WATER MANAGEMENT SYSTEM LIPTOVSKÁ MARA – BEŠEŇOVÁ IN THE CONTEXT OF CLIMATE CHANGE

The presented article briefly presents the impact of the Liptovská Mara – Bešeňová water management system operation on the runoff conditions downstream the dam. The analysis showed that its benefit is evident, especially in the period of extreme hydrological phenomena, with the occurrence of long-lasting low flows or in the event of short-term flood flows. In the area of Liptovská Mara and Bešeňová, this is evident mainly in winter and spring, because of a small amount of liquid precipitation in winter and melting snow in the surrounding mountain massifs in spring. Other hydraulic structures’ actual experiences also convince us that Slovakia’s variability of natural conditions requires artificial water sources. In the interest of raising living standards, protecting the environment, and sustainable development, their role is irreplaceable. In recent decades, there has been an increasing debate about the growing demands on water caused by registered climate change and its effects on weather extremes, as well as human demographic change. History confirms that water reservoirs can reduce negative impacts caused by extreme hydrological phenomena – floods and droughts. In line with the need for adaptation to climate change and sustainable development, in addition to measures close to nature, technical measures, reservoirs, and dams are also necessary.


Introduction
The Liptovská Mara -Bešeňová water management system is located in the northern part of central Slovakia, in the upper section of the Váh River. It was built between 1967 and 1975, and it is one of the crucial hydraulic structures of the Slovak Republic. Its purpose is to produce electricity for peak hours, flood control, and improve the flows of the Váh for industry and agriculture. The Liptovská Mara reservoir, together with the second largest Orava reservoir on the Orava River, which is the right-side stream of the Váh, as top reservoirs control flows in the Váh, starting from the riverbed downstream Bešeňová reservoir to the estuary to the Danube (Fig. 1). Both hydraulic structures use the rivers' energy potential, significantly reducing flood flows and reducing possible flood damage. Through coordinated action in the system, they improve the river's flow rates in favor of domestic water and industrial or irrigation. They also enable the dilution of wastewater and improve the self-purification of the river. They also play a significant role in ensuring a sufficient flow in the Váh and its derivation channels when using the hydropower potential at the Vážská cascade. Both reservoirs simultaneously compose the scenery of the landscape. These reservoirs are an integral part of the country, and they enable sports activities and createcenters of tourism. Recurring hydrological extremes are also an accompanying feature of registered climate change and global warming. Short-term flood flows vary with long periods of drought. Thus, the question of the large water reservoirs' influence on the hydrological regime of streams under dams comes to the forefront. In this article, we present the impact of the Liptovská Mara -Bešeňová water management system operation on the runoff conditions in the Váh River.

Basic parameters and a brief description of objects
The objects of the Liptovská Mara are the dam and the reservoir, appurtenant structures as intakes and outlets of the hydroelectric power plant, spillways, and bottom outlets, a channel downstream of the dam. The objects of the Bešeňová are the dam with a compensating reservoir, an associated appurtenant structure, and a channel downstream of the dam. The essential data of both structures are given in Table 1. Other objects of the water management system are dikes for the protection of adjacent municipalities, dikes of the canal between the Liptovská Mara power plant and the Bešeňová reservoir, and pumping stations (Bednárová et al., 2010).

Description of the Liptovská Mara dam
The Liptovská Mara dam is located on the Váh River. The dam is an earth-fill heterogeneous dam with silty sealing (Fig. 2). A double-layer filter protects the silty sealing. Stabilizing prisms are made of gravelly and sandy soils, excavated in the alluvium of Váh. The subsoil of the dam was sealed from the grouting gallery by a grout curtain. The dam's upstream slope has slopes of 1:2.25 to 1:3, in the lower part 1:5. The downstream slope inclines 1:1.8; for every 10 m, there are 4 m wide berms. There is a drain on the downstream toe of the dam. A riprap protects the upstream slope; the downstream slope is humidified and grassed. Appurtenant structures consist of a spillway in the area of the left valley slope. These are two fields, 11.0 m wide with a height of 3.6 m and a total capacity of 268.50 m 3 s -1 . The left tunnel and the right tunnel used as bottom outlets with a total capacity of 80-280 m 3 s -1 . The capacity depends on the water level in the Liptovská Mara and Bešeňová reservoirs. The hydraulic structure's integral part is a pumped-storage hydroelectric power plant with an installed capacity of 198 MW (2 x Kaplan + 2 x Dériaz, flow 2·140 m 3 s -1 + 2·130 m 3 s -1 ). The geological composition of the Liptovská Mara and Bešeňová consists of a Paleogene and a Quaternary. Slope sediments were formed mainly by weathering of the Paleogene. The Paleogene is formed mainly by flysch layers of claystone and various types of sandstones. Quaternary sediments form slope debris with a low content of silty components. Fluvial sediments occur in two terraces, covered with a thick layer of sloping silt or clay. Eolithic sediments form a thick cover of terraces. In many cases, they are moved by deluvial material (mainly deluvial clays).

Description of the Bešeňová dam
The Bešeňová dam is situated at the river kilometer of 335.22 (Fig. 3). The backwater level is at an elevation of 522.09 m a.s.l. and creates a reservoir that serves as a compensating reservoir for the peak load hydropower plant of the Liptovská Mara. It is an earth-fill heterogeneous dam with silty sealing. The sealing of the dam protected by a double-layer filter on both sides is embedded to the subsoil by a silty-concrete cutoff wall and grouting curtain. There is a railway at the crest of the dam. The hydroelectric power plant has two direct flow Kaplan turbines with a capacity of 2·20 m 3 s -1 and with a maximum output of 2·2.4 MW with bypassed alternators.

Hydrological parameters
Liptovská Mara and Bešeňová are in the Liptovská basin, in the valley of the upper section of the Váh River. They are surrounded on the northeast by the Western Tatras, on the south by the Low Tatras, west by the Great and Little Fatra, and the northwest by the Choč Hills. The catchment area of the Liptovská Mara dam profile is 1,481.90 km 2 , and the Bešeňová dam profile 1,612.23 km 2 . The average annual rainfall is about 710 mm. There are 11 tributaries to the reservoirs, but Váh, Demenovka, Palúdžanka, Lupčianka, Jalovský, and Kvačianka have a significant influence (Fig. 4). The measurement results for the period 1978-2019 (Fig. 5) show that the average monthly inflow into the reservoirs is 27.988 m 3 s -1 and the average monthly outflow from the Bešeňová reservoir is 26.184 m 3 s -1 . The difference between inflow and outflow can be attributed to vapor losses and seepage. The system of Liptovská Mara -Bešeňová, besides its activities (usage of hydropower potential, reduction of flood flows, flows improvement for the industry, agriculture, improving water quality, recreation, and sports, navigation, and fishing), has to provide a minimum flow of 15 m 3 s -1 year-round in the river Váh downstream the compensating reservoir Bešeňová. Exceptionally, the outflow can be reduced to 10 m 3 s -1 if the flow of the left-side tributary Váh -Revúca downstream the Bešeňová exceeds 5.0 m 3 s -1 , or if the level in Liptovská Mara falls below the level of 562.00 a.s.l. in the period from July to December (due to reserving a sufficient volume of water in the reservoir for safe winter operation) (Lukáč et al., 1996).

Influence of hydraulic structure on outflow downstream the dam
The operation's influence on the outflow conditions downstream of the dam can be presented by the probability curve of the inflow into the reservoirs (Qp) and the outflow into the riverbed downstream the dam (Qo) (Lukáč et al., 1991, Votruba andPatera, 1997). Fig. 6 shows the average monthly flows in the winter months (January February), in the spring months (April and May), in the summer months (July, August), and in the autumn months (November December). The presented graphs show the minimum flow of 15 m 3 s -1 and its reduced value of 10 m 3 s -1 . The average monthly flow analysis shows that the water management system's operation redistributes them during the year. While in the winter months, the flows under the dam are significantly improved, in the spring months, the higher    flows are reduced. In the summer and autumn months, the reservoir's impact on the runoff conditions downstream of the dam is less significant. The water management system's positive function on the outflow ratios in the Váh can also be registered in Fig. 7. There are presented inflows and outflows with a probability of 5% (increased flows), 50% (average flow values), and 90% and 95% (low flow values). We can register the positive impact of the reservoir's operation on the hydrological regime of the Váh downstream the Bešeňová dam profile. The positive effect of the Liptovská Mara operation on the redistribution of flows is evident from the fluctuating annual cycle of inflowsthe accumulation of water in the spring months and increasing outflows in  1979 02.1980 03.1981 04.1982 05.1983 06.1984 07.1985 08.1986 09.1987 10.1988 11.1989 12.1990 01.1992 02.1993 03.1994 04.1995 05.1996 06.1997 07.1998 08.1999 09.2000 10.2001 11.2002 12.2003

Time (month -year)
Qo -outflow Qp -inflow the winter months. This is also confirmed by the decrease in the coefficients of the variation value of the analyzed average monthly inflows and outflows with a presented probability of occurrence of 5% to 95% (Table 2).
The production of green electricity must be added to the positives of the Liptovská Mara -Bešeňová water management system to ensure minimum flows under the dam. The Liptovská Mara and Bešeňová hydroelectric power plants have produced more than 6,500 GWh of green electricity from a renewable sourcewater, with an average annual production of 152.8 GWh (134.5 GWh + 18.3 GWh). An essential function of the water management system is also to improve the flow for hydropower plants throughout the Vážská cascade.

Conclusion
The Liptovská Mara -Bešeňová water management system, which has been in operation for 45 years, reliably fulfills all planned purposes. These hydraulic structures have significantly reduced floods in the Váh basin since 1975. Using the energy potential of Váh, the water management system contributed to the energy network by producing more than 6,500 GWh of green electricity. There is enough water for industry and agriculture by improving the Váh River flows through the Liptovská Mara reservoir. Within the framework of registered climate change and global warming, with recurring hydrological extremes, the function of this water management system is increasing 10 m 3 s -1 under the Bešeňová dam. The analysis showed that the winter season is the most vulnerable. Without the accumulation storage of the Liptovská Mara reservoir, approx. 362 million m 3 , for almost two months a year, the minimum flow of 15 m 3 s -1 would not be ensured in the Váh River. The added value of the Liptovská Mara and Bešeňová reservoirs is also the completion of the landscape character, the development of tourism, the possibility of recreational and sports use of this area.