water Article Decision Making Methods to Optimize New Dam Site Selections on the Nitra River Igor Gacko 1,2,*, Zlatica Muchová 3 , L’ubošJurík 1 , Karol Šinka 3, Ladislav Fabian 2 and František Petroviˇc 4 1 Department of Water Resources and Environmental Engineering, Faculty of Horticulture and Landscape Engineering, Slovak University of Agriculture in Nitra, 949 76 Nitra, Slovakia; [email protected] 2 Vodohospodárska Výstavba š.p, 831 02 Bratislava, Slovakia; [email protected] 3 Department of Landscape Planning and Land Consolidation, Faculty of Horticulture and Landscape Engineering, Slovak University of Agriculture in Nitra, 949 76 Nitra, Slovakia; [email protected] (Z.M.); [email protected] (K.S.) 4 Department of Ecology and Environmental Sciences, Faculty of Natural Sciences, Constantine Philosopher University in Nitra, 949 74 Nitra, Slovakia; [email protected] * Correspondence: [email protected] Received: 5 June 2020; Accepted: 16 July 2020; Published: 18 July 2020 Abstract: Grouping both existing and newly planned reservoirs based on selected measurable characteristics allows to point out issues that are relevant to area management using experience obtained from the environment of other sites. Divisive hierarchical clustering has been deployed to find similarities between dam locations. The Nitra River Basin (located in Nitra District, Nitra Region in Slovakia) with its 54 reservoirs is the model area. Profiles for 11 potential new reservoirs have been developed. Partial river basins were identified for each of the existing and new reservoirs using a digital relief model. The area size, proportion of arable land, forestland and built-up area, degree of exposure to soil erosion and the volume of surface runoff have been used as parameters for comparisons. Six clusters have been identified containing similar existing as well as new locations, one of them being a special case. Keywords: retention reservoir; land development plan; water retention; cluster analysis; regionalization 1. Introduction Water has been a basic requirement for humanity from the very beginning, and it used to be drawn from natural resources such as springs, rivers and lakes. It was when humans began to settle in larger numbers in urban regions that they found themselves in need of constructing adequate water reserves. Another reason why dams and reservoirs started to be built was to protect against erosion, flooding and fire, as well as for recreation and fishing, and rivers were where the construction of dams and hydraulic construction started, with channel reservoirs, irrigation canals and dam reservoirs necessary for supplying water or hydro-energy to cities and towns. In [1] dams are defined as water-retaining obstacles used specially to accumulate surplus water as well as for managing and allocating water to particular regions. Reservoirs provide protection, redevelopment, seepage, reserves, retention, implementation, accumulation and clearing. Just et al. [2] commented that for a reservoir to fully function, it is crucial for it to meet these criteria of renewal and construction: (A) retaining flood discharge—the reservoir has a retention area, including appropriate functional objects; (B) improving of water quality, especially in case of flow-through reservoirs, which may favorably improve poorer-quality water and (C) support biodiversity. What is essential is their recreational, Water 2020, 12, 2042; doi:10.3390/w12072042 www.mdpi.com/journal/water Water 2020, 12, 2042 2 of 16 aesthetic and hygienic significance [3,4]. All of a country’s reservoirs fulfil certain major and many minor functions. These reservoirs contribute toward raising water quality in river basins and have a special indispensable significance in areas with narrow watercourses and a sparse hydrographic network, contributing considerably toward balancing water source capacity, water quality and everybody’s needs at any season or location [5,6]. In addition to a reservoir’s positive impact on the living and natural environment and on people [7,8], including provision of drinking water, there are also outcomes with a negative impact. For instance, polluted water may rapidly spread various infections and be a critical factor in epidemic outbreaks [9,10]. Jurík et al. [11] classify reservoirs into three groups. There are hydromelioration structures (necessary for managing ground water in soil and for water reproduction of plants and animals); hydrotechnical structures (solving the biggest and most crucial tasks related to major watercourses and considerable water reserves); health and water management structures (for water management in municipalities, towns and production units). Dams whose wall height is at least 150 m are categorized as “gigantic dams”, which are absent in the Slovak Republic. Four of Slovakia’s 231 dams reach the highest height of 15 m. Out of that number, Nitrianske Rudno (registered at ICOLD—International Commission on Large Dams), located in Prievidza District in Trenˇcín Region is a part of the Nitra river basin modeled. The other three reservoirs are located in Bratislava Region, Banská Bystrica Region and Trenˇcín Region. The others are small reservoirs (MVN) whose dam heights are less than 15 m, with 53 of them situated in the area to be modeled. This paper focuses on small reservoirs. Jurík et al. [11] define small reservoirs as an artificially or naturally created space filled and drained with water depending on the season. This term may define reservoirs as designed to retain or accumulate water, maintaining a potential imbalance between water offtake from the reservoir and water inflow into it. Small reservoirs are an integral part of agriculture, significantly aiding in environmental protection [12] and providing protection against flooding, space for redevelopment, seepage, reserve, retention, implementation, accumulation and clearing. In rural areas, they are a significant factor that may restrict the occurrence of floods, soil erosion and other hydrological risks [13,14]. In terms of water management, they are divided into three types: reserve (accumulation) reservoirs, protection (retention) reservoirs and multi-purpose reservoirs. STN 73 68 24 (Technical standard-Small reservoirs) defines small reservoirs according to how they fulfill several requirements: A reservoir volume which reaches the surface of controllable space not bigger than 2 millions cubic meters, the greatest depth of the water in the reservoir does not exceed 9 m, and the hundred-year flow rate (Q100) in the dam profile is not bigger than 60 cubic meters per second, or in case of reservoirs into where water is drawn artificially, this value should not exceed Q100 of the reservoir’s river basin and inlet capacity. Small reservoirs contribute toward improving water quality in river basins and have special indispensable significance in areas with narrow watercourses and a sparse hydrographic network, contributing considerably toward balancing water source capacity, water quality and everybody’s needs at any season or location [15,16]. In terms of its position on a water source, reservoirs are further classified into spring reservoirs, which are situated on the edge of the hydrographic network and supplied by the discharge from spring snow melt and heavy rainfalls. The dimensions of these reservoirs catch a substantial part of the entire inflow from major runoffs, heavy rainfall and spring melting. Czechoslovakia’s National Water Management Plan (1956) highlighted the gradual construction of small reservoirs and polders in the Nitra River Basin to satisfy the needs of the country’s manufacturing plants and healthcare facilities, for social and cultural aspects, and to protect the surrounding area against the harmful effects of water. The construction schedule for new reservoirs set out rules to create a system that would allow for purposeful water management. This is also evident in the 26 reservoirs that had been constructed and commissioned by the end of the 19600s. The number of new reservoirs had been slowly declining until the construction of the most recent dam and reservoir in Haláˇcovce, Water 2020, 12, 2042 3 of 16 which opened in 1990 [17]. While proposals for new reservoirs have not disappeared from water management plans, there has been no dam and reservoir construction since 1998. After roughly ten years, climatic changes started to emerge, of which global warming was the most profound, occurring in all locations and climatic areas around the world. In March 2019, Slovakia unveiled an action plan to address water scarcity and drought. Water is Valuable (in Slovak “H2O dnota je voda”) was approved in an effort to reduce exposure to water scarcity, drought and the impact on human health, economic activities, environment and cultural heritage [18]. An increase in average temperature reduces the harvest on farmland that is not irrigated [19]. Frequent and intensive heavy rainfall causes extreme phenomena to appear that accelerate soil degradation, negatively lowering the soil’s retention capacity [20]. As uneven rainfall over the year occurs ever more frequently, the resulting floods put property in danger, directly endangering human lives [21]. The issue of constructing new dams and reservoirs has recently come to the forefront, especially small reservoirs and polders which would cover for the lack of water during extreme weather conditions, either drought or heavy floods, with the water in the reservoir reserved for such situations. Support for nationwide action in areas affected