Where Rain Goes: The Sea Every Place in Britain, Ireland and the World Drains To

Every raindrop that lands on dry land either reaches the sea or ends in a basin with no outlet. I mapped which sea that is for every place in Britain and Ireland at about 180 m resolution, and for the world's land outside Antarctica at 0.05°, and published the result as an interactive map in which any spot can be clicked to see its sea, the length of the journey and the rivers on the way. For Britain I ran a priority flood from the coast over a terrain model with the Ordnance Survey's open river network burned in, assigning each coastal start to a sea area of the International Hydrographic Organization (IHO). For the world I traced HydroBASINS level 7 sub-basins to their sinks and outlets. 49.8% of Great Britain drains to the North Sea and 42% to the seas of the west; Wales sends none of its rain to the North Sea. The North Sea divide is about 2,660 km long on the grid, and I find 6 triple points where three sea areas meet, including Clyde Law at the heads of the Clyde, Tweed and Annan. The longest journeys in Britain are 326 km down the Severn and 323 km down the Thames. Worldwide, 23.6% of the land drains to no sea, by far the largest such basin being the Caspian. The model agrees with the river network along 99.9% of connected watercourse length and with HydroBASINS on 99.0% of Great Britain and 98.1% of Ireland. Interactive version: https://maps.tristanhotham.com/where-rain-goes/. Data and code: https://github.com/Tristan-Hotham/where-rain-goes. Blog post: https://blog.tristanhotham.com/2026/10/28/where-rain-goes/.