Why water storage?

Water is essential for our society. However, water availability is not always synchronized with our needs. Since the distribution of water in time and space is uneven, it is necessary to have water storage systems in place. Water storage systems store excess water, for example during floods, wet periods or when water need is lower than water availability, and release it during dry periods, or when water demand surpasses natural water availability, ensuring a steady supply of water for various uses. Natural water storage systems are represented mainly by lakes and groundwater, but are not always available, or can be exploited only to a certain extent. Therefore, additional artificial water storage needs to be put in place.

Artificial water storage in a European Union’s perspective

Artificial dams retain water, by creating a reservoir or artificial lake, to serve multiple needs including agriculture and electricity generation (hydropower), water supply for industrial and domestic uses, and flood control, or a combination of these. The management of water storage systems requires careful coordination and trade-offs among different uses and targets, particularly in the face of climate change. As the world adapts to the challenges posed by climate change, water storage systems are expected to play an increasingly important role in ensuring integrated water management and providing essential services to society. In the context of a Water Resilient Europe, water storage systems will be crucial in supporting the well-being of both the environment and human populations.

However, water reservoirs connected to rivers, especially when they are of large dimensions, can also cause environmental impacts, such as the interruption of the longitudinal connectivity of the river, with consequent biological, hydrological and morphological impacts. Hydromorphological alterations, due to, for example, water storage, can have major impacts on the aquatic ecology and are among the most widespread pressures on water bodies in Europe. Land use change may also cause social impacts. People reallocation may also be a problem in certain contexts, especially when very large projects are implemented, which is rarely the case of the European Union. Therefore, water reservoirs should be managed with a Water-Energy-Food (Society)-Ecosystem (WEFE) nexus approach. They, and all their interactions with the Earth’s spheres, are the subject of policies falling in the WEFE nexus realm, such as agricultural, energy and water policies.

Dams, and the associated reservoirs, can vary in size, from very small to very large. Large dams are defined as those that are higher than 15 meters, or above 5 meters and have a storage volume of more than 3 million cubic meters, according to the International Commission on Large Dams ( ICOLD, 2023 ). They generally interrupt a river and modulate the flow released downstream. There are approximately 4500 large dams in the EU, which store 290 billion cubic meters of water. 

A water reservoir can be conceived for a specific single use (e.g., hydropower, or irrigation), or for multiple uses (e.g., hydropower and flood control, agriculture and water supply). Following plots show the purpose of large dams in the EU, according to the data of ICOLD (2023).

Number of large dams for different uses and water stored in the associated reservoirs, according to ICOLD 2023

Use of the large dams according to ICOLD 2023 

Complex and controversial debates

Being of cross-sectoral interest, water reservoirs and large dams are complex topics, generally victim of conflictual and heated debates, especially from society. Some of these discussions have arisen in Spain after the Valencia flood in 2024, and in France, where retention basins have been planned for agricultural purposes.

However, generally speaking and with no specific link with the two previous examples, several of these debates often lack technical and scientific content and the conclusions they draw may be misleading and biased.

For example, some debates focus on the importance of dams in water scarce regions, which may not be always true, as other types of measures (e.g., water and wastewater reuse, water saving) may be more effective in some contexts. Other debates instead complain against dams and promote their removal, without a technical understanding of their important role in water supply, water storage in light of climate changes, hydropower supply for a resilient and flexible energy system and flood mitigation.

However, the effectiveness of dams in mitigating floods also depends on the flood intensity. Some debates accuse water reservoirs to be responsible of water losses due to evaporation, but do not consider the role they may have in groundwater recharge. The benefits and impacts of reservoirs have to be adequately and holistically acknowledged and considered, with a technical and scientific approach that takes into account all aspects of the WEFE nexus, and beyond.

Sustainable water&energy storage in closed open-pit lignite mines

Imagine transforming the deep scars of closed open-pit lignite mines into engines for a sustainable future. Instead of leaving these vast excavations idle, they can be repurposed as reservoirs for pumped storage hydropower (PSH) systems, as a giant, natural battery. When renewable energy from wind or sun is abundant, excess electricity pumps water into the upper reservoir. When demand peaks, that water flows back down, generating clean power instantly. Because lignite mines are already dug deep into the ground, they are perfectly suited for storing water on a massive scale without the environmental impact of building new dams. This dual-purpose approach offers more than just energy security. The repurposed sites can also act as crucial water reserves for local communities during droughts. Furthermore, by managing water levels effectively, these excavations can absorb excess rainfall and meltwater, providing vital flood protection for nearby towns and cities. This innovative strategy breathes new life into post-mining landscapes. It supports economic regeneration while re-using the land, and turns former industrial zones into essential infrastructure for clean energy and climate resilience. By combining energy storage with water management, a safer and greener environment can be created that benefits renewable energy production and storage as well as regional water management, and thus the people living around it. 

A recent study from the EU project Atlantis found that maximising power capacity for short-term load balancing yielded 9.6 GW power and 117 GWh storage, representing approximately 21% of current EU installed PSH capacity and a 45% storage increase. In contrast, maximising energy storage for seasonal demand resulted in lower power capacity of 5.4 GW (12% of EU total) but significantly higher storage capacity of 13 TWh. These findings highlight the substantial potential of utilising closed open-pit lignite mines for PHS to support the EU’s energy transition, directly aligning with the EU Green Deal and energy security objectives by a utility-scale solution that optimises renewable energy capacity utilisation.

The JRC critical and independent contribution to the debate

To promote a more informed discussion, the Joint Research Centre (JRC) has recently published several studies on water reservoirs and large dams. These publications aim to provide a more nuanced understanding of the complex issues surrounding water reservoirs and their management.

Did you know that water reservoirs store a huge amount of energy (in the form of potential energy of water used by hydropower systems), and are the main contributor in terms of energy storage and energy flexibility of the European Union's energy system? And did you know that some sustainable strategies to increase their water&energy storage are also possible (e.g., using abandoned mines, or increasing the volume of reservoirs by increasing the height of the dams)?

Did you know than one of the largest problems of reservoirs is sediment management and the filling of the reservoir with sediments, and that removing sediments can cost several billion euros per year?

Did you know that, in the era of digital transition, digital solutions and real-time control can also improve the environmental performance of reservoirs and hydropower systems, by mitigating their impacts, and not only enhancing energy generation and security?

Did you know that by retrofitting the existing hydropower fleet, it is possible to increase energy generation without using more water?

Did you know that water reservoirs can host floating photovoltaics, which reduce water evaporation and make available area for renewable energy production?

Did you know that European rivers are highly fragmented? Around 450,000 barriers have been identified in European Union's rivers. Explore the AMBER Atlas to learn more!

Did you know that water storage are also responsible of several impacts that can significantly affect the ecological status of rivers?

Working Group ECOSTAT report on common understanding of using mitigation measures for reaching Good Ecological Potential for heavily modified water bodies - Part 1: Impacted by water storage

Hydromorphological alterations due to e.g. water storage with major impacts on the aquatic ecology are among the most widespread pressures on water bodies in Europe. Many of these impacted water bodies have been designated as heavily modified water bodies, and many are expected to need mitigation measures to meet the good ecological potential (GEP) and some possibly good ecological status (GES) to fulfil the requirements of the WFD. Among the core activities for the CIS WG ECOSTAT from 2013 onwards, have been to assess the comparability of GEP among countries implementing the WFD. The aims of this activity and hence this report have been to develop a suitable method for assessing comparability (intercalibration), learn from each other to ensure common understanding and emerging good management practice across Europe, as a basis for supplementary guidance on HMWBs issues. In other words; agree on a common understanding on environmental requirements for water bodies impacted by water storage. Several workshops have been arranged and a series of templates have been circulated to national management experts to get a comparison of available measures in mitigation libraries and use thereof. In the present report, the assessment of comparability is based on returned templates from 23 countries. Response is lacking from four countries while three countries have informed ECOSTAT that finalising and use of national libraries is still ongoing. The outcome has been a common terminology and some key findings regarding key ecological functions normally expected to be mitigated by measures in water bodies impacted by water storage to meet the requirements of GEP.

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