How satellites measure dams
Satellites fly over the Mekong every few days. They take pictures of the reservoirs behind the dams.
One kind of satellite uses radar (Sentinel-1). Radar can see through clouds and works at night. Water looks dark in radar pictures, so we can draw the edge of the lake.
We measure how big the water surface is each month, in square kilometres. A bigger lake usually means more water is stored.
Another satellite, SWOT, measures the height of the water. We compare area and height to check our work. At big dams they match well. At small, narrow dams they match poorly.
Satellites cannot see inside the dam or its gates. Our numbers are good estimates, not exact readings. Always check official notices too.
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