Energy crisis in Brazil: Impact of hydropower reservoir level on the river flow

Water management strategies can have considerable impacts on the regional climate and hydrology. It is usually the case that the construction and operation of hydropower reduce the river flow downstream due to the increase in evaporation. However, this paper shows that in humid regions, such as in Brazil, the hydropower storage reservoirs contribute to increase the flow of the river. This observation has been tested with historical reservoir levels and river flow data from several dams in Brazil. It was found that the operation of reservoirs in Brazil has a considerable impact on its river flows. The higher the storage level at the beginning of the humid period, the higher the river flow during the wet period. The paper proposes strategies to allow the reservoirs to fill up and to maintain the reservoirs filled in the future, with the intention of increasing hydropower generation and reducing the intermittency of other renewable energy sources.


Introduction
Land use can have a substantial impact on the climate and precipitation profile of a region.
These impacts can be very diverse. For instance, conversion of forest into agricultural land affects precipitation patterns [1][2][3], deforestation can affect regional average temperatures [4] as have other impacts [5][6][7]. Apart from the changes in land use, water consumption patterns, which affect the evapotranspiration of a region, also affect the regional climate [8][9][10]. A particular area of research that has gained much attention is the impact of agriculture irrigation on regional temperature and precipitation [11][12][13][14]. Looking in detail into these studies, it can be concluded that agricultural irrigation increases the humidity of the soil and atmosphere, and, for evaporation to occur, water extracts heat from the air, which reduces the regional average temperature. In other words, land and water management have an important impact on regional climate [15,16]. This correlation between land and water management and climate has even been proposed as a regional adaptation measure for global warming [17] and included in weather forecast models [18].
After the end of the Soviet Union in 1991, the hydropower industry rapidly dwindled and was replaced by natural gas. The current need to reduce CO2 emissions is giving back the focus on hydropower generation. Even the IEA, has mentioned that hydropower will be a key electricity J o u r n a l P r e -p r o o f generation source in the future [19]. Future hydropower projects should be designed to mitigate major ecological impacts and to help countries and basins better cope with vulnerabilities caused by climate change, such as droughts and floods [20,21].
Looking at the impact of large reservoirs on the overall basin flow, large hydropower reservoirs result in high levels of evaporation, which reduces the overall yearly river flow downstream [22][23][24][25]. For example, the impact of the Keban Dam in Turkey on precipitation patterns was negligible [26] and the river flow downstream of the dam was reduced due to the evaporation in the reservoir. There are several research projects on the impact of climate change on hydropower generation [27][28][29][30][31][32][33]. Other studies on the impact of the hydropower reservoir on the river flow can be seen in [34][35][36].
In Brazil, the Southeast region presents two well-defined seasons, of which one is a dry season where relative humidity strongly decreases. Usually, relative humidity is at its minimum from August to the beginning of October, and evaporation peaks. Although this period is long, it occurs when reservoir levels and river flows are at their lowest levels [37], which reduces evaporation losses.
During the wet season in the Southeast region in Brazil, the average relative humidity of the atmosphere at the surface is very high, and this considerably reduces the reservoir evaporation. On the other hand, by increasing the regional humidity through evaporation, the evaporation contributes to the increase in precipitation in the region. This paper argues that in the Brazilian Southeast river basins, the larger the hydropower reservoir level, the higher the precipitation in the region and the larger will be the flow of the river. Many studies discuss this point and can be cited to back up this affirmation [38][39][40], particularly for the São Francisco river in Brazil [41][42][43].
A recently published study [44] proposed a possible explanation for the impact of reservoirs in humid climates is that, during the wet period, in the Southeast region of Brazil (between November and April), the average humidity is around 70% with low average wind speeds [45]. Thus, the evaporation is low and the additional evaporation contributes to increasing regional precipitation, which ultimately increases the river flow of the reservoir. When the storage reservoirs are full, the J o u r n a l P r e -p r o o f flooded area and the soil humidity surrounding the reservoir increase. This increases evaporation rates, which increases the humidity of the air and reduces the temperature of the regional climate.
With a more humid and colder atmosphere, when a warm and humid weather system reaches these reservoirs, the chance of precipitation increases. On the other hand, when storage reservoirs are empty, the flooded area and soil humidity surrounding the reservoir is lower. This reduces evaporation rates, which reduces the humidity of the air and increases the temperature of the regional climate.
With a less humid and warmer atmosphere, when a warm and humid front reaches these reservoirs, the chance of precipitation reduces. A visual representation of this phenomenon is shown in Figure 1. Historically, the Brazilian energy sector has been affected by multiple energy crises of different durations and geographic ranges such as the crises of the years 1924,1944,1955,1964,1986,2001 and 2014 [46,47]. In most cases, the causes of crises were associated with climatic conditions, which directly impact the hydro dominant pow J o u r n a l P r e -p r o o f er system of the country. In 2021 this trend is repeating, and the Brazilian Southeast has been suffering a considerable reduction of river flow and hydropower generation, a trend that has initiated in the 2014-2015 drought, as shown in Figure 2. Figure   The objective of this paper is to demonstrate that, on a yearly scale, the reservoir levels have a higher impact on the river flow than the impact of the river flow in the reservoir levels. In other words, if the reservoir is empty, there will be less precipitation in the basin and the river flow will reduce significantly. This is the first paper that analyses the impact of the operation of reservoir levels in the river flow. This paper is divided into four sections. Section 2 presents the methodology implemented in this paper. Section 3 presents the results of the paper. Section 4 discusses the findings of this research. Section 5 concludes the paper. The methodology applied in this paper is described in Figure 3 and consists of the following steps.
Step 1 consists of gathering historical data on the reservoir levels and natural river flow of the dams analyzed ( Figure 3a). Natural river flow is an estimation of the river flow assuming that there is no water extraction from the river, water storage, or evaporation in reservoir dams. This allows the natural river flow estimated in 1970 to be compared with the estimated natural river flow in 2020.
Note that there might be errors or changes in methodology for estimating the natural river flow during this period. The data sources and other details for the dams selected are described in Table 1. The source of data on the natural river flow of all dams is [48].
Step 2 consists of comparing the dam reservoir level at the end of the dry season (end of October), with the average flow of the following wet season (November to April). The reservoir level in October was selected because it is usually the lowest in the year, and it is just before the wet period starts. The average river flow from November to April (wet period) was selected because, it is close to October, which increases the influence of the level of the reservoir in October, and because it is the period when the river flows is usually at its highest in the Southeast region. These data are then plotted on a graph and a linear regression is created to estimate the impact of the reservoir level in the river flow.
J o u r n a l P r e -p r o o f Additionally, the selection of the monthly reservoir level and average river flow will vary from basin to basin.

Results
Step 2 results are shown in Figure 5 and Table 2.   Step 3 results are shown in Figure 6, Figure 7, Table 3 and Table 4. With the intent of estimating the increasing the hydropower generation, the head of the dam under analysis and the dams in cascade were added. The storage reservoir that has the largest hydropower generation head downstream is Nova Ponte with 641.6 meters, as shown in Table 3. The storage reservoir with the smallest generation head out of the selected dams is Sobradinho with 306.9 meters. Multiplying this generation head by the increase in river flow in Table 2  An important aspect to plan the order of filling the reservoirs is the energy storage capacity of the reservoir dams. If the hydropower generation is reduced to allow the reservoirs to recompose, the supply of electricity for the country will have to come from other sources, or the electricity demand be reduced. Out of the dams analyzed in this study, the one with the highest energy storage capacity is Serra da Mesa with 47.6 GWm (this is energetically equivalent to a power plant generating 47.6 GW of electricity continuously for one month). The dam with the smallest storage capacity in Paraibuna with 3 GWm. The dams that should be filled up first are the ones that will have the highest increase in hydropower generation with the least energy storage requirement. This is found by dividing the "Increase in cascade generation (GW)" columns by the "Reservoir storage capacity (GWm)" column. The higher the values, the higher the priority for filling up the dam.
After all reservoir dams in Brazil are filled up, the focus is to operate the reservoirs with the intention of increasing the river flow as much as possible, but at the same time minimize the losses in flow with spillage. The lower the level of the reservoir in October, the higher the capacity of the J o u r n a l P r e -p r o o f dam to store excess river flow and reduce spillage. However, if the reservoir level is too low, then the flow of the river significantly reduces, as shown in this paper. The higher the hydropower generation capacity factor during the wet period, the lower is the spilled flow. Figure 6 a assumes that Furnas dam operates at 100 to 30% of its generation capacity and shows the average wet period electricity generation in the dams in cascade assuming that the river flow follows the regression lines in Figure   5 and Table 2. Note that this is an average river flow, thus there will be years with higher flow and years with lower flow, which is not considered in this paper. Methodologies to further minimize spillage are proposed in [55,56]. Given that the hydropower potential in the future will be used to complement the generation with wind and solar power sources, a good generation capacity of the wet period is around 50% (green line in Figure 6a and b) [57,58]. In this case, the optimum level of the Furnas dam in October is 80%, assuming the existing generation capacity (Figure 6a

Flood control
One important aspect that should be considered in the operation of the dams and that is not considered in this paper is the use of the storage reservoirs for flood control. This issue is important because a high reservoir level contributes to increasing the river flow, as shown in this paper. Further study should be implemented to estimate the minimum storage capacity required to contain large floods in each of the dams analysed. In Brazil, it is usually assumed that the dam should be kept at a maximum of 90% during normal operation, with the intent of storing large discharges of water during flood events.

Increase in river flow vs evaporation
The average evaporation over the year for the Sobradinho reservoir operating with the full reservoir throughout the year is estimated to be 269 m 3 /s [59]. This is significantly smaller than the increase of 1600 m 3 /s in the São Francisco river flow as a result of operating it at a high level at the end of October, as shown in Figure 8.

Environmental river flow
It is of utmost importance to always maintain the environmental flow of the river downstream a dam to sustain a balanced aquatic and terrestrial fauna and flora [60,61]. To guarantee that the required river flow to sustain a healthy environment downstream the dam, the reservoirs should always operate at high levels in case of a drought happens, there will be water to maintain the river flow. Also, as shown in this paper. If the reservoir levels are maintained high, the chance of a drought reduces in the dams analyzed in this paper.

Power dispatch optimization
Another important reason for maintaining the hydropower reservoirs high is to guarantee that there will be water to operate the existing turbines to supply power to the grid, as shown in Figure 9a.
During the energy crisis of 2014, there was a lack of water in the reservoirs in the Southeast region to guarantee the power demand in the system, which raised alarms for the need for investment in technologies to supply power to the grid such as peaking thermoelectric plants and energy storage J o u r n a l P r e -p r o o f solutions. Another example of drought that impacted the capacity of the hydropower dams to supply power requirements in the South region is shown in Figure 9b. The grid operator should have conserved water in the reservoirs in the South during off-peak hours to guarantee that it could generate hydropower during peak hours. Brazil has an installed hydropower capacity of over 120 GW to supply a maximum electricity demand of 90 GW. If there is water stored to use the existing hydropower generation capacity, power will not be a problem for a while. This increases, even more, the importance of maintaining the existing hydropower reservoir levels high. Following this rationale, with the supply of water resolved, future power requirements in the country could be supplied with the increase in installed capacity in J o u r n a l P r e -p r o o f the existing dams and allow hydropower to complement solar and wind power generation.

Thermal electricity generation
Brazil has seen a major switch in the regulation in its electricity generation market in 2021.
The price of electricity switched from a weekly based cost to an hourly based. This change in regulation also resulted in a change in the operation of thermal electrical power plants. For example, Figure 10 shows  Brazilian river basins in [65][66][67]. Different newly proposed arrangements for SPHS can be seen on [68]. Another option for storing energy and water seasonally parallel to a major river is with energy crop storage, as shown in [69].
J o u r n a l P r e -p r o o f J o u r n a l P r e -p r o o f

Conclusion
This paper has shown that the reservoir levels of the hydropower plants have a significant impact on the river flow in the Southeast region in Brazil. On average, the impact of the reservoir level in October of the dams analyzed can be as much as an increase in 112% in the river flow. This aspect shows that the river inflow impacts the reservoir level at a weekly and monthly scale, however, looking at a yearly scale, it is the reservoir level that influences the river flow, as shown in Figure 2.
The dams that should be filled first to reduce the requirement for thermal electricity are This low capacity factor will allow the hydropower potential to generate electricity when there is no solar or wind power in the grid. Which in turn allows more solar and wind power to be added to the grid without the need for new storage solutions.
Brazil has a large potential for hydropower, which has not been explored to its fullest since the drought in 2014 and 2015. The country should focus on generating thermoelectricity, solar and wind power, and conserving energy to allow the reservoirs to rise so that the country can generate more hydropower with existing dams, reduce its electricity costs and reduce CO2 emissions from thermal electricity sources.