Water reservoirs are the basis of modern water management and an integral part of the global hydraulic infrastructure [1
]. The exploitation of existing reservoirs and their sustainable management is a challenge, resulting primarily from the growing demand for water, the observed effects of climate change, and the reduction of their capacity due to the accumulation of sediments in them [2
]. In addition, reservoir management is a complex process that requires reconciling various, often opposing, functions, that hinder operational activities [4
]. Worldwide, reservoir water supplies approximately 30–40% of irrigated land, provides 20% of the world’s electricity production through hydroelectric turbines installed on barrages, and serves many other purposes, including flood protection, counteracting the effects of drought, recreation and creating conditions for inland navigation [6
]. At the same time, the use of individual facilities and the fulfilment of the above functions are associated with the need to maintain proper damming levels. Sustainable water resources of reservoirs depend on the preservation of valuable retention capacity. For this reason, sediment management is a key task in the operation of reservoirs [7
Accumulation of sediments in the reservoir reduces its capacity, hinders its operation, and may accelerate the rate of wear of the hydraulic infrastructure located on the barrage, which reduces the efficiency of the performance of specific functions and may result in higher maintenance costs [8
]. In this respect, an important factor is to maintain the continuity of sediment transport in the river downstream of a given impoundment. Therefore, when designing retention reservoirs, appropriate solutions related to small hydropower plants, ecology, fisheries, flood protection, river morphology, tourism, etc., are used [9
]. Proper diagnosis of reservoir desludging needs and planning of this process, together with subsequent management, is important for maintaining appropriate parameters of individual reservoirs, as well as retention in a given area [10
The average degree of filling reservoirs with sediment in Europe is 0.73% of the initial capacity each year, which is mainly influenced by fluvial processes occurring in facilities located in the dry and sparsely vegetated southern part of the continent. Lower capacity loss values are recorded only in North America (0.68%), comparable in Central (0.74%) and South America (0.75%), and higher in other regions: Asia (0.79%), Africa (0.85%), Australia and Oceania (0.94%), and the Middle East (1.02%) [11
]. For comparison, the average annual silting rate of 47 large water reservoirs in Poland, which is crucial for water management, amounts to 0.2% of the loss of their initial capacity [10
]. Annual economic losses due to the loss of 1% of the water retention capacity of reservoirs range from USD 6 to USD 10 billion [12
]. Therefore, sediment management should be obligatorily taken into account when designing new hydraulic structures for river training, as well as applying to the existing ones. The principles of sediment management should also be included in strategic or planning documents at a national level [11
The main reasons for the decrease in the capacity of retention reservoirs include: the geological structure of the catchment, relief, climatic conditions, vegetation, hydrological conditions and anthropogenic elements, such as the size of the reservoir, hydraulic structures for riverbed training, and land development in the catchment area [13
]. Land use and land cover are considered to be the most important factors influencing the amount of surface runoff and related erosion processes. The main drivers of sediment supply in many areas are natural (e.g., geology, topography and climate) and anthropogenic (e.g., changes in land use and management practices), especially in mountain and foothill areas [14
]. The topographic features of the reservoir itself (riparian width, slope, and elevation) and the associated ecosystem may also be important [15
]. Taking into account these factors during the stage of planning the construction of a barrage is extremely important, because decisions on the location of a given reservoir determine its future performance to a great extent, i.e., the effectiveness of fulfilling the functions for which it was created. So far, to a great extent, individual objects have been located mainly on the basis of engineering, economic and often also political analyses. To ensure sustainable development, location decisions should take into account the wider spatial and temporal context [4
]. It is also essential to prepare more detailed water management programs and plans, which the European Union states are obliged to do under the Water Framework Directive [16
]. Drought is also an important issue [17
]. In response to this, the Drought Effects Counteracting Plan was developed and implemented in Poland. The document’s purpose is to indicate the most important directions of action to help prevent a water crisis [18
]. Flood, next to drought, is one of the most severe and dangerous natural phenomena. In order to increase safety, periodic flood risk management plans are developed [19
] in accordance with the provisions of the Flood Directive [20
] and the Water Law Act [21
The motivation to conduct research is, therefore, to determine the appropriate conditions in the catchment, provision and maintenance, which will consequently enable the appropriate use of dam reservoirs. Therefore, the main objective of the conducted research is to assess the factors influencing the changes in the capacity of artificial retention reservoirs located on rivers. This paper focuses on the analysis of conditions affecting the diversification of sediment transport, affecting changes in the capacity of retention reservoirs. Three mountain reservoirs (with catchments) located on the Outer Western Carpathians, in the southern part of Poland, were selected as the subject of the study: Goczałkowice on the Vistula River, Rożnów on the Dunajec River and Tresna on the Soła River. Taking into account the results of previous studies, it should be stated that they are characterised by a different degree and average annual rate of capacity loss (respectively, 0.027 million m3
, 0.02% of the initial capacity, 0.923 million m3
, 0.40% and 0.172 million m3
, 0.17 %) [10
]. In the analysis of the reasons for changes in the capacity of the indicated reservoirs, the natural factor, i.e., the geological structure, was taken into account, as well as very important aspects related to human activity, from which the following choice has been made: hydraulic structures (all structures located on the main rivers and its tributaries, as well as longitudinal reinforcement of the banks have been taken into account) and the land use structure. This made it possible to indicate the most important natural impacts and anthropogenic pressures, which translate into the volume of suspended load supplied to reservoirs and its accumulation. The results of the analyses made it possible to formulate recommendations for the management of sediments within the existing reservoirs and the construction of new facilities, thanks to which the volume of retained water resources will increase. The results can be used to prepare plans or programs in the field of counteracting the effects of drought or flood protection.
Water reservoirs can be used to counteract the effects of drought, but they are also designed to achieve many other goals, such as flood protection, irrigation, municipal and industrial water supply, hydropower, water quality management, and recreation. Therefore, management of a multi-purpose reservoir is a complex process due to the potential conflict of interest between these objectives [39
]. Particularly large reservoirs reduce the frequency of hydrological droughts, shortening their duration and severity by storing water during floods and releasing it during the dry season [40
]. Acting in this way, we increase water resources and reduce surface runoff in favour of ground runoff. Thus, during droughts, water reservoirs play an important role in regulating flows to meet the established water needs [41
]. Therefore, increasing the potential conditions for water retention, i.e., the systemic ability to accumulate water resources and keep them for a longer time in the biotic and abiotic environment, is the optimal action to adapt to the effects of climate change and to mitigate these changes [42
]. The climatic scenarios for Poland show that an intensification of extreme hydrological and meteorological phenomena, including long-term periods of drought, should be expected [43
The ongoing and forecast changes in meteorological and climatic conditions will potentially affect the performance of the above-mentioned functions by retention reservoirs. The processes taking place in the catchment, related to the supply of sediment to reservoirs and its accumulation, may also contribute to limiting the possibility of their implementation, affecting the loss of capacity of reservoirs located in southern Poland: Goczałkowice on the Vistula, Rożnów on the Dunajec and Tresna on the Soła.
There is no doubt that geological conditions are an important factor for the design of a given reservoir and its correct location, particularly affecting its stability during the period of use. The geological structure of the catchment basins of the analysed reservoirs does not differ enough to be the most important reason for the loss of reservoir capacity. Therefore, the conducted research shows that human activity is the basic factor of the observed changes. Anthropogenic impacts on the natural environment are noticeable in the transport of suspended load (constituting the main clastic material moved by the Carpathian rivers), showing that it is retained in the Rożnów and Tresna reservoirs. As a result of human activity, initially increased and then decreased soil erosion in catchments and watercourse beds is noticeable, as well as increasing land development and urbanisation of individual areas, but also building hydraulic structures in catchments and river beds [26
Land use, including, above all, the presence of forests in the catchment, is presented by many researchers as the basic element determining the limitation of sediment supply of water reservoirs, affecting their retention capacity. In forested areas, less sediment is displaced as a result of surface runoff than in areas used for agricultural purposes, and thus, the effective use of reservoirs located in the lower parts of the Outer Western Carpathians is much shorter than facilities built in higher parts, including the upper forested sections of the Beskids [35
]. Interestingly, in places of forest felling (with a high density of used dirt roads), similar results with the neighbouring agricultural areas may be recorded, and the most rapid changes occur as a result of the progressing urbanisation of the catchment [13
]. The obtained information shows that some human activities, carried out in the analysed area since the mid-20th century, resulted in positive changes, including, above all, afforestation of areas no longer used for agriculture and the creation of orchards and plantations of fruit bushes. Their result is a significant decrease in sediment transport in rivers [26
]. In addition, land use (both the entire catchment area and the areas directly adjacent to the reservoir) is an important causative factor also affecting the quality of water in dammed reservoirs [44
The obtained results show hydraulic structures in the catchment area upstream of the reservoir are a very important factor in the transport of sediment, both on the section of the river where the barrage is located, and on the tributaries. Densely located dams on the Vistula and its tributaries upstream of the Goczałkowice reservoir cause a significant reduction in the transport of suspended load. As a result, the average annual rate of capacity loss is 0.027 million m3
, which is 1.1% of the initial capacity [10
]. In the catchments of the Rożnów and Tresna reservoirs, there is a lower density of transverse and longitudinal developments in river, stream beds and floodplains, which results in a much larger volume of clastic material entering the reservoirs.
As indicated by Kondolf et al. [4
] and Randle et al. [3
], basic methods of sediment management assume three categories of approaches that focus on balancing sediment outflows and inflows to stabilise reservoir capacity:
Reduce sediment yield entering the reservoir (watershed management practices).
Route sediments through or around the reservoir to minimise sediment deposition within the reservoir (sediment pass-through or bypassing).
Remove sediments already deposited in the reservoir (drawdown flushing or dredging).
In connection to the data obtained for the studied reservoirs in Poland, in order to extend the service life of dam reservoirs, i.e., the possibility of retaining a certain amount of water for the purpose of performing the functions for which they were designed, the use of hydraulic structures should be considered, as exemplified by the effective operation of structures found in the catchment area of the Goczałkowice reservoir. They effectively reduce the transport of suspended load, therefore every effort should be made to maintain them properly. The recommended course of action is also the development of hydraulic structures in the catchment area of the Różnów and Tresna reservoirs, i.e., on the Dunajec and Soła rivers, along with their tributaries, respectively. This will result in a reduction of sediment supplied to both objects, which is consequently largely accumulated within them, according to the presented results. When implementing this proposal, it should be remembered that due to the location of a transverse structure on the watercourse, located in the bottom of the river, the transport of mineral and organic matter is disturbed. Therefore, it is necessary to take measures simultaneously to limit the interference of these types of hydraulic structures in the river ecosystem, as well as to provide for compensatory measures [45
]. At the same time, it is suggested to take a broader approach to the proposal of using technical measures in basins of reservoirs by drawing up a strategic or planning document, comprehensively discussing the issue of sediment management in river basins. In addition, when analysing the location conditions of a potential new retention reservoir, it would be necessary to combine the analyses at the pre-design and design stages, by proposing appropriate hydraulic structures in the catchment area, which would constitute a comprehensive investment measure affecting the operational efficiency of a given facility. An essential element in these considerations, despite the lack of clear indications in the present study, is also land use. The experience of other researchers speaks for the need to address this issue. For example, the silting rate of the Porąbka reservoir on the Soła decreased more than 20 times after filling the analysed Tresna reservoir [13
], which took over the main supply of suspended load and further reduced its transport to a minimum. Therefore, the construction of new, deep reservoirs could be one of the elements of preparation for the assumed climate changes and the risk of hydrological drought.
The above recommendations are proposed to be implemented in the context of counteracting the effects of drought. In this regard, it is advisable to maintain the initial parameters of retention reservoirs, i.e., to maintain the designed capacity, as far as possible. It is important for the performance of specific functions by a given facility related to water management and the implementation of economic and social tasks, as well as for the needs of subsequent shortages in dry periods and increasing the value of flows on sections of rivers located downstream of barrages. Such action is also desirable due to the observed delays in the reaction of the hydrological conditions in the catchment to the current meteorological situation, e.g., the occurrence of normal (multi-year average) rainfall may not lead to adequate flows in rivers [46
]. In addition, the technical measures of the catchment basins of individual reservoirs presented in the article could slow down the outflow of meltwater and rainwater, and maintain appropriate water conditions (soil moisture) necessary for agriculture and forestry in the long term to increase the retention capacity. However, this must be combined with continuous monitoring and forecasting the periods of occurrence of hydrological drought, their range and intensity (scale of the observed situation).
It should also be remembered that this type of research in reservoir catchments, apart from the indicated positive aspects, is also characterised by various limitations. The critical element is the already stated availability of data and their reliability. The authors’ most significant challenge was data unification between the Polish and Slovak sides. This mainly concerned database attributes. The databases of individual countries differ, and it would be a good practice to prepare supra-regional materials (in this case, for the European Union). It was also planned to publish a lithological map, but the terminology and graphic design used were different, making it impossible to publish it in a qualitative form. In studying river processes, the volume of sediment delivered to the reservoirs and the degree of its accumulation is also essential. In Poland, the hydrological service carried out the last measurements in the 1980s. Since then, no work has been carried out in this area, which means that it is necessary to rely on possibly outdated and unreliable data when analysing sediment transport.
The current practice of continuously filling the studied reservoirs with sediment is unsustainable. These reservoirs have no capacity to retain sediment indefinitely, and without proper management, sedimentation will eventually deprive them of their original functions.
Achieving sustainable use of domestic water resources requires better data collection from monitoring the capacity of reservoirs and the supply of sediments from catchments, changes in the operation of damming structures, and modification of the law regulating the use of the environment.
In the era of climate change, the construction and maintenance of multifunctional reservoirs are the basis for water retention in order to protect against hydrological drought. Therefore, in terms of the existing reservoirs in Poland, it is necessary to maintain adequate water storage capacity. Based on the analysed causes of changes in the capacity of dam reservoirs in Poland (Goczałkowice on the Vistula, Rożnów on the Dunajec and Tresna on the Soła), it was shown that the solution that effectively reduces the transport of suspended sediment and is worth implementing in other areas is the use of appropriate hydraulic structures in the catchments of the reservoirs.
The analysis of land use forms of the studied catchments did not clearly indicate the importance of this factor for the reservoirs in question. This does not mean, however, that this aspect has no impact on fluvial processes, but it emphasises the need to check land cover changes in a broader time perspective, including the verification of land use in various periods of operation for a given reservoir. It also shows that a catchment is a set of communicating vessels and all conditions should be considered comprehensively.
The development of GIS tools for conducting spatial analyses and hydraulic modelling is necessary to expand the scope of research into the causes of changes in the capacity of retention reservoirs. As our research has shown, the use of basic hexagonal grids, with the classification of their parameters using natural breaks, enabled us to determine both the degree of anthropogenic pressure on various elements and natural elements within the selected catchments.