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Water Cycle

The amount of water on earth is the same amount that has always been, and always will be.  Water is neither destroyed nor created; it is only transformed into different states as it moves through the environment in a process called the water cycle, or hydrologic cycle.  Water is the the only natural substance which exists at the surface of the earth in three states:  solid (ice), liquid (water), and gas (water vapor).  In order to change the state of water from one form to another, heat is needed.  In the natural environment this heat energy comes from the sun.

In its most simplest explanation, the water cycle can be described as water moving from the sky to Earth and back to the sky again.  This, however, is much too simple and therefore does not paint a true

picture of what a drop of water experiences as it goes through the water cycle.  Arbitrarily beginning with precipitation, water falls to the earth as rain, snow, sleet or hail.  Much of the precipitation falls on bodies of water considering 71 percent of the Earth's surface is covered by water.  Precipitation that strikes land can follow several different pathways, dependant on the characteristics of the surface it lands on and the environmental conditions at the time.
 
Precipitation, as it falls to land, is always subject to evaporation and sublimation.  Evaporation is when water changes from liquid to vapor without its temperature reaching the boiling point.  This of course occurs much more rapidly during warm sunny days, and it can greatly contribute to the movement of water through the water cycle.  Sublimation is a process in which ice is converted into water vapor directly, without passing through an intermediate liquid phase.  If laundry is hung in below freezing temperatures it will eventually dry, although very slowly.

The water molecules that do not evaporate or sublimate begin to follow two general pathways; water either infiltrates and percolates into the soil or it becomes surface runoff.  If water falls onto an impervious surface (hard surface), such as pavement, roof tops, etc., it will very quickly begin to move over the surface as directed by gravity---downhill.  In the natural environment, this overland flow or surface runoff occurs as well, although it is not as intense as when precipitation encounters impervious surfaces.  Conditions that may exist to cause surface runoff under natural conditions are already saturated soils in which it can not hold any more water or the actual characteristics of the soil.  Soils vary in permeability (the rate at which water passes through the soil).  This is in relation to the porosity of the soil, which is directly influenced by the mixture of the soil particles: sand, silt and clay.  Sand consists of large particles resulting in large spaces or pores between particles.  This property of sand allows water to flow through sand relatively easy.  Clay on the other hand consists of small particles, allowing the particles to pack more tightly together and thus reducing the amount of pores available for water to travel through.  Precipitation that falls on soils high in clay may directly become surface runoff or become detained in wetlands, lakes and ponds.
 
Precipitation that infiltrates and percolates into the soil also follows several different pathways.  Much water that falls during the growing season will be utilized by plants to transport much needed nutrients and carbohydrates from the root system to the above ground plant tissues. This water and carbon
dioxide are used in the process of photosynthesis, where plants make glucose for its energy needs.  Plants use capillary action to transport water up its stems against the force of gravity, however, to maintain a continuous flow of water plants must release water vapor into the air through its leaves, or small openings in the leaves called stomata.  This process lessens the pressure of its outer reaches (branches), thus causing water to be drawn from the roots where the pressure is greater.  The process of moving water through plants, and inevitably back into the atmosphere as water vapor, is called transpiration.  Transpiration can significantly influence the movement of water through its cycle, particularly during the summer.  Through transpiration an average sized maple tree lose more than 200 L (200 kg) of water per hour on a summer day.

 
Soil moisture not utilized by plants may remain shallow beneath the surface and resurface at springs or other bodies of water.  This shallow, sub-surface flow follows the contour or slope of the land until it reaches a point of discharge.  When precipitation is bountiful and transpiration is not too demanding, like in the spring, water percolating through the soil will eventually reach the zone of saturation, otherwise known as groundwater or the water table.  It is estimated that nearly a third of the precipitation falling on the Tunkhannock Creek Watershed flows into the groundwater system, replenishing the water that serves as a source for most of the residential water use within the watershed.

And now to the part of the water cycle of which are most familiar, surface water.  Surface waters consist of all above ground water bodies; these include lakes, rivers, streams, wetlands, icecaps and glaciers, and most extensively the oceans.  All inland bodies of water are recharged by surface runoff, groundwater, or direct precipitation.  Groundwater is a very important component of surface water recharge.  It has often been wondered how a stream continues to flow during weeks, sometimes months, of dry weather.  This is a result of groundwater recharging the streams at points where the stream bed encounters the water table.  In the Tunkhannock Creek Watershed it is estimated that 63 percent of the total surface water flow is from groundwater, thus making this component of the water cycle an important resource to protect.

Severing the link between precipitation and groundwater recharge is a very common, and often overlooked interruption of the water cycle.  Impervious surfaces, as mentioned earlier, do not allow water to infiltrate into the soil.  The construction of roads, cities, homes, driveways, sidewalks, et cetera, have enclosed precipitation into storm water management systems which directly release water into streams and rivers.  As a result, precipitation is not given a chance to infiltrate into the soil, and it is wisped away downstream.  As more impervious surface are constructed streams become flashier, meaning during storm events they flood with more intensity, and during dry periods they lack the groundwater recharge to maintain adequate flow for biological and human consumption.  In addition, WELLS GO DRY.

In addition to the surface water bodies that are so familiar, there also exits, way up north and way down south, icecaps and glaciers.  These frozen marvels contain approximately 235 times more water than
freshwater lakes and rivers combined, representing 2.14 percent of all the Earth's water (Table 1).  Water remains in the ice caps and glaciers for very long periods of time, however, through sublimation and variations in climate, these water bodies remain an important component of the water cycle.

Lastly, oceans are the largest component of the water cycle.  Oceans represent 97.24 percent of all the Earth's water.  All of the water in lakes and river will eventually end up in an ocean, and it is the oceans of the world which undoubtedly contribute most of the water vapor in the atmosphere.  As a result of the oceans' impact on the water cycle, it is no wonder why oceans influence weather and climatic condition of the globe, which is very evident in El Nino and El Nina year.
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: Global water distribution

Source: Nace, U.S. Geological Survey, 1967 and
The Hydrologic Cycle (Pamphlet), U.S. Geological Survey, 1984

With changing temperatures in the atmosphere, the water vapor begins to condense, forming clouds.  These condensation centers become saturated with water until it is released as precipitation.  Thus the water cycle has begun again.  For a depiction of the water cycle, please refer to the image below.

(Diagram courtesy of USGS)

Visualizations of the Water Cycle

(only recommended for very high speed internet access)

Image of Water Cycle.

Water Cycle Movie (with text ~45 Mb) | Water Cycle Movie (without text ~25 Mb)

Courtesy of Mark Malanoski, Visualization and Analysis Laboratory, NASAGoddard Space Flight Center.

What is a Watershed, Water Cycle, Habitats, Human Impacts on Water Quality, Global Water Issues, Macroinvertebrates, Geology, Groundwater