In brief: between October 2025 and August 2026, mainland Portugal received 140% of its normal rainfall, with floods in January and February. Meteorological drought disappeared from December to May, but returned in the summer and on 31 August was concentrated in the South. Floods and drought call for the same starting point: knowing how catchments and networks actually behave, and deciding with scenarios before investing.
What you should know
- National Water Day, on 1 October, marks the start of the hydrological year in Portugal.
- Above-normal rainfall does not rule out drought: there was no meteorological drought from December to May, but it returned in the summer; at the end of August, the North had recovered and the South was in drought.
- Average reservoir storage hides large differences between basins.
- Non-revenue water and real losses are different indicators and call for different diagnoses.
What did the 2025/26 hydrological year bring?
The 2025/26 hydrological year was very wet overall, but it did not stop drought from returning to the South in the summer.
Between mid-January and early February, four storms (Ingrid, Joseph, Kristin and Leonardo) crossed the mainland. In its review of flood management, the forecasts of the Portuguese Environment Agency (APA) pointed to a high risk of flooding on twelve rivers, from the Lima to the Sado. As storm Kristin passed, the Zêzere reservoirs retained around 130 million m³ and the Aguieira reservoir around 80 million. On the Tagus, releases were coordinated with Spanish dams so that the river flow would not exceed 3,500 m³/s.
From December to May there was no meteorological drought on the mainland; it returned from June. At the end of the summer, the North and the Centre-North recovered with the August rain, but the Alentejo and the Algarve remained in drought, and in the south-east the area in severe drought grew. In the interior of the Lower Alentejo, some municipalities had soil water below 10% of the capacity available to plants.
| Indicator | Value | Date | Source |
|---|---|---|---|
| Accumulated rainfall | 1,083.5 mm, 140% of the 1991–2020 normal | Oct 2025 – Aug 2026 | IPMA |
| Territory in meteorological drought | 70.5%, of which 16.4% moderate or severe | 31/08/2026 | IPMA |
| Storage in monitored reservoirs | 9,673 hm³, 73% of capacity | 28/09/2026 | APA/SNIRH |
| Basins below the September average | Lima, Cávado/coastal streams, Vouga, Mondego | 28/09/2026 | APA/SNIRH |
| Areas of potential significant flood risk | 63 in force (16 coastal); 18 more proposed | 2026 | APA |
Why is a national average not enough?
A national average hides the contrasts between basins, and water is managed basin by basin. On 28 September, the four basins below their September average were in the North and Centre, after a winter of floods, while the summer drought was concentrated in the South. Rainfall, meteorological drought, soil water and storage measure different things, and only read together do they describe the state of a basin.
Flood risk is also local: the 63 areas of significant flood risk in force cover 103 municipalities and around 102,900 people exposed in the 100-year return period scenario.
How do drought and intense rain affect water and wastewater networks?
Losses in water supply and undue inflows in wastewater systems call for different diagnoses; drought and intense rain make their consequences more critical.
In water supply, according to the annual report of the water regulator ERSAR (RASARP 2025), 27% of the water entering retail distribution systems in 2024 was non-revenue water: 224 million m³ out of 847 million. Non-revenue water combines real losses, apparent losses and unbilled authorised consumption. Real losses, in bulk and retail systems, totalled 187.3 million m³, and only reducing them cuts the water that has to be abstracted, treated and pumped.
In wastewater, sewers are being rehabilitated at a rate of 0.1% per year. In a scenario that eliminates 80% of real losses and undue inflows, ERSAR estimates that 213 million m³ a year would be avoided, 78 million of them from undue inflows. In intense rain, these inflows overload sewers and treatment plants.
What should be assessed in water infrastructure before winter?
A useful hydraulic study shows the data, scenarios and limits behind its recommendation. Five questions to assess a study or write its terms of reference:
- Are the data measured and recent? Measured flows and updated series can change a design.
- Are the scenarios appropriate? Events of different durations and probabilities, chosen according to the system and the consequences of failure. Drought, too, is analysed through scenarios.
- Has the model been calibrated and validated? Against which observed events, and with what error.
- Are losses and inflows separated? They are measured differently and fixed with different interventions.
- Does the study state its limits and trade-offs? It should say what would change the recommendation. In infrastructure with storage, water reserves and flood-attenuation volume compete; whether they can coexist depends on function, sizing and operating rules.
How IdroAM approaches these risks
IdroAM is a hydraulic and environmental engineering company that treats water as one system: catchment, network and treatment plant. Headquartered in Italy and present in Portugal, it carries out hydraulic and flood modelling, hydraulic risk assessment, and the design of water supply, drainage and wastewater networks and treatment plants.
Conclusion
The 2025/26 hydrological year showed that above-normal rainfall and drought can coexist in the same territory. The rain of the new year does not depend on those who manage water. The preparation does: knowing the catchment and the network, and deciding with scenarios before construction.
If you are preparing the terms of reference for a hydraulic study, get in touch.
Frequently asked questions
Is non-revenue water the same as water losses? No. Non-revenue water includes unbilled authorised consumption, apparent losses (unauthorised consumption and metering errors) and real losses (leaks and overflows). Each component is reduced with different measures.
What is an area of potential significant flood risk? It is an area identified in the preliminary flood risk assessment because it combines a probability of flooding with significant adverse consequences. That identification guides flood risk mapping and the flood risk management plans required by Directive 2007/60/EC.
Can the same year bring floods and drought? Yes. In 2025/26, the mainland had floods in January and February and, on 31 August, 70.5% of the territory in meteorological drought, mostly mild.
Sources
- IPMA, Boletim Climatológico Portugal Continental, Agosto 2026 (hydrological-year rainfall, PDSI drought index, soil water).
- IPMA, Observatório da Seca, PDSI evolution (no meteorological drought from December 2025 to May 2026).
- APA/SNIRH, Boletim semanal de albufeiras, water availability on 28 September 2026.
- APA, Balanço sobre a gestão de cheias em 2026 (storms, hydrological forecasts, Zêzere and Aguieira reservoirs, areas of potential significant flood risk).
- ERSAR, Relatório Anual dos Serviços de Águas e Resíduos em Portugal (RASARP 2025), 2024 data, presentation to the Environment and Energy Committee of the Portuguese Parliament, 20 May 2026.
Header image generated with AI; it does not show a real location.
