Hydrology

The Hydrologic Cycle

The hydrologic cycle describes where water is stored on the earth, how much water there is, and how water moves from one reservoir to another. Figure 1 is a diagram of the hydrologic cycle, where the arrows show the different pathways a drop of water can travel as it moves around the earth. The main storage reservoirs for water are in the ocean, in the atmosphere, in ice and snow, in lakes, and in aquifers (ground water). Water moves between reservoirs by processes of evaporation, condensation, precipitation, runoff, and infiltration.

The amount of water on the earth is fixed; only the volume stored in any particular reservoir or water source can change. Table 1 shows how much water is located in any given reservoir on average.

Water source Water volume, in
cubic miles		 Percent of
total water
Oceans 317,000,000 97.24%
Icecaps, Glaciers 7,000,000 2.14%
Ground water 2,000,000 0.61%
Fresh-water lakes 30,000 0.009%
Inland seas 25,000 0.008%
Soil moisture 16,000 0.005%
Atmosphere 3,100 0.001%
Rivers 300 0.0001%
Total water volume 326,000,000 100%
Table 1. Water distribution by volume.
[Source: Nace, U.S. Geological Survey, 1967]

Hydrologic Facts

  • 97% of the water on earth is in the oceans
  • Only 3% of the water on earth is freshwater
  • About 2.4% of the water on earth is permanently frozen in glaciers and at the polar ice caps
  • About 1/2 of 1% of the water on earth is groundwater
  • Only about 1/100 of 1% of the water on earth is in the rivers and lakes
  • It takes 39,090 gallons of water to make a new car, including the tires
  • Over 17,000,000 houses in the United States use private wells for their drinking water supply
  • A person can live about a month without food, but can live only about one week without water