Thames and Severn

The global water cycle is closed. July in the London basin is a local deficit covered by reservoirs in the upper Thames, the London ring main, and abstraction licences. The ocean store did not notice. The town would, if the taps failed.

The Thames is a lowland river with a large population on its floodplain. Groundwater in the chalk, soil moisture, and surface reservoirs are the stores that matter to a water company. Transfers are pumping and pipes as well as tributaries. The Severn, longer and wetter in its upland headwaters, is a different regime: more flashy in Wales, then a huge tidal range in the estuary — among the largest in the world.

A drainage-basin overlay on the water-cycle lab is this case. The watershed is the divide. Rain on one side goes to the Thames, on the other to a neighbour. Abstraction is a tap on a local store. Over-draw it, and the reservoir falls even if the ocean is unchanged.

Climate change moves the flows: heavier winter bursts, drier summers in the south-east. It does not invent a new cycle. Students who can name stores and transfers can then talk about drought without saying ‘the water ran out of the planet’.