Transformation of the water runoff of the Volga River after the creation of the Zhiguli hydroelectric complex

. For the Volga River, studies of interannual changes in water discharges in the period 1958-2021 were carried out. Systematic hydrological observations were carried out at the Zhiguli hydroelectric complex on the border of the Middle and Lower Volga. 97% of the water runoff of the entire Volga is regulated in this section in the seasonal, weekly and daily range. It was found that the average water flow was 7704 m 3 /s. The amplitude of interannual fluctuations was 5270-10431 m 3 /s. Temporal changes in the average annual water discharges were characterized by cyclicity, including the change of low-water and high-water phases. From 1958-1975 a low-water phase was observed, from 1976-1996 - high-water and since 1997-2021 - again shallow. A comparison of the historical (1877-1955) and modern (1958-2021) periods showed that over 144 years, the average water flow of the Volga River has not changed much. However, as a result of the regulation of water flow in the period 1958-2021 the value of water discharges in high-water years decreased, while in low-water years it increased.


Introduction
The Volga is the largest river in Europe, more than 40% of the population of Russia lives in its catchment area, and a powerful industrial and agricultural potential is concentrated here.Since the organization of hydrological observations on the Volga River in 1877, a real opportunity has arisen to study the dynamics of water runoff using instrumental measurement methods.With the growth of population and economic activity in the Volga basin, close attention was directed to assessing the impact of anthropogenic factors on the amount of water runoff [1][2][3].Further interest in the study of runoff dynamics was formed under the conditions of global climate warming that began in the 1970s [4][5][6][7].However, the question remains open -how the water flow of the river has changed in the modern period  after the creation of the Kuibyshev reservoir compared to the historical period .
The study of the dynamics of the water flow of the Volga River is necessary for assessing and predicting the quality of water resources, as well as in determining the anthropogenic impact, taking into account the natural features of the reservoirs.Therefore, the aim of this work is to quantify interannual changes in the period 1958-2021 and a multi-year trend since 1877.

Materials and methods
Regular measurements of water flow on the unregulated river in the village of Polyana named after Frunze began to be carried out since 1877 [8].Water consumption was measured using a hydrometric meter.Beginning in 1957, after the filling of the Zhiguli Sea, observations of the water flow of the Volga River were moved 40 km upstream from the village of Polyana named after Frunze (the site of the Zhiguli hydroelectric power station).After the creation of the reservoir, observations of the water flow of the Volga River began to be carried out in the hydroelectric complex.It is located 40 km upstream of the Volga River from the hydrometric site in the village of Polyana named after Frunze.At the Zhiguli hydroelectric complex, the water flow is determined by spiral turbine flow meters operating on the principle of measuring the pressure drop in a spiral chamber according to the Winter-Kennedy method.
The Volga River was dammed on October 31, 1955.The Zhiguli Sea was filled to the water level of 53.0 m BS on July 10, 1957.In 1958, the Zhigulevskaya hydroelectric power station, which controls 97% of the water flow of the Volga River, was put into operation.The Kuibyshev reservoir is the main regulator of seasonal water discharges in the lower reaches of the Volga River.The useful volume of the largest reservoir in Europe is 33.9 km 3 , and the total volume is 57.3 km 3 .The reservoir has a seasonal, weekly and daily mode of water flow regulation [9].
Through the hydroelectric complex, it is discharged into the lower pool along turbine ducts, weirs, transport structures (locks, raft walkers), special-purpose openings (fish passages, slush outlets, washing devices, etc.).The flow rate of water through the turbines of the hydroelectric power station is determined according to the data on the power developed by the unit and the operating flow characteristics of the unit.
Changes in water discharges of the Volga River were studied based on the materials of the state water cadastre [10] and data from the Zhigulevskaya HPP, a branch of JSC RusHydro, which regulates water flow based on current regulatory documents.The total water flow through the Zhiguli dam is determined by calculation and consists of the discharge of water through the turbines of the hydroelectric power plant, the spillway dam and lock chambers.
The values of the average annual water discharge were formed in series, which were subjected to statistical processing using the Statistica program.Modern statistical characteristics of the time series for the period 1958-2021 were compared with the characteristics of the historical time series for the period 1877-1955.

Results and Discussion
The catchment area of the Volga River in the alignment of the Zhigulevsky giroule is 1 210 000 km 2 .The number of years of observation -62.Average water discharge (Q) for the period 1958-2021 amounted to 7704 m 3 /s.The smallest annual water flow was observed in 1996 and amounted to 4954 m 3 /s, and the largest water flow was observed in 1990 and Amounted to 10431 m 3 /s and (figure 1).The coefficient of variation was 0.16, the coefficient of asymmetry was 0.13.Average flow Modulus (M) -6.36 l/s×km 2 .Based on the results of calculating the empirical provision of water discharge for the period 1958-2021, high-water and low-water years were determined.For dry years, there are "weak", "average" and "extreme" low water."Weak" water scarcity has a provision (P) of 50-75%, "medium" -75-90% and "extreme" -over 90%."Weak" high water supply has a provision of 50-25%, "medium" -25-10% and "extreme" -less than 10%.
Over the period of the Kuibyshev reservoir existence from 1958 to 2021, extremely high-water years (P ≤ 10%) include: 1990; 1994; 1979; 1991, 1966 and 1981.The range of average annual water discharges for these years is 9193-10431 m3/s.Extremely dry years (P ≥ 90%) include : 1975; 1973; 1996; 1967; 1976; 1977.The range of average annual water flow rates for these years is 5270-5984 m3/s.Over 62 years of observations, the number of extremely high-water and low-water years is the same and equals six.
For the period 1958-2021, undulating changes in water discharges were observed.Changes in Q were characterized by cyclicity, including the change of low-water and highwater phases.In the period 1958-1975 a low-water phase was observed (7184 m 3 /s).The low-water phase, if 1958 is conditionally taken as its beginning, lasted 17 years.The highwater phase was observed from 1976 to 1996 (8173 m 3 /s).It continued for 20 years.The subsequent less significant phase of low-water was observed from 1997 to 2021 (7653 m 3 /s).It has been going on for 24 years and so far its end has not been established.Longterm changes in annual water discharge are due to natural and climatic conditions in the Volga basin.The duration of periods with different water content averages 20.3 years and is close to the climatic cycle of solar activity with an average duration of 22 years.

Fig. 1. Fluctuations in annual water discharge
The performed calculations of water discharge (Q) and flow modulus (M) of different supply (P) (table 1) for the period of 62 years made it possible to distinguish extremely low-water and extremely high-water.The years 1975, 1973, 1996, 1967, 1976, 1977 should , 02 (2023) BIO Web of Conferences CIBTA-II-2023 https://doi.org/10.1051/bioconf/20237102020020 71 be attributed to extremely dry years (P> 90%).The years 1990The years , 1994The years , 1979The years , 1991The years , 1966 should be attributed to extremely high-water years (P <10%).In the period 1877-1955, systematic measurements of water discharge on the Volga River were carried out in the alignment of the village of Polyana named after Frunze (1).This site is located Downstream of the Volga River from the site of the Zhiguli hydroelectric complex (2) at a distance of 40 km.Therefore, the catchment area of the Volga River in the Frunze Polyana site is slightly larger than in the Zhiguli hydroelectric site.The catchment area for section 1 is 1 220 000 km 2 and for section 2 is 1 210 000 km 2 .The difference in catchment area is 0.08%.Between the indicated sections, the only river Sok flows into the Volga River with an average annual water discharge of 57 m 3 /s.
According to the data of observations of water discharges in 1877-1955, it was determined that the average discharge of the Volga River was 7740 m 3 /s in the alignment of the village of Polyana named after Frunze.The average runoff modulus is 6.33 l/s×km 2 .The coefficient of variation (KV) was 0.20, the coefficient of asymmetry (KS) was 0.07.According to the established runoff modules, the calculation of water discharges of various availability was carried out for two sections: the village of Polyana named after Frunze and the Zhigulevsky hydroelectric complex (table 2).Note: Q * -recalculated water discharges for the alignment of the Zhiguli hydroelectric complex The average values of water discharges and the Volga water runoff module for the period 1877-1955 at the observation point of the village of Polyana named after Frunze practically do not differ from the corresponding characteristics obtained at the site of the Zhiguli hydroelectric power station for the period 1958-2019.In the period 1958-2021, the KV values decreased from 0.20 to 0.16, and the KS values increased from 0.07 to 0.13 compared to the period 1877-1955.Comparing the empirical curves of water supply for two periods (figure 2) it was found that the amount of water discharge with low water supply decreased, while that of high water supply increased.

Conclusion
According to the results of the research, it was found that the average flow of the Volga River for the period 1958-2021 was 7704 m 3 /s.For this long-term period, fluctuations in water flow amounted to 5270-10431 m 3 /s.The dynamics of water runoff was characterized by cyclicity, successive change of low-water and high-water phases.In the period 1958-1975 a low-water phase was observed, in the period 1976-1996 -high-water phase and in the period 1997-2021again low-water phase.
Before the creation of the Kuibyshev reservoir, the average water flow of the Volga River in the alignment of the village of Polyana named after Frunze for the period 1877-1955 amounted to 7740 m 3 /s.This value differs slightly from the average water discharge (7704 m 3 /s) in the period 1958-2021 in the alignment of the Zhigulevsky hydroelectric complex.The average water consumption in the two periods differs by only 0.5%.But even this difference between water discharges will decrease when a correction is made for the difference in the catchment areas of the two sections.Consequently, over the period from 1877 to 2021, the average water flow of the Volga River at the site of the Zhiguli hydroelectric complex has not changed.However, when comparing empirical curves of endowments, differences between the two periods were established.In the period 1958-2021 due to the regulation of water flow at the Zhugulevsky hydroelectric complex, a transformation of the water flow availability curve occurred.Low water consumption decreased, while high water consumption increased.