Water cycle
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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 four 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. Along with evaporation can be counted transpiration from plants. Thus, this transfer is sometimes referred to as evapotranspiration. 90% of atmospheric water comes from evaporation, while the remaining 10% is from transpiration.
- 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.
- 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.
How much water
Clouds can hold only a limited amount of water. According to "Steam Tables. Properties of Saturated and Superheated Steam" (1940) by The Superheater Company Limited:
At 32° F = 0° C the Specific Volume is 3305.7 cu ft/lb At 120° F = 49° C the Specific Volume is 203.47 cu ft/lb
Converting the reciprocal to metric, 1 lb/cu ft = 453.59237 gram/cu ft = 453.59237/0.028316846592=16018.463374 g/m3
If a cloud hits a cold front, then it sheds water/hail/snow until the amount of water it holds is not above the saturation point at that particular lower temperature.
Curve fitting can help. To find out how much moisture a cloud can hold use x=°C at the cloud, and y=the maximum moisture (="at saturation") the cloud can hold in gram per cubic meter (g/m3) at that temperature, where E-6 means "times 10 to the power of minus 6", use:
y=3.18809058791E-6*x4+1.085260942615E-4*x3+0.01128820936197*x2+0.3271746184353*x+4.8520206433841
An Example: Assuming that it is known that at the cloud the temperature is 32°C, and it has just started to rain, it would be reasonable to conclude that the moisture contained by the cloud would be at the saturation level, which, at 32°C, is 33.8 g/m3. Later, when the temperature at the cloud has been reduced to 20°C, the maximum moisture the cloud can hold is only 17.3 g/m3. Therefore the amount of water that came down as rain so far is about 33.8-17.3=16.5 g/m3.
Similarly, to find out y=the minimum temperature in °C at the cloud at which the given x=amount of moisture in g/m3 can be retained by the cloud use
y=-4.9383889720942E-6*x4+9.8977592589601E-4*x3-0.073597683584735*x2+2.8358498562951*x-11.509289449903
See also
External Links
- Hydrologic Cycle from Earthscape
- Hydrologic Cycle from GROW
- United States Geological Survey Water Site