Brazil gets two-thirds of its energy from hydropower and has numerous large hydropower storage reservoirs that are fed by perennial rivers. Although it is usually understood that the construction and operation of hydropower systems reduce the river flow downstream due to the increase in evaporation, a recent study suggests that in humid conditions, as found in parts of Brazil, hydropower storage reservoirs actually contribute to increasing the flow of the river upstream of the reservoir.
“It is widely accepted that river flow impacts reservoir levels. This is true on a weekly and monthly scale. However, on an annual scale the level of the reservoirs actually has a greater effect on the inflow of the river in the southeast of Brazil,” explained study leader Julian Hunt, a researcher in the IIASA Energy, Climate, and Environment Program.
In fact, the study finds that large storage reservoirs in southeastern Brazil can double the flow of rivers if the reservoirs are full, when compared with the effect of empty reservoirs. This finding has significant implications for water and energy management in the region, which is currently experiencing a historic drought and associated energy crisis.
The researchers explain that humidity is high during the wet season in the south-eastern region of Brazil. This reduces evaporation from the surface of reservoirs but it also increases the chances of precipitation in the area because any additional evaporation from a large body of water (storage reservoir) adds significantly to the high moisture content of the environment. Therefore, the higher the hydropower reservoir level, the higher the precipitation and the larger will be the inflow of the associated river.
The researchers analyzed river flow into eight dammed reservoirs in the region, which is Brazil’s most important for the production of hydropower. They compared the natural inflow of the rivers prior to damming in 1970 with water flow today, during the wet and dry seasons.
The team then compared the dam reservoir levels at the end of the dry season (October), with the average inflow recorded at the start of the following wet season (November). The data was used in correlation analyses to understand the potential impact of the reservoir level on the incoming river flow.
The results show that the flow of the rivers into all dams increased by an average of 112% if the reservoir was full in October, compared to if the reservoir was empty.
The researchers conclude that hydropower storage reservoirs have a significant impact on river flow in this region of Brazil and that keeping them full will alleviate some of the effects of drought, including decreases in the production of energy and increases in the costs of food.
Additionally, the study authors recommend that in order to improve current conditions, the smaller reservoirs at the heads of inflowing rivers should be filled first, followed by the larger ones. They calculate that reservoir storage levels at the end of October should be 78 percent full and the hydropower plants in cascade should operate with a capacity factor of 50 percent.
The findings help to explain why the river flow in southeastern Brazil decreased by 60 percent within one year of the start of the drought in 2014. Reservoir levels were allowed to drop significantly at that time and this could have exacerbated the problem by reducing the flow of incoming rivers even further.
“We should realize that managing water, land and other resources is not just a way to adapt to a changing climate. It can also affect the climate itself,” said Hunt.
The study is published in the journal Energy.
By Alison Bosman, Earth.com Staff Writer