Planning
What more frequent data would cost
For the 5 big reservoirs, checking each tank's curve is free: the Irrigation Department publishes their gauge levels every day. The 8 village tanks have no gauges. Pick what you would add for them and see the cost.
An ESP32 board, a waterproof distance sensor, a 4G module and a small solar panel, all bought in Sri Lanka. It records the tank's height every day, so every satellite picture becomes a point on the curve.
Maps the depth of a tank bed and saves the points as a file, so it checks every layer directly. It needs a boat: we would use a local fisherman's boat with life jackets, because we never go into the water ourselves.
A commercial pressure logger, accurate to about 0.4 cm. The one-off extra is an air-pressure logger, a base station and software. Readings are collected by visiting the tank.
One Umbra radar image (1 m, 5 × 5 km) on a day we choose, through cloud. The free Umbra and Capella archives have no images over our tanks.
A DJI Mini 4 Pro Fly More Combo, to map the dry tank beds in the dry season. Flying needs Civil Aviation Authority approval and Ministry of Defence clearance, so borrowing one is better.
Free first
Free data we can add at no cost
Planet, for universities
3 m pictures almost every day, up to 3,000 km² a month, with a 30-day delay. Needs a university email.
NISAR radar
A new NASA and ISRO radar satellite that sees through cloud, every 12 days. Its data has been public since July 2026.
NASA HLS
Landsat and Sentinel-2 combined: a 30 m picture every 2–3 days on average, but clouds still block it.
Official gauges
Daily water levels for the 5 big reservoirs, saved automatically by our logger.
Sources
Where the prices come from
All checked on 27 October 2026. Prices change, so check again before buying.
- Logger parts: ESP32 board, waterproof sensor, 4G module, solar panel, battery and box; mobile data from Dialog.
- Sonar: Deeper PRO+ 2 (sold out on the US store on 27 Oct); its depth data download.
- HOBO logger: Instrumart.
- Radar images: Umbra price list.
- Drone: LuxuryX; rules: Civil Aviation Authority of Sri Lanka.
- Free data: Planet Education and Research, NISAR at ASF, NASA HLS.
- Exchange rate: ExchangeRate-API, 26 Oct 2026.