More than a third of the public water supply in England and Wales comes from groundwater. In parts of the South East, that figure is as high as 72%.

Groundwater is rain which has soaked through the soil into layers of permeable rock known as aquifers. Gravity and pressure draw the water through the rock, filtering it in the process.

This makes groundwater an excellent source of clean water as it does not require much treatment to bring it up to drinking-water standard. The water is accessed by boreholes.

Aquifers, which also feed rivers and springs, need to be replenished by winter rain.

Once the growing season begins, plants and trees soak up the water before it has had a chance to seep down to the rock.

And as the weather warms up, water evaporates more readily from the soil.