It is constantly changing its form. The direct process by which water changes from a liquid state to a vapor state is called evaporation . In transpiration , water passes from liquid to vapor through plant metabolism . To be considered an aquifer , the stored water must be accessible to people at a usable rate and volume. Aquifers can be confined or unconfined.
Confined aquifers have non-permeable layers above and below the aquifer zone.
An unconfined aquifer does not have confining layers that retard water movement. Water is continually recycled through aquifer systems. Groundwater recharge is any water added to the aquifer zone. Groundwater discharge is any process that removes water from an aquifer system. Natural springs and artificial wells are examples of discharge processes.