The water table

Water soaking down stops when it reaches an impermeable layer and fills the spaces above it from the bottom up. Below, every space is full of water: the saturated zone. Above it, the spaces hold some moisture and air. The top of the saturated zone is the water table; a well gives water only once it goes below it. The water table can be a few metres down near the Caspian and over 150 metres in Iran's dry plains; it rises in wet years and falls in dry ones. When more is pumped out each year than rain puts back, it sinks, shallow wells run dry, and the ground can subside.