Every time it rains hard in Spain, a silent network of sensors spread across the country starts measuring, transmitting and analysing hydrological data in real time. That network is called SAIH (Sistema Automático de Información Hidrológica, the automatic hydrological information system) and it is the backbone of river, reservoir and rainfall monitoring in the country. Without it, forecasting floods and managing them would be practically impossible.

This article walks through the history, the technical workings and the future of a system that, despite being critical infrastructure, remains largely unknown to the public.

Origins: a system born out of tragedy

SAIH grew directly out of the catastrophic floods that hit Spain in the early 1980s. The failure of the Tous dam in October 1982, the Basque Country floods of August 1983 and similar episodes exposed the complete absence of real-time hydrological information.

At the time, river levels were read manually by observers from staff gauges, at fixed times, once a day or at best every few hours. When a flood wave arrived, the authorities simply had no up-to-date data on which to base decisions.

The 1985 Water Act and the later National Hydrological Plan provided the legal basis for creating SAIH. The first systems were deployed in the late 1980s in the most flood-prone basins — Júcar, Segura, Ebro — and were progressively extended to the remaining river basin districts through the 1990s and 2000s.

Key point: SAIH was a European pioneer. When Spain rolled it out in the late 1980s, very few countries had national real-time hydrological monitoring networks. It remains one of the densest in Europe.

How SAIH works

SAIH is a distributed system operating on three hierarchical levels: the measuring points (field sensors), the concentration points (repeaters and communication nodes) and the processing centre (the control room of the relevant river basin authority). Here is each level in detail.

Level 1: sensors in the field

At the heart of the system are the measuring stations, installed on rivers, reservoirs, canals and strategic points of the hydrological network. Each station carries one or more types of sensor:

  • Automatic rain gauges. They measure accumulated rainfall with a tipping-bucket mechanism, where each tip corresponds to 0.1 or 0.2 mm of rain. More advanced models add heating to measure snow, plus anti-vandalism protection.
  • Water level recorders. They measure the water level in rivers and reservoirs. Traditional units use a float connected to a mechanical encoder; modern ones use submerged pressure sensors (piezoresistive transducers).
  • Ultrasonic level sensors. They measure the distance between the sensor, mounted above the channel, and the water surface using ultrasonic pulses. Because they need no contact with the water, maintenance is lower.
  • Radar level sensors. Similar to the ultrasonic type but using electromagnetic waves. They are less sensitive to ambient conditions such as temperature, wind or foam, and are increasingly the choice when SAIH stations are upgraded.
  • Discharge measurement equipment. Some points measure flow directly, using current meters, Doppler sensors (ADCP) or calibrated weirs.
  • Additional weather sensors. Some stations also carry anemometers, thermometers, hygrometers and pyranometers to put the hydrological data in context.

Every station has a data acquisition unit, or datalogger, that reads the sensors at regular intervals — typically every 5 or 15 minutes — and stores the readings until they are transmitted.

Level 2: communications

Getting data from remote stations to the control centres is a critical part of the system. Over its history SAIH has used several communication technologies, many of which coexist today:

  • VHF/UHF radio. The original SAIH technology. Stations transmit by radio to repeaters on high ground, which relay the signal to the control centre. It is reliable and self-sufficient, since it depends on no third-party infrastructure, but limited in bandwidth and range.
  • Satellite links. Used at very remote stations with no radio or mobile coverage. Systems such as Inmarsat or Iridium provide global connectivity, at higher operating cost.
  • GPRS, 3G and 4G. Progressive modernisation has added mobile communications, which offer more bandwidth and allow more frequent transmission. The drawback is dependence on the phone network, which can saturate or fail during an emergency.
  • Fibre and fixed lines. Some urban or easily reachable stations use high-capacity wired connections.
Critical redundancy: The most important stations have two communication paths — radio plus GPRS, for example — so that data keeps flowing if one channel fails. That redundancy matters most during a flood, which is exactly when the data is most needed.

Level 3: the processing centre

Incoming data reaches the data processing centre of each river basin authority, where it is validated, stored and displayed in real time. The technical staff of the SAIH control room — staffed around the clock during flood episodes — analyse the information in order to:

  • Spot rising trends in river levels and accumulated rainfall.
  • Compare observed values against predefined alert thresholds.
  • Feed hydrological models that forecast discharge.
  • Coordinate warnings with Civil Protection and AEMET.
  • Manage reservoir operations, including scheduled releases and spillway openings.

The network in numbers

Spain’s SAIH is run by the river basin authorities for basins that cross regional boundaries, and by the autonomous communities for basins contained within a single region. Taken together, the network comprises:

  • Roughly 1,800 measuring stations across the country.
  • Around 800 automatic rain gauges.
  • More than 700 gauging stations measuring level and discharge on rivers.
  • Some 300 stations on reservoirs, covering reservoir level and inflow and outflow.
  • Twelve main processing centres, one for each state river basin authority.

The main SAIH networks, by basin, include the Ebro (the largest by number of stations), Júcar, Segura, Guadalquivir, Duero, Tajo, Guadiana, Cantábrico, Miño-Sil, and the internal basins of Catalonia, the Basque Country, Galicia Costa and Andalusia.

Public access: SAIH data is public and can be consulted on each river basin authority’s website. WhatAWeather brings that data together to give a single, readable view of the hydrological situation across the country.

What the river basin authorities do

The river basin authorities are autonomous state bodies attached to the Ministry for the Ecological Transition (MITECO), responsible for managing water in basins that span more than one region. Their SAIH-related duties include:

  • Operating and maintaining the measuring stations and communication networks.
  • Running the control rooms and the technical staff on duty during episodes.
  • Setting the alert thresholds for each control point.
  • Coordinating reservoir releases with the concession holders.
  • Supplying hydrological information to Civil Protection, AEMET and other bodies.
  • Producing post-event reports analysing each flood episode.

Spain currently has the following state river basin authorities: Cantábrico, Miño-Sil, Duero, Tajo, Guadiana, Guadalquivir, Segura, Júcar and Ebro. The internal basins of Catalonia, the Basque Country, Galicia Costa and Andalusia are managed by their respective regional governments, which run their own SAIH systems.

From sensor to warning: how the data flows

To understand what SAIH is for, it helps to follow the information from the sensor beside the river to the decision to issue a warning:

  • Measurement. The sensor records the parameter — rainfall, level, discharge — and sends it to the station datalogger.
  • Transmission. The datalogger forwards the data to the control centre over the communications network. The standard interval is 15 minutes, but it can drop to 5 minutes during a flood.
  • Validation. Incoming data passes through automatic filters that detect anomalies: out-of-range readings, abrupt jumps, missing values. Suspicious data is flagged for manual review.
  • Display. Validated data appears in real time on the control room screens, as time series and synoptic maps.
  • Analysis. Technical staff assess the situation by comparing observations against alert thresholds and weather forecasts.
  • Modelling. The data feeds hydrological models — rainfall-runoff and flood routing — that estimate how discharge will evolve at critical control points.
  • Decision and communication. If thresholds are exceeded or forecasts indicate risk, Civil Protection is informed and the relevant emergency protocols are activated.

Modernisation and current challenges

SAIH is a mature, reliable system, but it faces several challenges that have driven successive rounds of modernisation.

Ageing technology

Many stations still run equipment installed in the 1990s or early 2000s. Dataloggers and communication hardware have a limited service life, and the lack of spare parts for discontinued technologies forces complete replacements. Successive modernisation contracts aim to bring the equipment up to current standards.

Integrating new data sources

Traditional SAIH is built on point measurements at specific locations. Combining it with distributed data — rainfall estimates from AEMET weather radar, satellite products, numerical weather models — gives a complete spatial picture and greatly improves how far ahead a flood can be anticipated.

Cybersecurity

Connecting stations to IP networks exposes the system to cybersecurity risks that did not exist with the original closed radio networks. Protecting this critical infrastructure is a growing priority.

Maintenance in harsh conditions

Keeping stations running in remote ravines, mountain headwaters or areas exposed to frequent flooding is a serious logistical challenge. Floods often destroy the very sensors meant to measure them, leaving gaps in the record at exactly the most critical moments.

A telling detail: During the October 2024 DANA in Valencia, several SAIH stations in the Júcar basin stopped transmitting after being swept away by the flood they were measuring. Losing data at the worst possible moment made the case for more resilient stations and for complementary sources of information.

SAIH and WhatAWeather

WhatAWeather uses public SAIH data as one of its main information sources. Combining it with weather forecasts from Open-Meteo, official warnings, reservoir levels and radar imagery makes it possible to offer a joined-up view of hydrological risk anywhere in Spain.

Where SAIH is a technical system designed for the professionals who run the river basin authorities, WhatAWeather translates that information into a visual, intuitive format that any member of the public can use to check the state of rivers and reservoirs nearby.

Conclusion

SAIH is critical infrastructure that, for almost four decades, has made a decisive contribution to flood prevention in Spain. Its network of sensors, communications and control centres has saved lives by making it possible to anticipate floods and manage reservoirs on the basis of real data. The challenges of this century — climate change, ageing technology, cybersecurity — call for sustained investment in keeping it modern.

Understanding how the system works helps us appreciate why hydrological monitoring matters, and what tools society already has to protect itself from floods.