The hydrologic cycle -- The Earth's water transfer
system is called the hydrologic cycle. The water system is powered
by the sun. 97.2% of the Earth's total water is in the oceans.
2.2% is in ice caps and glaciers. 0.6% is in groundwater, rivers,
lakes and the atmosphere.
Freshwater budget -- Groundwater is 14% of freshwater
in hydrosphere. The average rate of groundwater exchange is 280
years. Exchange means the transfer from the atmosphere to the
ground and back to the atmosphere. This exchange rate gives an
indication that groundwater movement is slow.
Water table -- The water table defines the top of
the zone of saturation, where groundwater occurs. Porosity governs
the amount of groundwater that a rock material can hold. The water
table is not flat. The water table is higher under hills than
under valleys. Groundwater flow is impeded by indirect flow around
Permeability -- Permeability defines the ability
of a porous rock to transmit a fluid. Permeable layers are called
aquifers. Impermeable layers are called aquicludes. Springs arise
from a perched water table. Otherwise groundwater discharges into
streams as a consequence of flowing from a higher water under
hills towards the lower water table in valleys. If water discharges
into a stream, the water table comes to surface right at the stream.
In this case the water table slopes downhill toward the stream.
Mining Groundwater -- Wells are drilled into the
zone of saturation below the water table. When wells are pumped,
the water flows to the well which locally depresses the water
table (drawdown) and creates a cone of depression.
Groundwater is an exhaustible resource.