Water and the Water Cycle

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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I. Location of Water resources:  97.2% ocean, 2.15% permanently frozen, rest (0.65%) is groundwater, lakes, streams, air  II. Water transformation Liquification: change from solid to liquid state

Evaporation: change from liquid to gaseous state 

Transpiration: evaporation which occurs through plants (important in vegetated areas; ususally combined with evaporation to make evapotranspiration)

Sublimation: change from solid to gaseous state

Condensation: change from gaseous to liquid state

Solidification: change from liquid to solid state

III. Latent heat of water. Heat is required to make ice become water, and water become steam (water vapor).. Depending upon the energy level, water at the freezing point (32 F) can be solid or liquid, or at the boiling point (212 F) can be liquid or gaseous. Thus, extra heat is required to melt ice, or to boil water.

Latent heat of Fusion (solid-liquid) = 79.7 calories/g

Latent heat of Vaporization (liquid-gas) = 540 calories/g

This is reason why sweating will help cool body, as water on skin surface will absorb 540 cal/g of water evaporated

Conversely, when water vapor condenses, 540 calories is released; and when water freezes, 79.7 calories is released per gram of water

This released energy is what creates winds, storms, lighting, etc.

IV. Cloud Formation Water will transform from gaseous to liquid state when the air is fully saturated, in other words whrn the relative humidity = 100% 

Saturation concentration of water vapor in air varies with air temperature, so that cold air can hold much less water vapor than warm air

Creating saturation can occur in a number of ways:

First, by increasing the amount of water vapor in the air. However, this requires movements of air masses, and will take hours to days to happen.

Second, by keeping water vapor levels constant, but changing the temperature. Warm air which is not saturated will become so if the temperature falls. This happens when air rises. 

The three major ways in which this can occur are:
  (1) convection (fair weather summer clouds)

(2) frontal boundaries (cold front, warm front)

(3) orographic lifting (air moving up a mountain slope)

This last method is responsible for the formation of rain shadows in mountainous regions. When moist air is pushed up a mountain by prevailing winds, the air cools, relative humidity reaches 100%, and clouds form. These clouds create rain, which takes water out of the atmosphere. As air decends the downwind side of the mountain, the temperature of this water-depleted increases. Relative humidity falls to much less than saturation, clouds disappear, and rain fall decreases. At the latitude of the US and Europe, prevailing winds are from teh west. Thus, rainshadows are more pronounced on the eastern sides of mountains. However, at more southerly latitudes (Mexico, for instance), prevailing winds are from the east, creating rain shadows on the western side of mountains.

 V. Groundwater resources. When rainfall exceeds evapotranspiration, some of the excess water is drawn by gravity into the ground. Approximately 95% of the liquid water on land occurs as groundwater. 


Zone of aeration = region in soil horizon in which air is found in spaces between particles 

Zone of saturation = region in soil horizon in which all of spaces between particles are filled with water

Water table = the line which separates the zones of aeration and saturation

Aquifer = layer of sediment or rock that is permiable and transmits water

An artesian well is formed when an aquifer is sandwiched between inpermiable layers, allowing pressure to develop on water in an aquifer

Points of discharge occur when groundwater or aquifers intersect the ground surface

Rates of groundwater flow are quite low (50 feet per year). As such, it is possible to remove groundwater more rapidly than it can be replaced. This can cause nearby shallow wells or springs to run dry.


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Created 2 September 2011, Last Update 02 September 2011

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