Every raindrop that lands on Britain or Ireland runs downhill into one of seven seas. Every drop on the world's land ends in one of a hundred seas, or in a lake with no way out. Which one does yours reach, and how far does it travel first?
Stand anywhere in Britain when it rains and the water at your feet has already chosen a sea. On the Cotswold edge near Birdlip, a drop can set off down the Churn and the Thames to the North Sea, 323 km away. A drop landing 0.2 km to the west runs down to the Severn instead and reaches the Bristol Channel after only 83 km. The line between them is a watershed, and Britain is criss-crossed with them.
I followed the rain downhill for every 180 metre square of Britain and Ireland, using a model of the land surface and the Ordnance Survey's open map of every named river and stream, and for the whole world using HydroBASINS, a global atlas of river basins. Each place is coloured by the sea its water reaches, using the International Hydrographic Organization's map of the world's seas. Click anywhere, or search for a place, to see where its rain goes, how far it travels and which rivers carry it.
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Great Britain's rain splits between seven of the IHO's sea areas. Almost exactly half of the island, 49.8%, drains east to the North Sea. The western seas together take 42%: the Irish Sea (13%), the Bristol Channel (13%), the Inner Seas off the west coast of Scotland (13%), the open Atlantic (2.2%) and the Celtic Sea (1.1%). The English Channel takes the rest, 8%.
The nations are very different. 58% of England drains to the North Sea, but Wales sends none of its rain there at all: 52% goes to the Bristol Channel and 43% to the Irish Sea. Scotland is split, 50% east and 36% to the sea lochs and sounds of the west. Most of Northern Ireland (75%) drains by Lough Neagh and the Bann, or by Belfast Lough, into the North Channel, which the IHO counts as part of the Inner Seas off the West Coast of Scotland. More than half of Ireland (56%) faces the open Atlantic.
The line that splits North Sea rain from everything else runs the length of the island. On my grid it is about 2,660 km long, wriggling from the north coast of Scotland to the Kent coast near Kingsdown. It reaches its highest point, 1,119 m, beside Ben Alder in the Central Highlands, and only 5% of it lies below 100 m. Away from the coast its lowest point is just 31 m, near Corsback Hill in the flat country of Caithness.
Some towns sit right on a watershed, so that rain falling at one end of the high street goes to a different sea from rain at the other. On my grid 7 of Britain's 1,426 OS towns and cities have their centre on a cell next to one that drains to another sea, and so do 128 villages. Four of the seven are in the Black Country, where the ridge between the Severn and the Trent runs through Dudley, Sedgley, Rowley Regis and Blackheath. The others are Bedworth, Market Drayton and Cumbernauld, which sits on the divide between the Forth and the Clyde.
In a few places three watersheds meet, and a raindrop could set off for any of three seas. I found 6 on mainland Great Britain and its islands. The best known is in the Southern Uplands, where Clyde Law looks down on the heads of the Clyde (west to the Inner Seas), the Tweed (east to the North Sea) and the Annan (south to the Irish Sea). Another is a low, quiet one in north Staffordshire near Swallow Hill, at 179 m, where the Tern sets off for the Severn, the Lea for the Weaver, and Meece Brook for the Sow and the Trent.
Half of Britain is within 40 km of the sea by the route its water takes, and 20% is within 10 km. But 21% of the island sends its rain more than 100 km. The two longest journeys end in different seas: 326 km from near Dylife, under Plynlimon, down the Severn to the Bristol Channel, and 323 km from near Birdlip in the Cotswolds down the Churn and the Thames to the North Sea. Llanidloes is the town whose rain travels furthest (304 km), ahead of Cirencester (300 km) and Swindon (298 km).
Across the world's land outside Antarctica, the Atlantic and its seas take the most rain: 33% of the land, including the Mediterranean, the Black Sea, the Gulf of Mexico and the Baltic. The Arctic takes 16%, much of it through the great Siberian rivers into the Kara and Laptev seas, the Pacific 14% and the Indian Ocean 12%. But 24% drains to no sea at all. Its water ends in lakes and salt pans with no outlet, or simply evaporates: the Caspian Sea (by far the largest, taking the Volga), Lake Chad, the Tarim basin, Lake Eyre and the Okavango delta.
The world's longest journeys in the model are continental: 6,777 km from the hills of Burundi down the Kagera and the Nile to the Mediterranean, 5,893 km from the Tibetan plateau down the Yangtze to the East China Sea, and 5,388 km from the Altai mountains down the Irtysh and the Ob to the Kara Sea. The longest journey that ends inland runs 3,755 km down the Oka and the Volga to the Caspian. Moscow's rain goes that way too.
Only one place on the world grid touches the Atlantic, Pacific and Arctic drainages at once once tiny closed hollows are set aside: the mountains of Glacier National Park in Montana, at 48.6° N, 113.5° W. That is where the famous Triple Divide Peak stands, sending water to the Pacific, to the Gulf of Mexico and to Hudson Bay, which the IHO's 2002 draft treats as part of the Arctic. Indonesia's islands drain to 17 different IHO sea areas, more than any other country; Russia and Canada drain to 12 each. At the other extreme, 7 countries and territories send essentially none of their rain to the sea, all of them in Central Asia and the Caucasus, around the Caspian and the Aral basin.
For Britain I checked the model two ways. First, against the river network itself: following each OS watercourse downstream link by link to its mouth, 99.9% of the 138,242 km of watercourse that the network connects to the sea runs through cells my terrain model sends to the same sea. Second, against HydroBASINS, built from a different elevation model (HydroSHEDS) by different people: the two maps agree on 99.0% of Great Britain and 98.1% of Ireland. The largest differences are where two neighbouring sea areas meet at the coast, such as the Bristol Channel and the Celtic Sea, rather than on inland watersheds.
Britain and Ireland. I built a grid of web-mercator cells about 180 m across from AWS Terrain Tiles (zoom 10, averaged to zoom 9). Every OS Open Rivers watercourse in Great Britain except canals was burned into the surface, deeper for bigger rivers, so that paths keep to the mapped channels. A priority flood then climbs inland from the sea all round the coast, always taking the lowest cell next; each land cell inherits the sea of the cell it was reached from, which is the next step down its path. The sea of each coastal cell is the nearest IHO sea area. Path lengths add up the steps (diagonal steps count √2). River names come from the OS watercourses the path follows for at least about 1 km (2 km for brooks, drains and ditches). Ireland and the Isle of Man use the terrain alone.
The world. HydroBASINS level 7 (57,646 sub-basins, version 1c) rasterised at 0.05°. Each sub-basin is followed downstream to its next sink. If that is an endorheic sink, or the shore of the Caspian Sea, the water stays inland; otherwise it reaches the IHO sea nearest to the basin's single pour point. Lumped coastal sub-basins, made of many small streams, are assigned cell by cell to their nearest sea, as are a few far-flung islands that HydroBASINS lumps with another coast. Distances are HydroBASINS river distance to the sink plus the straight line to the sub-basin's own outlet. I group the IHO seas into oceans with a simple convention: the Mediterranean, Black, Baltic and Caribbean seas and the Gulf of Mexico count with the Atlantic; the Greenland Sea, Baffin Bay, Hudson Bay and the seas north of Russia and Alaska with the Arctic; the seas of the Indonesian archipelago with the Pacific, except the Timor, Arafura and Savu seas.
Limits. This is a model of where water would flow over the land surface, not a hydrological survey. It ignores groundwater (chalk and limestone can carry water under a surface divide), pumped drainage, canals, water transfers and reservoirs that pipe water to other catchments. On flat land, such as the Fens, the Somerset Levels and Ireland's central lowlands, a 180 m terrain model cannot always see which way water goes; the burned-in rivers fix this in Britain but not in Ireland. The OS network has small gaps; I bridged those of up to 300 m. Shares are of the grid's land, which is a few per cent larger than official land areas because coastal cells and tidal flats count in full. HydroBASINS counts every closed hollow as a sink, so its inland share is higher than one that counts only large closed basins. Mexico City, for instance, sits in a valley that was naturally closed and is now drained by tunnels, and the model leaves it inland. River names for the world come from Natural Earth's generalised rivers, so many small rivers have no name.