Most aquifers are replenished naturally by infiltration of
water from precipitation of rain and snow in the recharge area
which may be many miles from the point of withdrawal from wells.
If water is pumped from many wells at a withdrawal rate in an
aquifer that exceeds the natural recharge rate, the water table
drops. As shown in the diagram, a cone of depression may form
around a well. Depending upon the depth, other wells in the area
may go dry. If this situation prevails for any significant amount
of time, this is called water "mining". This may happen
from rapid withdrawals for irrigation purposes from so called
"fossil aquifers" which get very little if any recharge.
Such "fossil aquifers" underlie the Sahara and
Kalahari deserts in Africa, the Great Artesian Basin in Australia,
the central Asia basins, and the Ogallala Aquifer in the western
Mid-West of the United States.