IBERDROLA AND THE RESERVOIRS IN SPAIN

How reservoirs promote sustainable water use and help mitigate the effects of drought in Spain

The reservoirs in which Iberdrola has hydroelectric power plants play an important role in the efficient use of water in Spain, especially in times of scarcity, by storing water from rivers, streams and rainfall and allowing us to regulate its flow with precision through the dams. In this way, while generating clean, renewable energy, we are helping to reduce the impact of droughts and floods in river basins.

Hydroelectric dam in a deep rocky canyon with intense blue water, captured from an aerial perspective

The Aldeadávila Dam is a concrete arch-gravity dam located on the Douro River on the border between Spain and Portugal.

Key conclusions on reservoirs in Spain

Reservoirs make it possible to store and regulate water, facilitating renewable hydroelectric generation and electricity supply.
In 2025, Spain had more than 17,000 MW of installed hydroelectric capacity and 32,500 hm³ of stored water.
The good condition of reservoirs in Spain helps mitigate droughts, regulate flows and ensure water and energy supply.
The Water Act establishes water-use priorities and the authorities set the maximum and minimum operating levels for each reservoir.
Hydroelectric power plants can quickly restore electricity supply through autonomous start-up systems.

Hydroelectric power plants have existed in Spain for more than a century, and installed capacity at the beginning of 2026 exceeded 17,000 MW (according to data from Red Eléctrica). At Iberdrola España, we have been committed to hydroelectric energy since our origins as a key foundation for a greener future. However, the meteorological and hydrological variability of recent years in the Iberian Peninsula, with alternating periods of prolonged drought and episodes of torrential rainfall, has created a challenging context in certain sectors that has led to significant misinformation about electricity companies and reservoirs in Spain.

In this context, the current situation shows that the good health of reservoirs in Spain is a guarantee for the future, as they promote the sustainable use of water and help mitigate the effects of drought in Spain.

Iberdrola España's purpose, as a company that forms part of the Iberdrola Group, is clear: to drive the energy transition in order to combat the effects of climate change, building a more electrified, healthier and more accessible energy model based on renewables.

Drought is not a concern only in Spain. It is a global challenge

Water scarcity remains a priority on the international agenda. According to data from the Joint Research Centre of the European Commission, although the situation varies depending on the season, a significant part of the European Union’s territory recurrently faces drought conditions. At a global level, data confirm that the frequency of flash droughts has increased across 74% of the planet’s surface, making it a shared challenge that transcends national borders.

Meteorological drought is a natural phenomenon that occurs as a result of low rainfall and high temperatures, which hydroelectric power plants do not influence at all and from which they do not derive any benefit. In fact, the electricity sector is particularly vulnerable to increasing water stress, as hydropower production can decrease significantly where water flows are likely to decrease. The decline in supply of this renewable energy, in turn, can create a gap in the market for other more expensive and polluting energy sources.

The movement of dammed water: regulated and transparent

There is significant state control over where, how and how much water is released from Spanish dams (which is called turbining the water, in terms of electricity generation). As a result, energy companies operate with a narrow margin, subject to very strict regional and state regulations. There is a requirement that there must always be a significant reserve of water that is not turbined, while guaranteeing an adequate volume of flow downstream (the "ecological flow" set by the Hydrographic Confederations that protects fish life and riparian vegetation).

The Water Law sets the priorities for water use and the Administration sets the maximum and minimum levels at which each reservoir moves. But there are many more public bodies involved in one way or another in the regulation and control of hydropower generation in Spain.

Public bodies responsible for dams and reservoirs in Spain

These are the main public bodies - state, regional and municipal - with different competences in order to control that the creation, modification and management of reservoirs and dams comply with environmental, safety, sustainability and competence requirements at all times.

Minister for Ecological Transition and the Demographic Challenge
State Secretariat for Energy

Directorate-General for Energy Policy and Mines

Authorisation, inspection and possible sanctions of power plants
State Secretariat for the Environment

Directorate-General for Water

Review of safety and proper functioning

Directorate-General for Environmental Quality and Assessment

Files of environmental nature
Hydrographic confederations

Granting or termination of concessions. Supervision and control

Ministry of the Interior
Under-Secretariat for Home Afairs

Directorate General for Civil Protection and Emergencies

Implementation of emergency plans for dams

Autonomous communities
Central authorisation, modification, transfer or closure of regional competence

Town halls
Town planning, environment, health, local police, civil protection, fire prevention and extinguishing

National Commission for Markets and Competition (CNMC)
Penalty system. Capacity investment monitoring

The purpose of dams is precisely to make a regulated use of an irregular resource such as water - dammed and released water - so that the functions established by law can continue to be fulfilled downstream. In order: water for drinking, for irrigation, for industrial uses and, where possible, for recreational uses. This is without prejudice to the fact that there are reservoirs with these uses that were built specifically for hydroelectric production. If there is water and turbines are available, it is mandatory by law to offer the energy to the market.

It is also important to be aware in this context of the obligation to comply with the Albufeira Agreement, which governs coordination in the management of the rivers shared by Spain and Portugal (Minho, Douro, Tagus and Guadiana) and requires the delivery of certain volumes of water. In Iberdrola's case, this regulation particularly affects the reservoirs it operates in the Tagus basin, such as the Alcántara reservoir and the Cedillo reservoir (both in Cáceres), and in the Duero basin, such as the Aldeadávila power station (Salamanca) and the Ricobayo reservoir (Zamora).

Sustainable water management in reservoirs in Spain

A basic principle of hydropower generation is that water is neither wasted nor lost in the process, only its energy potential is used to transform it into electricity. Therefore, reservoirs in Spain can never "waste" water, they only regulate its flow, in a transparent and strictly regulated manner.

Moreover, hydroelectric regulating reservoirs make a very important contribution to regulating (storing) water and are therefore important elements that help to combat the effects of droughts. At the same time, they prevent overflows. All this with an exhaustive control of the flow that allows the river ecosystem to be protected, both in periods of drought and in periods of extreme rainfall. Furthermore, in order to be able to continue providing its energy service in the event of droughts, there is hydraulic pumping technology, which, with a closed water cycle, can make the plant independent of rainfall.

Capacity figures in Spain in context

In order to give a real value to the data we read about water movements or transfers, it is important to know that Spain had, for example, at december 2025, a water reserve of more than 32,500hm3 of water in reservoirs, just over 56% of the total capacity of 56,043hm3, according to data from the Ministry for Ecological Transition and Demographic Challenge.

This scenario is particularly positive in the river basins where Iberdrola España has the greatest presence, such as the Tagus and the Douro, which exceed 56% and 62% respectively, and the Miño-Sil basin, which exceeds 62% of its capacity. These levels ensure water supply and allow our plants to operate at full capacity, consolidating their role as a fundamental and stable renewable backup for the electricity system.

Droughts also affect hydropower production

Another important issue is that many electricity companies, including Iberdrola España, have part of their electricity production pre-negotiated with long-term contracts. If hydropower production decreases sharply and unexpectedly, this is detrimental to the business as they have to supply, if necessary, the missing energy from other sources.

Hydroelectric power in Spain: cheap and clean

Hydroelectric energy, in addition to being renewable, clean and emission-free, is key to covering electricity demand and avoids the use of more expensive and polluting technologies. In the Spanish energy system, the price per kWh for each hourly slot of each day is set the day before, determined by the Iberian Energy Market Operator based on the balance point between the sales offer of the generators and the purchase offer of the marketers and large consumers. Therefore, the more renewable energy that is offered, including hydro (with a variable cost that is much lower than that of fossil fuels), the more the final price will be reduced.

In this regard, at Iberdrola España we demonstrate the strategic value of this technology with figures. During 2025, hydroelectric power remained the source that contributed most to our national production, representing 33.31% of the total energy we generated (64,678 GWh), according to information submitted to the National Securities Market Commission (CNMV). This firm commitment is supported by a significant reservoir capacity that allows us to guarantee key support for the stability of the Spanish electricity system.

Status of Iberdrola España’s hydraulic reserves

Data as the end of the last quarter of 2025

5,657

Potential electricity generation capacity

50.2

Hydraulic reserve capacity

The key role of reservoirs in energy recovery during the massive outage in Spain

The hydroelectric plants of Iberdrola España played a crucial role in restoring the electricity supply in Spain following the power outage on 28th April 2025. Plants such as Aldeadávila (Duero Basin), Ricobayo (Esla Basin), Cortes-La Muela (Júcar Basin), José María de Oriol (Tajo Basin), and Puente Bibey (Sil Basin) were essential in restoring the flow of electricity to the affected regions with their storage systems and robust infrastructure. These facilities demonstrated their resilience in times of crisis, emphasising the importance of hydroelectric power in the country’s electricity supply.

This was only possible because the hydroelectric plants have an autonomous start-up system—known as "black start"—which allows them to restart without any external support. This process can be carried out using a battery system, autonomous generators, or the water flow from their own reservoirs.

Thus, in situations of disconnection from external energy, these facilities are able to produce the initial energy needed to gradually reactivate other power plants, in a coordinated manner with the protocols established with the state operator, Red Eléctrica Española (REE). Without the hydroelectric energy produced by the plants, the restoration of the electricity supply on 28th April would not have been achieved in the timeframe it was.