A peer-reviewed study has found that 214 rain gauges across northern Thailand disappeared from the country’s public rainfall archive after a 2018 data-system change, leaving years of readings out of maps used to plan drainage and flood protection. In the Ping River basin, the omitted gauges recorded 31% more days of extreme rain than the official dataset showed, the paper says.
The findings, published on Thursday in the Journal of Monsoon Systems, draw on seven years of hourly readings from gauges run by provincial offices and universities. Researchers matched 1.8 million measurements against Siam Monsoon Service records and found that the agency’s public maps marked dozens of still-operating stations as inactive or unreliable.
The gauges that went missing
Lead author Zula I. Sporer, a hydrologist at Northern Basin University, said the stations were not hidden in locked rooms: many continued sending data to local offices, but their readings no longer appeared in the central archive. Her team checked station logs, maintenance dates and duplicate measurements at 46 sites before concluding that the gap was systematic.
In the Mae Chaem district of Chiang Mai province, the paper found, six gauges logged rainfall above 100 millimetres (3.9 inches) in 24 hours on 19 occasions from 2019 to 2025. The national archive recorded only 11 such events in the same area. Researchers say the missing measurements matter because intense rain can vary sharply between steep valleys only a few kilometres apart.
The country was planning for rain that its own gauges had already measured.
Zula I. Sporer, hydrologist
The paper says a software update introduced a new “calibration confidence” flag, and stations without a recent central inspection were filtered out of public products. The authors found no evidence that officials deliberately removed the readings. But a former archive contractor told the researchers that local offices received no clear notice that their gauges had stopped feeding into national maps.
Plans built on thinner data
Siam Monsoon Service director Clara Rubio said the agency had asked its regional offices to verify the affected stations and expected an initial review within 60 days. She said the 2018 filter was intended to prevent faulty readings from distorting forecasts, and added that “the paper identifies a gap we need to check carefully”. The agency did not provide a count of stations already restored.
The omission could affect drainage work across three provinces, where planned flood-control and reservoir projects are valued at 6.8 billion baht (about €177 million or $197 million). The researchers have not calculated how much any individual project would need to change, but say engineers should rerun rainfall models before final designs are approved.
Hannan Ooi, a civil engineer who reviewed the paper for the Thai Association of Water Planners, said the result did not prove that existing structures were unsafe. It did, however, make some design assumptions difficult to defend without fresh calculations. The association has urged provincial planners to publish the station data they hold and compare it with the national archive.
The research team has released a station-by-station list and asked the service to restore the measurements in downloadable form, not just revise future forecasts. Sporer said the next test would be whether planners recalculate risk maps before the next wet season, rather than treating the missing years as a historical footnote.
