Reservoirs of Spain: how much water is left today

Spanish reservoirs: what the hydrological bulletin measures and how to read it

The weekly hydrological bulletin published by Spain’s Ministry for the Ecological Transition is the official record of reservoir levels. Dam by dam, it lists the volume stored in every mainland reservoir above 5 hm³ of capacity, and it comes out every Tuesday with Monday’s reading. It is the figure the river basin authorities work from, the one quoted in cabinet meetings whenever drought comes up, and the one headlines call “Spain’s water reserve”.

That national percentage is a weighted average: total stored water divided by total capacity. A very large dam therefore counts for far more than a small one, and the national number can hide very different situations. A mid-range national reserve is perfectly compatible with a comfortable Atlantic basin and a Mediterranean one in emergency: the Duero or the Miño-Sil often run well while the Segura or the Catalan inland basins sit far below their own average.

Hydrological drought is not declared by looking at that national figure. Each demarcation has its own drought plan, with indicators per operating system combining stored water, river inflow, groundwater and accumulated rainfall. Those produce the normal, pre-alert, alert and emergency states, declared system by system rather than nationally. So it pays to look at the basin — and the individual dam — before the headline.

Comparing one week with the last says little: in a country this seasonal, almost every reservoir falls in summer and rises in winter. The useful comparison is against the same week in previous years, which is what every page in this section provides, alongside the average monthly curve and the highs and lows of the whole record since 1988.

Quick glossary

• hm³ (cubic hectometre): one million cubic metres, or a billion litres. The standard unit for stored volume.
• Usable capacity: the maximum volume that can be stored between the minimum intake level and the normal maximum operating level.
• Spillway: the structure that releases water once the reservoir reaches its maximum level, keeping it from overtopping the dam.
• Release: controlled outflow, whether for irrigation, supply, environmental flow or hydropower.
• Environmental flow: the minimum volume a dam must release downstream to keep the river alive.
• Demarcation: the territorial unit for water management, made up of one or more basins plus their associated coastal and groundwater bodies.
• Transfer: a statutory move of water between basins. The best known is the Tagus-Segura, drawn from the headwaters at Entrepeñas and Buendía.
• Hydropower reservoir: the bulletin flags dams whose main use is electricity generation. Their level follows power demand rather than the irrigation calendar.

Sources

• Weekly hydrological bulletin and reservoir archive of Spain’s Ministry for the Ecological Transition (MITECO), complete series since 1988.
• River basin authorities of the Ebro, Duero, Tagus, Guadiana, Guadalquivir, Júcar, Segura, Miño-Sil and Cantábrico; the Catalan Water Agency and the Andalusian agencies for Guadalete-Barbate, Tinto, Odiel and Piedras, and the Andalusian Mediterranean basin.

The bulletin covers mainland Spain. Reservoirs in the Balearic and Canary Islands are managed by the island administrations and are not part of this series, so they do not appear on these pages.