Himalayan flood study estimates 293,800 people in potential exposure zone

A Scientific Reports study published on 8 October estimates that 293,800 people in Himalayan catchments spanning Nepal and parts of India and China live within a zone of potential flood exposure. Researchers also found that building area in the zone grew by about 46% between 2016 and 2023, raising the importance of accurate maps for communities downstream of glacial lakes.

The population figure covers about 18% of residents in the catchments examined. It counts people estimated to live in buildings up to 50 metres vertically above nearby river channels. That measure helps identify places that may be exposed to flooding; it does not mean all 293,800 people are expected to be flooded, or face the same level of danger.

How the Himalayan flood exposure estimate was made

C. Scott Watson and colleagues studied areas up to 50 kilometres downstream of 1,361 Himalayan lakes. They combined building footprint data with population projections and measures of proximity to river channels. In mountainous valleys, vertical height matters: a building can be close to a river on a map yet sit well above it on a steep slope.

The researchers used height above nearest drainage, or HAND, to identify buildings within 50 vertical metres of a channel. Watson told EL PAÍS that this differs from drawing a fixed horizontal distance around a waterway. The resulting population total is a broad screening estimate, rather than a count of residents inside the mapped path of any particular flood.

For Nepal, the study estimates that 108,500 people lived within 50 vertical metres of glacial-lake flow paths in 2026. Of those, 22,800 were estimated to live within 10 vertical metres. The wider 293,800 figure includes the study catchments across Nepal and neighbouring areas of India and China.

What the estimates cannot establish

The authors compared five building datasets with manually mapped reference buildings at 13 sites. Accuracy and completeness varied across the datasets. Google Open Buildings Temporal had the highest median F1 score, 0.68, but the validation still revealed omissions and false detections. Such errors can change estimates of how much development lies in potentially exposed terrain.

The population estimate relies on WorldPop projections rather than a census of every building in the zone. The researchers checked those projections against census data at one point in time in Nepal. They caution that uncertainty in future population projections is unknown and that gridded data may miss some rural residents.

Nor does exposure alone establish likely losses. Flood depth, water speed, the chance of an event, building occupancy and residents' circumstances all affect what a flood could do. The study does not fully model those factors for each location. Its estimate should therefore guide closer investigation, not be read as a prediction of casualties or property damage.

Why detailed flood maps matter

Watson told EL PAÍS that the findings call for planning based on risk and described the method as a way to identify exposure hotspots. Detailed local flood mapping would be needed to judge particular places. The study also projects 17,900 more people, a 6% increase, in the 50-metre vertical zone between 2015 and 2030; that figure is a projection, not an observed outcome.

An earlier global study estimated 15 million people potentially exposed to glacial lake outburst floods, including 9.3 million in High Mountain Asia. Watson told EL PAÍS that its horizontal distance method differs from the vertical measure used here. The two totals therefore describe different populations and cannot be compared as though they were updates to the same count.