Experts estimate that probably only a fifth of all rainfall is "effective" - that is, it reaches subterranean storage areas. The rest evaporates, is taken up by plants and trees, or runs off down rivers. After prolonged dry spells, such as we have had in recent years, the top layer of soil must become saturated before moisture can start dripping down into the lower strata.
Then gradually water sinks into porous rock such as sandstone, whose individual grains are circular, or into the cracks and fissures of hard rock like limestone. Depending on depth, and on the density of the stratum, the process can take from weeks to tens of thousands of years.
The amount of water stored beneath Britain is immense. The main chalk aquifer alone stretches from the Dorset coast all the way to North Yorkshire. It has been calculated that, in the chalk and sandstone, the top 20 metres alone contain about 20 times as much water as all the surface reservoirs in the kingdom put together.Reuse content