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NASA's SWOT satellite can measure water levels along most of 800,000 km of Asian canals
University of Washington researchers say NASA's SWOT satellite can measure water levels on more than 85% of 800,000 km of Asian irrigation canals. The mission was never designed for canals, so the map points to monitoring from hardware already in orbit, with its scores checked against US canal measurements.
The Scientist · Science desk
What happened
- Lead author Mridul Sharma overlaid SWOT radar data on GRAIN, a global canal map that co-author Faisal Hossain and collaborators published in 2025.
- The strongest readings came from wider, well-organized canals on smooth slopes with open surroundings, and dense vegetation was the satellite's biggest obstacle.
- Compared with real canal measurements in the United States, the team's confidence levels matched what the satellite could actually do.
- The University of Washington study was published on Sept. 30 in Geophysical Research Letters.
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Why it matters
- capability Water managers who now depend on scattered gauges, inspections and local reports could get water-level readings along most canal kilometers without installing new equipment.
- constraint Roughly 124,800 km of the studied canals scored as poorly observable, so those stretches still need ground gauges or inspections for reliable readings.
- decision An agency weighing satellite readings against new gauges would want comparisons on its own canals first, since the confidence scores were validated against US measurements.
Each kilometer of canal got its own confidence score [9], so canal length is the denominator to check. The high and moderate bands add to 84.4% of the kilometers studied [1]. The summary figure is "more than 85%" of locations [2]. The two numbers may be counting different units, and the gap is under a percentage point. In length, about 300,000 km rated highly observable and about 375,200 km moderately observable [3][4].
The sample is large and regional. The 800,000 km across 22 Asian countries is about 21% of the 3.8 million km that the GRAIN registry charts worldwide [8][6][2]. The team treated Asia as a proof of concept, in a region where irrigation helps feed roughly 3 billion people [8].
It is a tidy design. "GRAIN mapped where canals are. SWOT revealed what was happening inside them," Sharma said [15]. The score for each kilometer rested on two tests: how closely the satellite's view of the water matched the known contours of the land, and how strong the signal was against the noise [9]. Both tests are drawn from the satellite data itself, so neither one confirms a reading against the ground.
"It was quite the accidental and pleasant discovery to see that SWOT is able to track flow direction and water levels in most of the canals skillfully," Sharma said [14]. The accident is genuine. SWOT was built to track oceans, lakes, rivers and other large bodies of water [4]. Irrigation canals are often narrower than 20 meters, at the extreme low end of what the satellite can observe, and the mission's architects assumed changes in small canals would be invisible to it [5].
The thing this doesn't tell you is whether readings arrive often enough to run a canal. SWOT's interferometric radar compares scans over time to detect changes in elevation, and it observes surface water at least once every three weeks [3]. The observability score measures whether a pass can see the water. How often the passes come is a separate limit for a canal operator, and the study as described does not test it.
Faisal Hossain, a UW professor of civil and environmental engineering and a co-author, went further. "It may represent a fundamentally new way of managing the water conveyance systems that sustain modern agriculture," he said [16]. I think the evidence so far supports a narrower claim: a kilometer-by-kilometer map of where an instrument already in orbit can see canal water [9]. The team expects the satellite's performance to improve as researchers build better algorithms and analysis methods [13].
What to watch
- Comparisons between SWOT readings and ground gauges on Asian canals, especially vegetated ones, would test whether the US-validated scores hold in the region studied.
- An extension of the kilometer scoring beyond Asia's 800,000 km to the rest of the 3.8 million km GRAIN map.
- Whether new SWOT processing algorithms shrink the 15.6% of canal length now rated poorly observable.