The Hydrologic Cycle
The major components of the hydrologic cycle
Looking at an aquifer as an
example, percolation of water into the ground is an inflow to the aquifer.
Discharge of ground water from the aquifer to a stream is an outflow (also
an inflow for the stream). Over time, if inflows to the aquifer are greater
than its outflows, the amount of water stored in the aquifer will increase.
Conversely, if the inflows to the aquifer are less than the outflows, the
amount of water stored decreases.
The hydrologic cycle consists of inflows, outflows, and storage. Inflows add
water to the different parts of the hydrologic system, while outflows remove
water. Storage is the retention of water by parts of the system. Because
water movement is cyclical, an inflow for one part of the system is an
outflow for another.
Ground water is water stored in the aquifer and accounts for .6% of the
earth's water. Precipitation and stream flow recharge the groundwater in
western Montana. Water well drilling draws the groundwater from the aquifer
for household, agricultural, or industrial use.
Over 50% of people use
groundwater as a source of drinking water, therefore groundwater quality is
important. While surface waters are easily polluted, groundwater is somewhat
protected by the soil that not only covers it, but also acts as a filter
when groundwater passes through it. However, soil cannot filter out many of
the pollutants humans generate, and groundwater is at greater risk of
According to the United
States Geological Survey (USGS) figures, groundwater provides an
22% of all freshwater withdrawals.
37% of agricultural use (mostly for irrigation)
37% of the public water supply withdrawals.
51% of all drinking water for the total population.
99% of drinking water for the rural population.
A Few Facts About
Percentages of Water On Earth: