The amount of water
on earth has remained unchanged over time.
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 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 an explanation, 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, or 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 which can not hold any
more water or the physical 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 easily. 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 their 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 their leaves, or small openings in their leaves called
stomata. This process lessens the pressure of their 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 loses 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, as 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 whisked away downstream. As more impervious surfaces 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 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 rivers 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 globes, which was very evident in an 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)
