About Danube Alert

Glossary of hydrological terms

Eighteen terms that appear on the level, alert, model and navigation pages, briefly explained and each with a real example from a gauge. Every term has its own link, so you can point straight to the one you need.

Water level

The water level is the height read at the station, in centimetres above the station’s gauge zero: it says how high the water is, not how much water flows.

Example: At Belgrade, on 24 October 2026, the water level was 134 cm, 5 cm higher than the day before.

See also: Discharge · Gauge zero (PNP)

LDC — Low water

LDC is the minimum navigation level of the Danube Commission, reached or exceeded on 94% of the ice-free days in a 30-year period.

Example: At Kienstock, LDC is 161 cm; on 27 October 2026 the level was 151 cm, 10 cm below it.

HDC — High water

HDC is the maximum navigation level of the Danube Commission, exceeded on only 1% of the ice-free days in the same 30-year period.

Example: At Kienstock, HDC is 601 cm, 440 cm above LDC.

ENR / HNN

ENR is the Romanian low-water level exceeded on 94% of days, the LDC equivalent. HNN is the Romanian high-water level exceeded on only 1% of days, the HDC equivalent.

Example: At Calafat, ENR is −5 cm and HNN is 657 cm above gauge zero.

Gauge zero (PNP)

PNP is the gauge zero, the reference datum of the level staff against which readings are measured, not a threshold.

Example: At Smederevo the gauge zero is 65.36 m above sea level; a reading of 421 cm means 69.57 m.

Staff gauge

A staff gauge is a graduated ruler fixed in the river, from which the water level is read.

Example: At Komárno the gauge zero is 103.335 m above sea level, and on 22 October 2026 the water stood at 23 cm.

Rkm (river kilometre)

Rkm is the river kilometre: the distance in kilometres from the mouth of the arm (Sulina), not from the source. On the Danube, rkm increases upstream, so a larger number means farther from the mouth.

Example: Calafat sits at rkm 795, i.e. 795 km from the mouth of the Sulina arm.

Discharge

Discharge is the volume of water passing a river cross-section in one second, measured in cubic metres per second (m³/s). A level says how high the water is; discharge says how much water is flowing.

Example: At Smederevo, on 22 October 2026, the discharge was 1200 m³/s at a level of 421 cm.

Return period

A return period shows how rare a level is: a 100-year flood has a 1% chance in any year, not a promise that it comes exactly once every 100 years.

Example: At Achleiten, the 100-year flood level is 914 cm.

Flood-defence level

A flood-defence level is the state-set level at which flood measures begin. Romania has three stages — CA, CI, CP; Hungary has stages I–III, and Serbia has redovna and vanredna odbrana.

Example: At Isaccea, the stages are CA 380 cm, CI 508 cm and CP 520 cm.

The “backup” state

The level comes from a neighbouring backup gauge: the station’s usual source is missing or stale, so this value is a substitute brought onto the same gauge datum, not the station’s normal reading.

Example: At Budapest, on 22 October 2026, two sources gave 410 cm and 12 cm for the same hour; such a mismatch is not mixed, and when the usual source is missing a neighbouring gauge takes over, marked “backup”.

Model or measurement

The forecast of how much water will flow is not a measurement.

Example: At Bezdan, on 22 October 2026, the gauge read −146 cm and the forecast for 23 October was −158 cm: the first is a measurement, the second a calculation.

2-, 5- and 20-year return levels

The 2-, 5- and 20-year levels are the return periods the model watches in each river cell. They are discharges, not water levels, and follow the same return scale as the 100-year flood: the longer the period, the higher the threshold.

Example: At Achleiten, LDC is 258 cm, HDC 510 cm and HW100 914 cm; the 2-, 5- and 20-year levels are discharges, not water levels, and are measured in m³/s.

See also: Return period

Forecast ensemble

An ensemble runs the same model several times with small variations in the starting data and gives a range instead of a single number. The site keeps the median and the P25/P75 quartiles, not every member.

Example: The forecast ensemble has 50 members and the seasonal one 51; the site keeps the median and the P25/P75 quartiles, not the 50 members, and the weekly bulletin prints only the direction, with no model figure.

Model forecast and official forecast

Two forecasts appear on the chart and are not mixed: the official one for the gauge level, and the model one, computed separately. Model forecast: The discharge forecast is computed by a hydrological model from rainfall and catchment data; it is not a gauge reading. The model has its own uncertainty and a limited lead time, so it can differ from the actual discharge.

Example: At Bezdan, on 22 October 2026, the official forecast for 23 October was −158 cm and the one for 24 October −163 cm; the model forecast is computed separately from rainfall and catchment data and can differ.

See also: Model or measurement · Forecast ensemble

Model quality at a station

The model quality label says, for each station, how well the model matched the measured discharge in the past: good, mediocre or poor. The model forecast is reliable here up to +N days.

Example: At Smederevo, on 22 October 2026, the measured discharge was 1200 m³/s; the quality label does not look at the official forecast but compares the model with that measurement, on 1–5-day horizons.

See also: Model or measurement

Drought in the model indicators

Model-based indicator, updated every 10 days. On the map, each area gets a single stage: rainfall below normal, dry soil or affected vegetation; the warmer the colour, the further the drought has gone. On the map, low-flow rivers are shaded from yellow to red; red marks the most severe water shortage, and grey means the river is not assessed.

Example: On the Drava, the 2026 drought brought the record low on 22 October, and the drought indicators are recalculated every 10 days.

Observed flooded areas

Observed flooded areas are outlines taken from satellite images on a given date: they show the water seen then, not an alert and not the official limit of a flood.

Example: In the flood of 21 September 2024, one satellite scene showed 628 water pixels, i.e. 219.54 km² of flooded area.

Data sources

DanubeAlert shows the official water levels that each country publishes.

• Germany: Wasserstraßen- und Schifffahrtsverwaltung des Bundes (PEGELONLINE)
• Germany: Bayerisches Landesamt für Umwelt (Hochwassernachrichtendienst)
• Germany: Bayerisches Landesamt für Umwelt (Gewässerkundlicher Dienst, GKD)
• Austria: viadonau (DoRIS)
• Austria: Amt der Oö. Landesregierung, Hydrographischer Dienst
• Austria: Amt der NÖ Landesregierung, Hydrographischer Dienst
• Slovakia: Slovenský hydrometeorologický ústav (SHMÚ)
• Hungary: Országos Vízügyi Főigazgatóság (vizugy.hu)
• Croatia: Državni hidrometeorološki zavod (DHMZ)
• Serbia: Републички хидрометеоролошки завод Србије (RHMZ)
• Romania: Administrația Fluvială a Dunării de Jos Galați (AFDJ)
• Romania: Administrația Națională „Apele Române” (ANAR)
• Romania: Institutul Național de Hidrologie și Gospodărire a Apelor (INHGA)
• Bulgaria: Изпълнителна агенция „Проучване и поддържане на река Дунав” (ИАППД)
• Bulgaria: Национален институт по метеорология и хидрология (НИМХ)
• Moldova: Serviciul Hidrometeorologic de Stat
• Ukraine: Український гідрометеорологічний центр
• Switzerland: Bundesamt für Umwelt (BAFU), Hydrologische Daten
• Italy: Autonome Provinz Bozen – Südtirol, Hydrographisches Amt
• Czechia: Český hydrometeorologický ústav (ČHMÚ)
• Slovenia: Agencija Republike Slovenije za okolje (ARSO)
• Bosnia and Herzegovina: Federalni hidrometeorološki zavod BiH
• Bosnia and Herzegovina: Републички хидрометеоролошки завод Републике Српске
• Bosnia and Herzegovina: Agencija za vodno područje rijeke Save
• Montenegro: Zavod za hidrometeorologiju i seizmologiju Crne Gore (ZHMS)
• Poland: Instytut Meteorologii i Gospodarki Wodnej (IMGW-PIB)

DanubePortal (Danube FIS) fills in only the days without an official reading. Weather data comes from Open-Meteo.

For Komárno and Štúrovo, the records before 2024 are built from the Hungarian measuring point at the same bridge (Komárom, Esztergom; OVF). We take the Hungarian level and add the difference between the two points. We use the middle value of that difference for each water year, from autumn to autumn.

Data transparency

About the Alert System

Danube Alert watches the water all the time. When the water reaches a level you set, you get an email.

Types of Alerts

• Alerts for low and high water: You get a message when the water comes close to LDC or HDC. You can also set your own level.
• Your own alert: Choose a water level. When the water reaches it, you get a message. You can change the level at any time.
• No repeated messages: We send a message only when the water is close to the level you chose, within 10–15% of it. A 2% safety gap stops false alarms.

How you get the message

Email: Alerts arrive at the address you confirmed. You can also ask for a daily report with the stations you follow.

More alert settings

• How often we write: Choose the maximum number of messages per day.
• Choose the level: Set how close the water must come to LDC or HDC before we write to you.
• Choose the kind of level: a share of LDC or HDC. You can also choose the official flood levels, where they are published, or your own level.

The alert system is built to be fast and exact. It helps you see dangerous conditions in time.