How to read a station page

A station page shows four things at once: the current level, the trend of the past days, the station thresholds and the forecast. We take them one by one so you know what you are reading.

Glossary of hydrological termsLDC — Low waterHDC — High waterGauge zero (PNP)Flood-defence levelModel or measurement

The level: the water now

The level is the water height read at the gauge, in centimetres. It is measured from the gauge zero, a fixed reference of the station, not from the river bed. A level of 300 cm therefore does not mean the water is 3 metres deep.

Next to the level is the status against the thresholds: “below LDC (low water)”, “within the normal range” or “above HDC (high water)”. If the station has no recent reading, the page says “no reading in the last 48 h”, and the figure is left out of the trend and of alerts.

ScreenshotThe screenshot shows the top of the station page: the station name, the level in large centimetres, the status label against the thresholds and the trend arrow. The chart of the past days sits below them.

The trend: where the water is going

The trend compares the current level with the level 24 hours ago and with the past days. “Rising”, “falling” or “stable” describe the direction, not a promise for tomorrow.

A single reading does not make a trend. If the water rose sharply today but was higher yesterday, the page may still show “falling”: look at the chart, not only at the arrow.

ScreenshotThe screenshot shows the station chart: the line of levels from the past days, the LDC and HDC threshold lines and, where available, the forecast line. The selected range, 7 or 30 days, is also visible.

The thresholds: LDC and HDC

A threshold is a reference level of the station, not today’s level. LDC is the low-water threshold: below it, the water is low. HDC is the high-water threshold: above it, the water is high.

Between LDC and HDC the page shows where the level sits on the scale: “at 40% of the LDC–HDC range”, “30 cm below LDC” or “20 cm above HDC”. The percentage is not a probability, it is a position on the scale.

Some stations also have flood-defence stages: alert level, flood level, danger level. They are thresholds too: reference levels, not today’s measurements.

ScreenshotThe screenshot shows the threshold area of the station page: the LDC line, the HDC line, the position of today’s level between them and, where the station has them, the flood-defence stages. The distance in centimetres to the threshold is visible too.

The forecast: a model, not a measurement

The forecast is computed by a model from rainfall, catchment data and what has been measured so far. It is not a gauge reading and not a measurement taken in the future.

It has a lead time, usually 24–120 hours, and its own uncertainty. That is why it can differ from the level that will actually be measured: the longer the horizon, the larger the difference can be.

When the forecast shows a threshold being crossed, an alert can arrive 1–3 days ahead. When forecast and measurement disagree, the measurement is what happened; the forecast is what might happen.

ScreenshotThe screenshot shows the forecast on the chart: the dashed line or shaded band that continues the measurement line, with the 24–120 hour horizon. The “model forecast” note is visible too, saying the figure is computed, not measured.

Keep in mind

In short: the level is measured now, the trend says where it is going, the threshold is a reference, and the forecast is an estimate. None of them replaces the official notices.

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