Discharge hydrology and hydroelectric generation: the balance between water, energy, and sustainability

Water is a fundamental resource for modern society. As well as domestic, industrial, and agricultural use, water is essential for generating electricity. For more than a century, rivers and reservoirs have been an indispensable part of the Spanish electricity system.

In the 1940s, hydroelectric energy was the key component, accounting for over 90% of the electricity consumed. Today, the situation has changed, and hydropower coexists with technologies like solar, wind, nuclear, and thermal power. Although its share of the energy mix has decreased to 13.3% in 2024, it remains an essential energy source because it is renewable—emitting no greenhouse gases—and flexible, which enables it to respond to changes in demand in minutes or even seconds.

How electricity is produced from water

The operating principle of this type of energy lies in the movement of water flowing through rivers or stored in reservoirs and later released. The amount of water that moves towards a turbine is directly related to how much electricity can be produced, and this is known as hydrological discharge.

In other words, when we talk about discharge, we are referring to the hydraulic flow rate that passes through a turbine, turning its runner. In this process, potential energy (from the height the water falls from) and kinetic energy (from the speed at which it arrives) are transformed into mechanical rotational energy, which is finally converted into electricity by the generator coupled to the turbine.

Hydrological discharge and its impact on the electricity system

Although the term "hydrological discharge" seems like a simple concept, it has a direct impact on the security of supply, the price of electricity, and the manageability of hydropower when operating alongside other electricity generation technologies.

In Spain, in 2024, the installed hydroelectric capacity was 17,097 MW, 12.91% of the total installed capacity (132,343 MW). It generated 34,912 GWh, representing 13.3% of the total electricity produced. The relationship between water and energy is very close, especially in rainy years like 2024, which saw a 35.5% increase in hydroelectric generation compared to the previous year.

Types of hydroelectric plants and flow management

Hydrological discharge is a critical concept for hydropower plants, and it is not the same for all of them. There are cases where production depends on the amount of water available at a specific point and time; in other words, the available flow in the river. In other cases, the stored water is managed by releasing a hydraulic flow to the turbine, taking into account various technical, economic, and environmental variables such as electricity price and demand, weather forecasts, stored water capacity, and ecological flows, among others.

Furthermore, not all rivers have stable flows that allow for water to be harnessed; they tend to be more stable in wet areas like Galicia or Cantabria, and more irregular in areas like eastern or southern Spain, where droughts are common and rainfall is intense.

Run-of-river plants: generation without storage

Therefore, discharge hydrology dictates the operation of the hydroelectric plant. As mentioned earlier, there are two main types, known as run-of-river plants, which divert water from the river to the turbine and return it to its course immediately afterwards. These types of plants have almost no storage capacity, apart from small regulating ponds, so if it rains less, less is produced, and if it rains more, more is produced. They are facilities that take advantage of steep gradients and stable rivers but have very little capacity for regulation.

Dam-based plants: the great hydraulic battery

Very different are dam-based plants, which are associated with large reservoirs where water is stored and released when the operator decides. These plants act as a large hydraulic battery, capable of storing potential energy and converting it into electricity when it is most needed. This flexibility is crucial in an electricity system with a large share of intermittent renewables, such as wind and solar power, which depend on the wind and sun and require backup when those resources are unavailable.

Geographical distribution and installed capacity in Spain

Spain has great hydrological diversity. In the Atlantic area, the great rivers—Duero, Tajo, and Guadiana—have large basins and frequent inflows, which allows for the construction of large dams and hydroelectric plants. On the Mediterranean side, in contrast, the rivers are usually shorter, with highly variable flows and extreme peaks. The Ebro is the main exception, with an average flow rate of nearly 600 cubic metres per second at its mouth.

Most of the hydroelectric capacity installed in Spain is concentrated in a few regions: Castilla y León, with 25.7%, mainly in the Duero basin. Galicia, with 21.8%, due to its abundant rivers and mountainous terrain with steeper gradients than other regions. Regions like Extremadura, Catalonia, and Aragon also account for 30% of the installed capacity. In other words, just five regions have almost 80% of the country's installed hydroelectric capacity.

How hydraulicity influences production and electricity prices

Data from Red Eléctrica Española (REE) clearly show how hydraulicity—which depends on rainfall and rainfall patterns and translates into the water available in rivers and reservoirs—affects energy production. In 2023, a dry year, hydropower generated 25,273 GWh of electricity, accounting for 9.5% of the total. During that year, gas and coal plants played a larger role in covering demand, which resulted in an average price of €87.1/MWh.

In 2024, the situation changed. Hydroelectric production rose by 35.5% to 34,912 GWh, 13.3% of the total. At the same time, generation from coal and gas-fired combined-cycle plants decreased by between 25% and 30% respectively. Considering the emission factors for natural gas combined-cycle plants and coal (0.35 tCO₂/MWh and 0.90 tCO₂/MWh respectively), this greater contribution from hydropower made it possible to avoid around 3-4 million tonnes of CO₂.

Furthermore, the so-called hydroelectric producibility index—which compares how much electricity is generated with the available water versus what would be normal in an average year—went from 0.94 in 2023 to 1.24 in 2024. In other words, there was much less water in 2023 than in 2024, a year in which reservoirs ended at 52.3% of their capacity, providing some leeway to turbine at times of greater need. This greater hydraulic availability helped lower market prices, resulting in a lower average price of €76.3/MWh.

However, it should be noted that this price drop did not depend exclusively on the increased share of hydroelectric plants. It was also influenced by the trend in natural gas prices, which often set the marginal cost of the daily market, as well as by the contribution of other renewable technologies, although at certain times hydropower played a significant role in setting the marginal price.

Hydrological discharge as a strategic resource

This set of figures clearly shows how hydrological discharge is converted into electricity and euros. In rainy years, hydropower becomes an ally of the system, reducing emissions and helping to lower prices. In dry years, the system is forced to depend more on fossil fuels, which increases bills and raises emissions. Hydrological variability is simultaneously an obstacle and an advantage. It implies needing a diversified mix, but when water is available, it becomes a strategic resource, capable of providing clean, flexible energy just when it is needed most.

It is important to remember that the hydroelectric system provides flow regulation and safety services in addition to energy. Dams make it possible to control floods and ensure ecological flows in rivers. Furthermore, from the electricity system's point of view, they enable coverage of sudden peaks in demand and contribute to the system's frequency and voltage regulation, which is particularly important with the progressive electrification of the economy. In this context, hydropower acts as a kind of insurance. Although it accounts for around 13% of annual production, on certain specific occasions it can be the difference between a balanced system and one under stress.

Climate change and challenges for hydropower

It must also be considered that climate change is altering rainfall patterns on the Iberian Peninsula, where less rain,  longer droughts, and more concentrated downpours are expected. As noted by the Centre for Public Works Studies and Experimentation (CEDEX), inflows in basins like the Tajo, Guadiana, and Segura could decrease by between 7% and 14% by 2050. However, in the northern basins, the decrease is expected to be more moderate. This trend will require more advanced water management and improved connections between regions so that hydropower can remain a fundamental component of the electricity system. It will be necessary to combine hydrological forecasting capacity with new digitalisation, remote sensing, and climate modelling technologies to better anticipate flow scenarios.

The future of discharge hydrology in Spain

Discharge hydrology, therefore, is the bridge between nature and energy. Every litre of water that flows down a river in Spain has the potential to become renewable energy. Its variability influences the price of electricity, contributes to the security of supply, and is linked to strategic, environmental, and economic decisions. For decades, Spain has been able to benefit from this connection by building dams, power plants, and a hydraulic regulation network that is a model at an international level. The challenge now is to preserve and improve this legacy in a scenario of greater climate uncertainty, with more intermittent renewable sources on the grid, and with growing social and environmental awareness to ensure sustainability.