From water height to water volume

Tank A is a made-up village tank with round numbers, so the maths is easy to follow. Drag the water up and down, or play a whole year and watch the satellite updates arrive.

SWOT measures the height
Sentinel measures the area
The curve links height and area
Thin layers add up to the volume

Water height 101.25 m

Swipe sideways on the picture to see the whole tank. Not to scale. Real tanks are hundreds of metres wide but only a few metres deep, so the depth is stretched here to make it visible.

The sum, layer by layer

What Sentinel sees from above

When the water drops, the edge moves in. The dashed line shows the tank when it’s full. Floating plants can hide part of the water: a plain water map then shows too small an area, and too little water. Our AI model has a “water under plants” class for this.

The curve that links them

Each faint dot is a past day when SWOT and Sentinel both saw the tank. There are no dots below 100 m, so that part of the curve is only a guess.

A year of updates

Sentinel-1 radar, about every 12 days (sometimes 6), sees through cloud
Sentinel-2 photo, every 5 days, but only about 1 in 5 is clear
SWOT pass, about twice every 21 days; on a tank this small it is typically off by about 13 cm
Real water, for comparison

Add a water-level logger in the tank. It records the height every day (we match its zero to SWOT once), so every Sentinel picture becomes a new dot on the curve.