An Overview of Hydropower Reservoirs in Brazil : Current Situation , Future Perspectives and Impacts of Climate Change

Global climate change, related to the greenhouse gases emission, impacts hydroelectric power generation mainly due to the increase in air temperature and changes in the precipitation patterns. As a consequence, it affects basin evapotranspiration process, runoff, sediment transport as well as evaporation of reservoirs. This study analysed the current and future Brazilian context of hydroelectric reservoirs and investigated the potential impacts of climate change on hydropower generation and possible mitigation adjustments, giving relevant examples from around the world. Moreover, it is key to consider other factors that influence the availability of water resources such as: (a) upstream development of reservoirs, mainly the development of agricultural systems, which can contribute to increased water demand; (b) changes in land use, which can have an impact on soil degradation and sediment transport. Hydroelectric dams are a possible alternative to mitigate the impacts of climate change considering that those impacts could generate the need of adaptive actions. The assessment of climate change impacts’ projections anticipates possible future scenarios and can assist in strategic planning together with the definition of adaptive operational policies.


Introduction
Hydroelectric plants are the main sources of electricity in some countries (e.g., Paraguay, Norway, Brazil, Austria and Canada).Hydropower contributes about 85% of global renewable electricity [1,2].It is the main source of renewable energy in South America, followed by biofuels [3].In several regions of the world (e.g., Southeast Asia and South America), the hydroelectric plants are one of the pillars for the production of renewable energy, but, at the same time, they are vulnerable to the impacts of climate change [4].These changes are causing the majority of the variations in hydroelectric potential [5].Other factors, such as changes in land use and demand for upstream irrigation, can also influence water availability for energy generation [5][6][7].
Brazil has one of the largest hydroelectric potentials and it is believed that hydroelectricity will continue to play an important role in the next ten years [8].In the country, 80% of the energy generated comes from renewable sources [9].The Brazilian energy matrix is composed mainly of hydroelectric plants (60.9%), followed by fossil fuels (16.11%), biomass (8.74%), eolic (7.52%), import (4.91%), nuclear (1.20%) and solar (0.61%) [10].The 85 solar power plants account for 1,021,602 kW of the installed capacity, and only 0.61% of the energy matrix [10].Even though the country has areas with high solar irradiation, there are no available technologies for its deployment, showing a clear need for investments in national technologies [11].Most of the equipment required for the installation of solar photovoltaic and concentrated solar thermal plants is imported, which implies high costs [12].
Regarding energy sources, hydroelectric plants are likely to be more affected by changes in annual and seasonal precipitation as well as temperature increase [13,14], with smaller impact on other energy system sources [15].Two opposite sides must be considered in terms of energy generation and climate change: on one hand, there is energy generation producing emissions (including hydroelectric; through the decomposition of organic matter in reservoirs), which requires mitigation strategies; and, on the other hand, the impact of climate change on renewable energy sources, requiring adaptation strategies [13].
Water resource use for generation of energy is dependent on hydrological and climatic conditions [4][5][6]8,[15][16][17][18][19].Global warming may influence the intensity and duration of rainfall, the discharge volume of rivers, the increased evaporation rate of reservoirs and the amount of evapotranspiration of plants [20].Thus, these factors interfere in the water availability for energy generation and may trigger indirect effects such as changes in land use and an increased demand for irrigation [18].
Several impacts of climate change on hydroelectric reservoirs can be considered: hydrological changes, variation in sediment load due to flooding, variation in volume and river flow due to changes in precipitation, and the occurrence of extreme events (e.g., droughts and floods) that can generate the need for dam design and safety improvements [1][2][3][4][5][6]8].
Hydropower reservoirs are less susceptive to climate change than run-of-river schemes due to its storage capacity as well as being less seasonal flow dependent [18].This resilience depends on dry season duration [13].The changes in the flow affect the level of reservoir and, consequently, the capacity of energy generation [18,19].In Brazil, there are expectations that future hydroelectric installations will be of run-of-river due to environmental restrictions for the construction of large reservoirs, resulting in the use of the total installed capacity of water flow schemes only during the rainy season.In the dry season, other plants, such as thermoelectric plants, would need to be used to meet the energy demand [21].
Climate change must be taken into account in the process of water resources planning and management, so that measures are taken to adapt to future climate scenarios [6].Adaptation strategies for the operation of reservoirs should be implemented [5,16,22], since hydroelectric plants are fundamental for electricity in Brazil [19].
Through a review of the literature, this study aims:

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To provide an overview of the current state of hydroelectric plants in Brazil, focusing on reservoir plant types, providing insights into recent policies for the development of hydropower (Section 2);

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To assess the potential for new hydroelectric plants in the country (Section 3);

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To put the current state of hydroelectric plants in Brazil into context in comparison with other countries (Section 4);

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To present the possible impacts of climate change on hydroelectric power generation.To analyse and compare methodologies and results of studies that evaluate the impact of climate change on energy generation by hydroelectric reservoirs (Section 5);

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To identify measures to adapt to future climate scenarios that have been the focus of studies in Brazil and in other countries.(Section 6);

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To present a framework directed to the study of climatic and environmental impact on the generation of hydroelectricity (Section 7).

Current Situation of Brazilian Hydroelectric Plants
Brazil has a great hydroelectric potential, totalling 101,268,561 kW, which corresponds to 60.9% of the energy matrix.There are 1313 operating plants ranging from hydroelectric generation centrals (with up to 1 MW of power installed), small hydropower plants (between 1.1 MW and 30 MW) and hydroelectric power plants (with more than 30 MW).The 218 hydroelectric power plants (HPP) are responsible for 60.5% of the total installed capacity in the country, adding up to 95,619,468 kW.The small hydroelectric plants (SHP), 428 in total, answer for 3.2% of the total installed power.The 667 hydroelectric generation centrals (HGC) in operation account for only 0.4% of the installed power [23].
The largest Brazilian hydroelectric power plant is Itaipu, located in the Paraná River [8].After its construction, the Belo Monte hydroelectric in the Amazon basin will be the third largest hydroelectric power plant in the world (11,233 MW).The second largest is Itaipu, producing 14,000 MW, and the largest in the world is the Three Gorges hydroelectric plant in China, producing 22,500 MW [2,8].The main Brazilian plants with regularization reservoirs are presented in Table 1 and sorted by region and importance in useful volume.The Serra da Mesa hydroelectric power plant on the Tocantins River has the highest useful volume, at 43.25 km 3 ; followed by the Tucurui plant, in the same river, at 38.98 km 3 , which stands out in terms of installed power at 8370 MW [24].Usually the levels of the reservoirs are higher in the summer, from December to March (more abundant rainy period), and lower between April and November, when there is the dry season [8].
Recent studies addressed the current situation of SHP in Brazil, exposing regulations and policies for the growth of the number of plants and the perspectives for the sector [25].Brazilian SHPs are expanding due to their characteristics of small area occupancy and minor environmental impact, besides the possibility of installation closer to the places of consumption, which reduces the loss of energy in the transmission and, consequently, results in a reduction of costs for the consumer.
The main Brazilian hydroelectric reservoirs are in the Paraná river basin.The concentration of hydroelectric power plants is higher in the Midwest, South and Southeast regions (Figure 1).The Southwest region accounts for 70% of the energy storage capacity.The largest hydroelectric reservoirs are installed near the regions of greatest demand.When full, these reservoirs can store energy to meet the demands for five months, a much shorter period than that observed in the 1970s, when the energy stored in the reservoirs, at maximum capacity, could meet the demand for three or four years [26].These changes arise as a result to the increase in energy demand caused by population growth, in addition to climate and environmental changes.

Recent Policies for the Development of Hydropower
In Brazil, renewable energy sources are considered fundamental for sustainable development [28].Two strategies with regard to the energy sector have been incorporated in Brazil, one focusing on a clean and renewable energy matrix, mainly hydroelectric, and the other promoting efficient use and conservation [11].However, the country needs to review current energy policies in the context of economic growth and also to consider climate change, with investments in energy efficiency, renewable sources and the necessary technological improvements [29].
Brazil has opted to increase energy security levels, increase energy generation capacity, and, with this expansion, environmental and social problems have arisen.In order to avoid greater environmental impacts, the main strategy goes towards new projects for run-of-river.However, this policy ends up affecting the generation of energy during dry periods, due to lesser river flow [30].To achieve energy security, the country has invested in hydroelectric and thermal plants, in order to compensate for the lower generation of hydropower during dry periods [29].
In run-of-river schemes, energy costs are higher due to the possibility of inactivity in dry periods caused by the absence of a reservoir.The construction time is shorter, on average two years.With regard to environmental impacts, the cumulative effects of several run-of-river installed in the same basin must be considered [8].In addition, the implementation of run-of-river hydropower

Recent Policies for the Development of Hydropower
In Brazil, renewable energy sources are considered fundamental for sustainable development [28].Two strategies with regard to the energy sector have been incorporated in Brazil, one focusing on a clean and renewable energy matrix, mainly hydroelectric, and the other promoting efficient use and conservation [11].However, the country needs to review current energy policies in the context of economic growth and also to consider climate change, with investments in energy efficiency, renewable sources and the necessary technological improvements [29].
Brazil has opted to increase energy security levels, increase energy generation capacity, and, with this expansion, environmental and social problems have arisen.In order to avoid greater environmental impacts, the main strategy goes towards new projects for run-of-river.However, this policy ends up affecting the generation of energy during dry periods, due to lesser river flow [30].To achieve energy security, the country has invested in hydroelectric and thermal plants, in order to compensate for the lower generation of hydropower during dry periods [29].
In run-of-river schemes, energy costs are higher due to the possibility of inactivity in dry periods caused by the absence of a reservoir.The construction time is shorter, on average two years.With regard to environmental impacts, the cumulative effects of several run-of-river installed in the same basin Water 2018, 10, 592 5 of 18 must be considered [8].In addition, the implementation of run-of-river hydropower plants can also contribute to the increase of emissions indirectly, with the need to complement the hydroelectric potential with thermoelectric plants [29].
In contrast, large reservoirs maximise electricity production, ensuring greater energy security of the system during periods of drought [5,31].However, some factors may affect the decision on the construction of hydroelectric reservoirs such as: building time; greenhouse gas emissions; fragmentation of the rivers; changes in water quality; relocating or displacing humans; interference in migration and spawning fish.
The average building time of large dams is 8.6 years [32].Since the 1990s, possible emissions from reservoirs have attracted the attention of several researchers [33][34][35][36].Even though it is a renewable source of energy, there are potential greenhouse gas emissions from reservoirs, such as: methane (CH 4 ), nitrous oxide (N 2 O), and carbon dioxide (CO 2 ).Large amounts of organic matter remain in the reservoir when the area is flooded, leading to the production of these gases.One of the major concerns is the emission of methane [37].According to Zarfl et al. [38], the quantitative emissions depend on how water is released in dams.The release of deep water contributes most significantly for emissions.
Studies carried out in areas in riparian and grassland environments of the Belo Monte hydroelectric complex (Pará) indicate that the most significant concentrations were CO 2 with a daily mean value of 10,448.41± 3036.48 mg m −2 for pasture and 8004.50 ± 1314.98 mg m −2 for forest [2].The Belo Monte mill may produce 1 million tons of CO 2 per year during the first ten years [12,39].Dams such as Jirau and Santo Antônio will have lower emissions, due to the size of the reservoirs [40].
The amount of emissions varies according to the age of the reservoir, the type of land use before impoundment, management practices, climatic conditions [1,2] and/or the location and morphometry of the reservoir [38].Moreover, it should also be noticed that greenhouse gas emissions in reservoirs are higher in the first 10 years [39,40].
The construction of reservoirs also contribute to the fragmentation of the rivers, relocating or displacing humans (especially of indigenous) [38], changes in water quality [41], interference in migration and spawning fish, flooding and drainage of areas for fish production [42].Pringle et al. [43] relate some fish-related effects: population fragmentation and isolation, migration interference, increases in exotic lentic-adapted species (non-native fishes) and the threat of extinction of some species.Therefore, dams are fundamental for the maintenance of society; however, one must think about environmental sustainability and socio-economic impacts, i.e., the best way to build, operate and maintain dams [44].

The Potential for New Hydropower Plants in Brazil
Considering the focus of the hydroelectric expansion in the Amazon, due to the nearly exhausted hydroelectric potential in other Brazilian regions [30], important environmental concerns are raised regarding the hydroelectric potential in the Amazon region and studies reporting the impacts caused by dams in this region, such as the emission of greenhouse gases due to flooding of forest areas, the loss of conservation areas, changes in water quality, interference with the displacement and reproduction of aquatic animals, a decline in biodiversity as well as social and cultural impacts [29,31].
The socio-environmental constraints and vulnerabilities of the use of hydroelectric potential, for each Brazilian region, include flood risks in large areas (Northern region), few perennial rivers (Northeast and South), border rivers, floods and agro-industrial pollution (Southeast) and border rivers with high levels of sedimentation (Midwestern region) [30].Most of the hydropower plants under construction are located in the state of Mato Grosso and Parana (Table 2).The hydropower reservoir (Sinop) and run-of-river (São Manoel) located in the Amazon basin are scheduled to start operating in 2018 [45].

Comparison to Other Countries
Most of the large dams around the world were built between the years of 1950-1989 and are related to population growth and consequently increased energy consumption.Regarding their main use, 39% were built for power generation, 29% for irrigation, 14% for flood control, 8% for water supply and 10% for other uses [44].The reduction in growth rate of number dams built since the 1990s is due to social and environmental concerns, high financial costs and reduction of favorable locations [44].In Brazil, there was an increase in the deployment of large hydroelectric plants in the 1960s and 1970s due to changes in tax criteria, followed by a reduction in construction of new developments in the 1980s and 1990s due to higher inflation and two oil shocks [46].
The five largest producers of hydroelectricity in the world are: China, Canada, Brazil, the United States and Russia [47].According to estimates, hydroelectric production may double by 2050, contributing to the reduction of CO 2 emissions from fossil fuel plants [1].When predicting population growth, water, food, energy consumption and dam development from 2010-2050, Chen et al. [44] foresee about 36,813 dams worldwide, with a total reservoir capacity of 9204 km 3 in 2050, and, in 2010, there were 32,473 dams with a capacity of 7975 km 3 .In terms of energy consumption, the estimate is an increase from 134,000 to 183,000 TW h.Future dams will be concentrated in developing countries and emerging economies (Southeast Asia, South America and Africa).Approximately 75% of the dams will be of medium and small size (1-100 MW) and 93% of the future hydroelectric power capacity will be provided by large dams (more than 100 MW).In Brazil, the dams under construction are concentrated in the North and South regions and those planned are in the South and Southeast regions of the country [38].

Possible Impacts of Climate Change on the Generation of Hydroelectricity
The increase in greenhouse gas emissions and consequently climate change demonstrates having an impact on energy production in Brazil, as well as in other countries that rely heavily on hydropower [29].Climate change affects the performance and safety of dams in various ways [14], such as: increased sediment load of rivers, impacting turbine operation and loss of storage; increasing reservoir surface evaporation, reducing the amount of water available; generating changes in rainfall and river flows, interfering with the turbines full capacity operation; and increasing the magnitude of floods that can lead to failures in dams.The main factors that interfere in the generation of hydroelectric power globally [13] are:

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Reduction in average precipitation that implies a reduction in runoff (three to four times).

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Increase in average precipitation.Depending on storage and turbine capacity, increased precipitation can lead to an increase in power generation potential.However, this does not always occur, considering that plants are designed for a certain river flow, and the increase of this flow would only lead to unproductive spills.

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Increase in average temperature.Rising temperatures will affect soil moisture levels, interfering with the runoff and storage of water in dams.In addition, increase of temperature causes changes in atmospheric pressure and wind patterns.These changes can alter the precipitation patterns [30].
• Extreme droughts.Depending on the duration of the drought period, the reservoirs act as a buffer, maintaining normal power generation capacity.

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Flooding.It entails sediment loads beyond what is expected, and depending on installed capacity, they do not always bring benefits in terms of increasing the potential for generating power.
There are also the indirect impacts of climate change: water scarcity and competition among other sectors (e.g., agriculture, industries, urban consumption); soil degradation and siltation, depending on the vulnerability of the soil to the action of rainwater and river, scarcity of vegetation and soil desiccation during periods of drought, promoting erosion and transport of sediments to reservoirs [13].
The most cited studies in the literature on the impact of climate change on hydroelectric power production are that of Christensen et al. [48] and Lehner et al. [49].Lehner et al. [49] analysed the effects of global changes on Europe's hydroelectric potential by applying the WaterGAP model (Water-Global Assessment and Prognosis) [50] for runoff calculations and compared current climate and water use conditions with future scenarios.The study pointed out severe future changes in the discharge regimes that will lead to instability in the European hydroelectric potential that could be reduced by 6% by the 2070s.Christensen et al. [48] simulated future climatic conditions and their effects on the Colorado River basin.The results of this study indicated warming, reduced precipitation, decreased runoff and a significant reduction in annual hydroelectric power generation: 56% (2010-2039); 45% (2040-2069); and 53% (2070-2098) in relation to the simulated history.
For South America, the projections indicate an increase in temperature and an increase (or decrease) in precipitation up to 2100; for some regions, there are large uncertainties related to precipitation.Warming varies from 1.7 to 6.7 • C by 2100 and precipitation varies according to the region (−22% in Northeastern Brazil, +25% in South-Eastern South America).In Brazil, the Amazon is highlighted as a hotspot of global warming, a region most vulnerable to climate change [3].
In Brazil, few studies have evaluated the impacts of climate change on hydropower reservoirs [21,51].Some studies have more generally evaluated the impact on river basins, not directly evaluating hydroelectric reservoirs [52,53].Other studies analysed the vulnerability of hydroelectric generation to changes in climate [30,54].
Soito and Freitas [30] reported the risks to energy security in Brazil; the generation of electricity depends significantly on the hydrological availability that can be threatened by increasing water demands (driven by demographic growth and economic development), droughts in the Northeast, and degradation of rivers in the Southeast region (pollution, silting, etc.).In a more recent study, Sorribas et al. [53] examined projections of climate change (2070-2099) and their impacts on discharges from the Amazon basin and identified that, for some regions of the basin, there will be an increase in river discharge (Northwest, annual mean +9%) and, for other regions (central Amazon, annual minimum −15.9%), there will be a possible reduction of flow in dry seasons, which may impact the generation of energy from future hydroelectric plants.
Lucena et al. [21] evaluated the impacts of climate change on Brazilian river basins.The results point out a tendency for reduction in average annual flow in the period 2071-2100, mainly in the Parnaíba (−10.3%),São Francisco (−26.4%)and Tocantins-Araguaia (−15.8%)basins (North and Northeast regions of Brazil).In the South and Southeast regions, impacts will not be significant, so, in these regions, reservoir management would mitigate the effects of climate change.According to the results of an investigation by Ribeiro Júnior et al. [51], if climate change scenarios are confirmed, Furnas reservoir levels may decline sharply.The authors proposed a methodology that considers the determination of the length and transgression frequency of level to minimise the conflicts of water use and the impacts on the generation of energy.
Analysis conducted by Nóbrega et al. [55] in the Grand river basin showed an increase in average river flow at the Agua Vermelha reservoir (+10% in the A2 Scenario, from 2508 m 3 s −1 to 2748 m 3 s −1 ).Still, according to the study, the increase of 3 • C in global mean temperature can result in an increase in average river flow by 24% (from 2475 m 3 s −1 to 3070 m 3 s −1 ) generating flood risks.However, some projections indicate a reduction in average river flow by up to 20%.In assessment of climate change impacts on river basins, there are uncertainties about greenhouse gas (GHG) emission scenarios and global circulation models (GCMs), as well as uncertainties regarding downscaling and hydrological modelling [6].GCMs produce climate projections, providing information on climate uncertainties and water availability more roughly, and are used directly in forecasting runoff in more general studies [14].The simulations of climate change impacts are generally carried out with a control period and future scenarios of 30 years, comprising three phases of simulation, namely: model development, control simulation (base period), and simulation of future scenarios [4], in addition to using different variables (Figure 2).
Water 2018, 10, x FOR PEER REVIEW 8 of 19 increase in average river flow by 24% (from 2475 m 3 s −1 to 3070 m 3 s −1 ) generating flood risks.However, some projections indicate a reduction in average river flow by up to 20%.In assessment of climate change impacts on river basins, there are uncertainties about greenhouse gas (GHG) emission scenarios and global circulation models (GCMs), as well as uncertainties regarding downscaling and hydrological modelling [6].GCMs produce climate projections, providing information on climate uncertainties and water availability more roughly, and are used directly in forecasting runoff in more general studies [14].The simulations of climate change impacts are generally carried out with a control period and future scenarios of 30 years, comprising three phases of simulation, namely: model development, control simulation (base period), and simulation of future scenarios [4], in addition to using different variables (Figure 2).Assessing the impact of climatic changes in impoundment schemes is rather complex [4,18], requiring information such as inter-basin transfers and design specifications.The existing literature generally evaluates single locations, not establishing how to include multiple sites at national or regional levels, due to the need for detailed information on the operational behavior of each site.As reservoirs can serve different uses (e.g., irrigation), the indirect impacts of climate change on hydroelectric dams should also be evaluated [19].
The meteorological variables temperature and precipitation are important in climate change studies for hydroelectric management [13,16,20,56,57].Some authors only model precipitation, considering it the higher impact factor on the availability of water for power generation [21].Other authors propose a combined study between climatic changes and changes in land use [6,7] or between climate changes and demands for irrigation [5].The inclusion of sediment load assessment in hydroelectric basins tends to be a significant contribution to the study of hydrological models.Another approach complements the gap: the need for an analysis of water demand, mainly for irrigation.
Some studies adopt only one scenario, not considering the different views of the general circulation models on the evolution of the climate and its repercussion in terms of the river basins, different from the one done by Haguma et al. [6], López-Moreno et al. [7], Gaudard et al. [17] and Parkinson et al. [57].The use of several scenarios assists in the estimation of different uncertainties involved in hydro-climatic modelling [57], such as parameter uncertainty and model structural uncertainty.Climate models that consider a multi-model set, such as ENSEMBLES or CMIP5, provide more reliable assessment since they include different uncertainties [4].
Table 3 presents some insights on the possible climatic changes and their consequences in the runoff, flow of the rivers and generation of energy in hydroelectric reservoirs.The table includes the methodology used in each study to evaluate how different uncertainties (observational uncertainty, parameter uncertainty and model structural uncertainty) were included.A comparison between Brazil and other countries was also carried out.
In order to assess the impacts of climate change on water resources, it is necessary to predict the runoff of the basin.A model commonly used is the IHACRES, introduced by Jakeman and Hornberger [58], which interfaces climate change input data (temperature and precipitation).The IHACRES is a rainfall-runoff model used to calculate the flow of a river, considering climatic changes.
In some regions, such as in Canada, Manic-5 and Toulnustouc reservoirs, studies indicated positive effects in relation to climate change and power generation.These results are associated with the specific characteristics of the basins; however, as highlighted by the authors, in the long-term, global warming will provide potential negative effects on energy generation [6].

Adaptations to Reduce Vulnerability and Increase Resilience
Soito and Freitas [30] suggested that, in addition to the management of Brazilian reservoirs in terms of optimisation and adaptation to climate change, other measures should be taken into account for planning, execution and control phases of the plants, namely: establishment of an orderly use of the soil in the drainage basin; conservation of vegetation growth; regulation of the rivers flow and their tributaries; and the controlled disposal of industrial waste.It should also be considered the monitoring of sediment transport [70].An overview of different adaptation strategies to reduce vulnerability of hydroelectric reservoirs to climate change is given in Table 4.

Study
Adaptations to Reduce Vulnerability of Hydroelectric Reservoirs to Climate Change [13] Obtaining information on the climatic impacts (monitoring of the climate and the runoff of the basins) in the hydroelectric generation and incorporating climatic risks in its management; Investments in adequate infrastructure, with equipment designed to operate in different climate conditions (e.g., turbine types); Modernization of plants to enable long-term sustainability; Development of drought management plans to deal with water competition; Modifications to operating rules; Management of land use to reduce soil erosion in the basin and reservoir; Consideration of the diversification of the energy matrix.
[22] Dynamic management with water allocation adjustments. [ Controlled disposal of industrial waste; The conservation of vegetation growth; Regulation of flows of rivers and their tributaries.
[57] Adaptations between hydroelectric and alternative scenarios involving other sources of energy.
[6] Adaptation of the operational policy to the future hydrological regime with adjustments in reservoir levels in seasons.
[7] Simplified water management schemes based on the operational history of the dam, applying restrictions to the releases of water from the dam.
[17] Operational rules optimized for balancing power generation (optimization through hydraulic loading).
[4] Flexibility in the storage of water obtained through pump-storage HPP systems.[51] To apply restrictions to the releases of water from the dam.
[26] Store energy by pumping water to a new reservoir during the wet period and generate energy by releasing the stored water during the dry period (Enhanced-Pumped-Storage).
The adaptability of hydroelectric schemes to climate change is dependent on local characteristics, specifically: project specifications; storage volume; morphological regime and environmental legislation [4,18].For example:

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Reservoirs with smaller surface area and greater depth tend to be less affected by global warming, which increases evaporation rate of reservoir (according to previous study, 1.1 m depth on average per year) [13].

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Some hydropower systems offer flexibility of water storage, pump-storage HPP.In periods of lower demands, the water is rebounded to a reservoir with higher elevation, and, in periods of higher demand, the water is released by the turbines.In this case, the generation of hydroelectricity is less dependent on changes in the hydrological regime [4].The limitations for the construction of these hydroelectric schemes in Brazil are related to the establishment of regulatory bases and economic viability.

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Optimised operating rules can balance power generation [7,16,17,20,57].An example of such operational policies would be raising and lowering reservoir level during some seasons.Haguma et al. [6] proposed as adaptive operational policies for Manicouagan Reservoir (Canada) the lowering in reservoir water level during winter and raising during spring.For the Furnas HPP reservoir (Brazil), Ribeiro Júnior et al. [51] proposed an operative rule that contemplates the time and the cycle of attendance to a certain level of the reservoir, to attend to the generation of energy and to guarantee the sustainability of multiple uses of water for the future climate.
Analyses indicate that hydroelectric managers usually consider historical hydro-meteorological data in the planning and design of new hydropower stations, but data should be also included on the possible impacts of climate change [14].In Brazil, this new strategy is also necessary.With the inclusion of climate change impacts in planning, even if there are changes in flows, the plant can act in a balanced way, reducing losses in energy generation.
According to Lumbroso et al. [14], this strategic management leads to a set of possible options for hydroelectric schemes and impacts the amount of energy generated and future economic return.In Brazil, there is a need for climate and hydrological forecasting to anticipate hydrological risks and to assess the impacts and vulnerability of hydroelectric plants considering changes in climate and adaptation measures [30,51].However, the influence of the actions taken is greater during the hydroelectric planning and design phases [14].
The diversification of the energy mix is considered a form of adaptation [13].Thus, in Brazil, different sources of renewable energy can be managed in parallel:

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Encouraging the use of solar and wind energy can contribute to the reduction of greenhouse gases and to Brazilian social and economic development [28].Biomass as an energy source is also interesting due to the amount of waste from agriculture and forest products industries, extracted mainly from the sugar and alcohol industry [11,12].

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Sources such as biomass (from bagasse and sugar cane straw) and wind power can contribute significantly to the generation of energy in periods with lower rainfall intensity, as it coincides with the more intense potential of these sources [8].It can complement the generation of hydroelectricity in the period of greater fragility, especially the run-of-river hydropower plants.

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The great annual variability in the hydrological conditions and the intense rain seasonality can put at risk the Brazilian hydrothermal energy generation system.However, a complementarity between hydroelectric, wind and solar photovoltaic systems could contribute to the stability of production and a decrease in thermoelectric generation [71].

Discussion
The assessment of projections of climate change allows better understanding of the magnitude of water and energy risks as well as the adoption of adaptation measures to meet future energy demands.The vulnerability and resilience of each hydroelectric plant depends on specific characteristics such as the type of plant (reservoir, pumped-storage or run-of-river), operating strategies, turbine resistance to sediments and the vulnerability of the hydrological regime.
Studies of the impact of climate change on the hydrological regime anticipated possible future developments, but interpretation should cover modelling uncertainties, especially related to climate change scenarios.These studies provided an overview of the trends and enable the establishment of hydroelectric reservoir management strategies.The impacts on the availability of water to the reservoirs can be predicted, which facilitates the designation of a plant operation strategy in order to mitigate the effects throughout the year or the adoption of other measures to guarantee demand to be met.
When establishing adaptive strategies, it is also necessary to investigate the impacts of adaptive actions on ecological integrity of rivers downstream of the reservoir, and there should be concerns related to maintenance of environmental flows, as highlighted in Spalding-Fecher et al. [5].
The analyses of the temperature and precipitation variables are fundamental in projections of climatic changes and in the evaluation of impacts on the generation of energy by hydroelectric plants.Precipitation directly influences the refilling of aquifers and river flow, i.e., the availability of Water 2018, 10, 592 14 of 18 water resources.Temperature influences the evaporation of the reservoir and the evapotranspiration of the vegetation-also impacting the availability of water in the reservoir.Variations in temperature also impact the behavior of other climatic variables (wind speed, humidity, precipitation).Investigations also highlighted the need for joint evaluation of temperature and precipitation, since evapotranspiration and evaporation may counterbalance precipitation [5,6,30].That is, even if there is an increase in rainfall in some regions, higher temperature would increase the rates of evaporation of the reservoir and evapotranspiration in the basin, generating loss of water to the atmosphere.Other variables that can complement the analysis: demand for irrigation and land use-related to the silting of river banks and changes in evapotranspiration rates.
In Brazil, climate change trends indicate that the regions that will suffer the most changes will be the Amazon and the Northeast.Considering the Amazon, strategies should be implemented, due to the clear tendency, for the region, of construction of run-of-river plants, the kind of power plant in which the impacts of global warming are most significant.Considering the Northeast, in strategic terms, the country can opt for complementarity with the development of other renewable sources of energy, such as wind and solar, given its enormous potential for generation, as evaluated in the study of Jong et al. [12].
Based on the study of several articles dealing with the impact of climate change on the management of hydropower reservoirs and other related articles, a framework was elaborated (Figure 3).be met.
When establishing adaptive strategies, it is also necessary to investigate the impacts of adaptive actions on ecological integrity of rivers downstream of the reservoir, and there should be concerns related to maintenance of environmental flows, as highlighted in Spalding-Fecher et al. [5].
The analyses of the temperature and precipitation variables are fundamental in projections of climatic changes and in the evaluation of impacts on the generation of energy by hydroelectric plants.Precipitation directly influences the refilling of aquifers and river flow, i.e., the availability of water resources.Temperature influences the evaporation of the reservoir and the evapotranspiration of the vegetation-also impacting the availability of water in the reservoir.Variations in temperature also impact the behavior of other climatic variables (wind speed, humidity, precipitation).Investigations also highlighted the need for joint evaluation of temperature and precipitation, since evapotranspiration and evaporation may counterbalance precipitation [5,6,30].That is, even if there is an increase in rainfall in some regions, higher temperature would increase the rates of evaporation of the reservoir and evapotranspiration in the basin, generating loss of water to the atmosphere.Other variables that can complement the analysis: demand for irrigation and land use-related to the silting of river banks and changes in evapotranspiration rates.
In Brazil, climate change trends indicate that the regions that will suffer the most changes will be the Amazon and the Northeast.Considering the Amazon, strategies should be implemented, due to the clear tendency, for the region, of construction of run-of-river plants, the kind of power plant in which the impacts of global warming are most significant.Considering the Northeast, in strategic terms, the country can opt for complementarity with the development of other renewable sources of energy, such as wind and solar, given its enormous potential for generation, as evaluated in the study of Jong et al. [12].
Based on the study of several articles dealing with the impact of climate change on the management of hydropower reservoirs and other related articles, a framework was elaborated (Figure 3).This research provides several conclusions related to climate and environmental changes in the context of hydropower reservoirs, which are useful for the academic and industry communities.A summary of the relationship between different variables is presented (see also   This research provides several conclusions related to climate and environmental changes in the context of hydropower reservoirs, which are useful for the academic and industry communities.A summary of the relationship between different variables is presented (see also Figure 3):

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Global climate change affects power generation in hydropower reservoirs;

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The increase in temperature causes changes in atmospheric pressure and wind patterns and consequently in precipitation and humidity patterns; The combination of changes in precipitation and temperature affect the moisture levels of soil; • Increase of temperatures results in an increase in potential evaporation; • Changes in wind speed and humidity may compensate for or amplify the increase in temperature, which may interfere with the evaporation rate of the basin and reservoirs; • Precipitation is the climatic variable that most affects the flow of rivers; • Increased precipitation can lead to a rise in river flow; however, the increase in temperature may counterbalance the effect of this rise, as it increases the rate of evaporation of the reservoirs and the evapotranspiration in the basin; • Global warming increases the demand for water for agricultural exploitation (mainly irrigation) and other socio-environmental demands; • Changes in land cover alters the rate of evapotranspiration, which may imply changes in runoff characteristics; • Some changes in land cover can lead to soil degradation (silting), which can affect both the basin and the reservoir level due to sediment transport-aggravating the negative impacts of climate change; • Hydrological impacts vary according to: precipitation intensity, basin characteristics, type of vegetation and/or changes in land cover; • As a chain effect, changes in runoff can affect the production of electricity.

Conclusions
In summary, climatic and environmental variables impact the volume of water available for the generation of energy in hydropower reservoirs, hence the need for combined studies (considering these different variables) to develop reliable scenarios for the future availability of water resources for hydroelectric power generation and for the establishment of rules for flexible reservoir operation.

Figure 2 .
Figure 2. Scheme for simulating the impact of climate change from Schaefli [4].Figure 2. Scheme for simulating the impact of climate change from Schaefli [4].

Figure 2 .
Figure 2. Scheme for simulating the impact of climate change from Schaefli [4].Figure 2. Scheme for simulating the impact of climate change from Schaefli [4].

Table 4 .
Adaptations to reduce vulnerability of hydroelectric reservoirs to climate change.

Figure 3 .
Figure 3. Framework-Climate change and reservoir management.

Figure
affects power generation in hydropower reservoirs; •The increase in temperature causes changes in atmospheric pressure and wind patterns and consequently in precipitation and humidity patterns;

Figure 3 .
Figure 3. Framework-Climate change and reservoir management.

Table 3 .
Methodologies and main results of studies on impacts of climate change on reservoir management.
a Hadley Centre Coupled Model, version 3 (Met Office Weather Centre-UK); b IPCC's Special Report on Emissions Scenarios (SRES) contains data on the scenarios of greenhouse gas emissions (GHG) and presents different trends, such as low emission or optimistic scenario (B2), moderate emissions (A1B) and high emissions or pessimistic scenario (A2)[68]; c IHACRES model-Identification of unit Hydrographs and Component flows from Rainfall, Evaporation and Streamflow data; d Drawn from the European Union (EU) WATCH3 dataset of statistically downscaled climate models; e IPCC's Fifth Assessment Report-CMIP5: established new scenarios of climate change: high emission scenario (RCP 8.5), medium stabilization scenarios (RCP 4.5 and RCP 6.0) and mitigation scenario (RCP 2.6) [69]; f HEC-ResPRM-US Army Corps of Engineer's Reservoir Evaluation System Perspective Reservoir Model; g SUISHI-O-Modelo de Simulação a Usinas Individualizadas de Subsistemas Hidrotérmicos Interligados developed by CEPEL-Centro de Pesquisas de Energia Elétrica (Electric Power Research Centre), Brazil; h MGB-IPH-Modelo Hidrológico de Grandes Bacias (Hydrological Model of Large Basins) developed by IPH-Instituto de Pesquisas Hidráulicas (Institute of Hydraulic Research) of the Federal University of Rio Grande do Sul, Brazil.