How the Three Gorges monitor works
The Live Monitor is a real pipeline, not a picture. Every point on its chart is measured from a Sentinel-2 satellite pass over the reservoir, pulled from open data with no download step, indexed for water, and checked against the season. Here is each stage, exactly as the code runs it.
Find the passes in the open archive
We query the Sentinel-2 L2A catalogue over the dam at Sandouping through the Earth Search STAC on AWS Open Data. No login, no bulk download. We filter to the one map tile that fully covers the reservoir reach (49RDQ) and keep the clearest scene per month, from January 2023 to the most recent cloud-free pass.
Read only the pixels over the reservoir
Each scene is a cloud-optimised GeoTIFF. Instead of pulling the whole image, we read just the window over the reservoir straight from the file on the web, at roughly 60 m. That turns a gigabyte-scale scene into a few megabytes and is why the whole history runs in minutes.
Bought infrastructureMask the cloud, then index the water
We drop cloud and shadow using the scene classification band that ships with every Sentinel-2 tile, and throw out any pass with less than 80 percent clear sky over the reservoir, so partial cloud cannot fake a drop. On what is left we compute the NDWI water index, (green − near-infrared) / (green + near-infrared), and count the pixels above zero as open water. That is the reservoir surface, in square kilometres, for that day.
Learn what the season should look like
A reservoir is supposed to rise and fall on a yearly rhythm, so a raw drop means nothing on its own. We fit a seasonal curve to the whole water-area history, a Fourier model in day-of-year that captures the reservoir's sharp yearly drawdown, so that for any date we know the level the season predicts. That fitted curve is the dashed line on the chart.
Rheality's engineFlag only what breaks the pattern
Each pass is scored by how far it sits from the seasonal expectation, using a z-score built to resist outliers. Anything past three standard deviations is flagged for a human to explain. On the current run that is a single point, the unusually deep drawdown of May 2025, which is a hydrology story rather than a structural one, and exactly the kind of thing the flag is meant to surface.
Rheality's engineRheality provides decision-support monitoring data, not a safety guarantee. See
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Step photos via Wikimedia Commons: NASA/JPL-Caltech, NASA and NASA GSFC (public domain); Christian David (CC BY-SA 4.0); U.S. Navy (public domain).