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Hydrologic Cycle
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Hydrologic Cycle

The Hydrologic Cycle

The major components of the hydrologic cycle are:

  • Evapotranspiration

  • Condensation

  • Precipitation

  • Infiltration

  • Percolation

  • Runoff

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 becoming contaminated.

According to the United States Geological Survey (USGS) figures, groundwater provides an estimated:

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:

  • OCEANS 97.5%

  • FRESH WATER: 2.5%

  • Glaciers / Polar Ice Caps 79%

  • Groundwater 20%

  • Lakes, Streams, etc. 1%

  • ATMOSPHERE .001%




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