12]. The work also revealed the intra-annual redistribution of the river runoff of the Upper Don. It is noted that this is typical for modern climatic conditions, which are expressed by an increase in both the average annual air temperature and the air temperature of the cold period of the year, as well as an increase in the number of winter thaws.