The
water
cycle—technically known as the
hydrologic
cycle—is the circulation of water within the earth's
hydrosphere, involving changes in the
physical state of water between liquid,
solid, and gas phases. The
hydrologic cycle refers to the continuous exchange of water between
atmosphere, land,
surface and subsurface waters, and
organisms. In addition to storage in various compartments (the ocean is one such "compartment"), the multiple cycles that make up the earth's water cycle involve five main physical actions:
evaporation,
precipitation,
infiltration,
runoff, and subsurface
flow:
- Evaporation is the transfer of water from bodies of surface water into the atmosphere. This transfer entails a change in the physical nature of water from liquid to gaseous phases. Evaporation is closely related to
transpiration from plants, as well as, to a lesser degree,
perspiration from land mammals and
marsupials. Thus, this transfer is sometimes referred to as
evapotranspiration. 90% of atmospheric water comes from evaporation, while the remaining 10% is from
transpiration.
- Condensation is the accumulation of water vapor in the air to form clouds that will do
precipitation.
- Precipitation is atmospheric
moisture that has previously condensed to form clouds (changed from the gas phases to a liquid or solid phase), falling to the surface of the earth. This mostly occurs as
rainfall, but snow,
hail,
fog drip, and other forms participate as
well.
- Interception is precipitation trapped by vegetation instead of falling directly onto the
soil.
- Infiltration into the ground is the transition from surface water to groundwater. The infiltration rate will depend upon soil or rock
permeability as well as other factors. Infiltrated water may reach another compartment known as groundwater (i.e., an
aquifer). Groundwaters tend to move slowly, so the water may return as surface water after storage within an aquifer for a period of time that can amount to thousands of years in some cases. Water returns to the land surface at lower elevation than where it infiltrated, under the force of
gravity or gravity induced
pressures.
- Runoff includes the variety of ways by which land surface water moves down slope to the oceans. Water flowing in
streams and
rivers may be delayed for a time in
lakes. Not all precipitated water returns to the sea as runoff; much of it evaporates before reaching the ocean or reaching an
aquifer.
- Subsurface
flow incorporates movement of water within the earth, either within the
vadose zone or
aquifers. After infiltrating, subsurface water may return to the surface or eventually seep into the
ocean.
| Reservoir |
Volume (million cubic
km) |
Percent of
total |
| Oceans |
1370 |
97.25 |
| Ice Caps and
Glaciers |
29 |
2.05 |
| Groundwater |
9.5 |
0.68 |
| Lakes |
0.125 |
0.01 |
| Soil
Moisture |
0.065 |
0.005 |
| Atmosphere |
0.013 |
0.001 |
| Streams and
Rivers |
0.0017 |
0.0001 |
| Biosphere |
0.0006 |
0.00004 |
| Reservoir |
Average Residence
Time |
| Glaciers |
20 to 100
years |
| Seasonal Snow
Cover |
2 to 6
months |
| Soil
Moisture |
1 to 2
months |
| Groundwater:
Shallow |
100 to 200
years |
| Groundwater:
Deep |
10,000
years |
| Lakes |
50 to 100
years |
| Rivers |
2 to 6
months |
