Summary of the
Hydrology is the science
that studies the Earth's water molecules and their
movement through the hydrologic cycle. The Earth and
its various abiotic and biotic systems are greatly
influence by water. Water is essential for life and
plays an important role in atmospheric and lithospheric
The hydrologic cycle is
used to model the storage and movement of water molecules
between the biosphere, atmosphere, lithosphere and
hydrosphere. Water is stored in the following reservoirs:
atmosphere, oceans, lakes, rivers, glaciers, soils,
snowfields, and groundwater. It moves from one reservoir
to another by processes like: evaporation, condensation,
precipitation, deposition, runoff, infiltration, sublimation,
transpiration, and groundwater flow.
Water molecules are stored
in the atmosphere in all three states of matter. Water
vapor in the atmosphere is commonly referred to as
humidity. If liquid and solid forms of water can overcome
atmospheric updrafts they can fall to the Earth's surface
as precipitation. The formation of ice crystals and
water droplets occurs when the atmosphere is cooled
to a temperature that causes condensation or deposition.
Four processes that can cause atmospheric cooling are:
orographic uplift; convectional uplift; air mass convergence;
and radiative energy loss.
Precipitation can be defined as any aqueous
deposit, in liquid or solid form, that develops in
a saturated atmospheric environment and generally falls
from clouds. A number of different precipitation types
have been classified by meteorologists including rain,
freezing rain, snow, ice pellets, snow pellets, and
hail. Fog represents the saturation of air near the
ground surface. Classification of fog types is accomplished
by the identification of the mechanism that caused
the air to become saturated.
The distribution of precipitation on
the Earth's surface is generally controlled by the
absence or presence of mechanisms that lift air masses
to cause saturation. It is also controlled by the amount
of water vapor held in the air, which is a function
of air temperature. A figure is presented that illustrates
global precipitation patterns.
In certain locations
on the Earth, acid pollutants from the atmosphere
are being deposited in dry and wet forms to the Earths
surface. Scientists generally call this process acid
deposition. If the deposit is wet it can also be
called acid precipitation. Normally, rain is slightly
acidic. Acid precipitation, however, can have a pH
as low as 2.3. The formation of acid deposition in
the atmosphere is explained in detail. The process
of lake acidification by acidic pollutants is also
Evaporation and transpiration
are the two processes that move water from the Earths
surface to its atmosphere. Evaporation is movement
of free water to the atmosphere as a gas. It requires
large amounts of energy. Transpiration is the movement
of water through a plant to the atmosphere. Scientists
use the term evapotranspiration to describe both processes.
In general, the following four factors control the
amount of water entering the atmosphere via these two
processes: energy availability; the humidity gradient
away from the evaporating surface; the wind speed immediately
above the surface; and water availability. Agricultural
scientists sometimes refer to two types of evapotranspiration:
Actual Evapotranspiration and Potential Evapotranspiration.
The growth of crops is a function of water supply.
If crops experience drought, yields are reduced. Irrigation
can supply crops with supplemental water. By determining
both actual evapotranspiration and potential evapotranspiration
a farmer can calculate the irrigation water needs of
The distribution of
precipitation falling on the ground surface can be
modified by the presence of vegetation. Vegetation
in general, changes this distribution because of
the fact that it intercepts some the falling rain.
How much is intercepted is a function of the branching
structure and leaf density of the vegetation. Some
of the water that is intercepted never makes it to
the ground surface. Instead, it evaporates from the
vegetation surface directly back to the atmosphere.
A portion of the intercepted water can travel from
the leaves to the branches and then flow down to the
ground via the plants stem. This phenomenon is
called stemflow. Another portion of the precipitation
may flow along the edge of the plant canopy to cause
canopy drip. Both of the processes described above
can increase the concentration of the water added to
the soil at the base of the stem and around the edge
of the plants canopy. Rain that falls through
the vegetation, without being intercepted, is called
Infiltration is the movement of water
from precipitation into the soil layer. Infiltration
varies both spatially and temporally due to a number
of environmental factors. After a rain, infiltration
can create a condition where the soil is completely
full of water. This condition is, however, only short-lived
as a portion of this water quickly drains (gravitational
water) via the force exerted on the water by gravity.
The portion that remains is called the field capacity.
In the soil, field capacity represents a film of water
coating all individual soil particles to a thickness
of 0.06 mm. The soil water from 0.0002 to 0.06 mm (known
as capillary water) can be removed from the soil through
the processes of evaporation and transpiration. Both
of these processes operate at the surface. Capillary
action moves water from one area in the soil to replace
losses in another area (biggest losses tend to be at
the surface because of plant consumption and evaporation).
This movement of water by capillary action generally
creates a homogeneous concentration of water throughout
the soil profile. Losses of water stop when the film
of water around soil particles reaches 0.0002 mm. Water
held from the surface of the soil particles to 0.0002
mm is essentially immobile and can only be completely
removed with high temperatures (greater than 100 degrees
Celsius). Within the soil system, several different
forces influence the storage of water.
Runoff is the surface
flow of water to areas of lower elevation. On the
microscale, runoff can be seen as a series of related
events. At the global scale runoff flows from the
landmasses to the oceans. The Earths continents
experience runoff because of the imbalance between
precipitation and evaporation.
Throughflow is the horizontal subsurface
movement of water on continents. Rates of throughflow
vary with soil type, slope gradient, and the concentration
of water in the soil. Groundwater is the zone in the
ground that is permanently saturated with water. The
top of groundwater is known as the water table. Groundwater
also flows because of gravity to surface basins of
water (oceans) located at lower elevations.
The flow of water through a stream channel
is commonly called streamflow or stream discharge.
On many streams humans gauge streamflow because of
the hazards that can result from too little or too
much flow. Mechanical gauging devices record this information
on a graph known as a hydrograph. In the online notes
there is a representation of a hydrograph showing some
of its typical features.
Oceans cover most of the Earth's surface.
On average, the depth of the world's oceans is about
3.9 kilometers. However, maximum depths can be greater
than 11 kilometers. The distribution of land and ocean
surfaces on the Earth is not homogeneous. In the Southern
Hemisphere there is 4 times more ocean than land. Ratio
between land and ocean is almost equal in the Northern
Hemisphere. Geographers recognize three major ocean
basins: Pacific; Atlantic; and Indian.
The water found in the ocean basins is
primarily a byproduct of the lithospheric solidification
of rock that occurred early in the Earth's history.
A second source of water is volcanic eruptions. The
dissolved constituents found in the ocean come from
the transport of terrestrial salts in weathered sediments
by leaching and stream runoff. Seawater is a mixture
of water and various salts. Chlorine, sodium, magnesium,
calcium, potassium, and sulfur account for 99% of
the salts in seawater. The presence of salt in seawater
allows ice to float on top of it. Seawater also contains
small quantities of dissolved gases including: carbon
dioxide, oxygen, and nitrogen. These gases enter the
ocean from the atmosphere and from a variety of organic
processes. Seawater changes its density with variations
in temperature, salinity, and ocean depth. Seawater
is least dense when it is frozen at the ocean surface
and contains no salts. Highest seawater densities occur
at the ocean floor.
Atmospheric circulation drives the movement
of ocean currents. Within each of the three ocean basins,
the patterns of these currents are very similar. In
each basin, the ocean currents form several closed
circulation patterns known as gyres. A large gyre develops
at the subtropics centered at about 30 degrees of latitude
in the Southern and Northern Hemisphere. In the Northern
Hemisphere, several smaller gyres develop with a center
of rotation at 50 degrees. Similar patterns do not
develop in the middle latitudes of the Southern Hemisphere.
In this area, ocean currents are not bound by continental
masses. Ocean currents differ from each other by direction
of flow, by speed of flow, and by relative temperature.
The cyclical rise and fall of seawater
is known as a tide. Tides develop because of gravitational
interactions between the Earth, Sun, and moon. The
timing of tidal cycles is related to the rotation of
the Earth on its axis and the revolution of the moon
around the Earth. Extreme tidal events, known as spring
tides, occur when there is an alignment of the gravitational
forces of the Sun and the moon. When the Sun and moon's
forces are perpendicular to each other, moderate neap
tides develop. The nature of the three major types
of tides is described in detail.
List of Key Terms
Shock, Acidic, Actual
Mass, Alkaline, Ammonia, Ammonium, Aquifer, Artesian, Artesian
Dioxide, Condensation, Condensation
Nuclei, Conduction, Confined
Groundwater, Conglomerate, Coniferous, Convectional
Nuclei, Dew, Dew
Point, Dissolution, Disturbance, Diurnal
Rotation, Eddy, Element, Evaporation, Evaporation
Capacity, Fog, Freezing, Freezing
Rain, Friction, Frontal
Lifting, Frontal Precipitation, Frost, Frost
Glacier, Granite, Gravel, Gravitational
Water, Gravity, Groundwater, Groundwater
Hail, Humidity, Hydrograph, Hydrostatic
Pressure, Hygroscopic Coefficient, Hygroscopic Water,
Pellet, Igneous, Infiltration, Infiltration
Lake, Leachate, Leeward, Lithosphere,
Matter, Orographic Precipitation, Orographic
Water Table, pH, Potential
Fog, Rain, Rainfall, Rainshadow
Humidity, Rill, River, River
Sandstone, Saturation, Saturation
Mixing Ratio, Secondary
Tide, Sleet, Snow, Snowfall, Snowfield, Snow
Pellet, Soil, Soil
Heat, Spring, Stemflow, Stomata, Stream, Stream
Flow, Sublimation, Sulfur
Acid, Supercooled, Super-Saturation, Surface
Inversion, Throughfall, Thunderstorm, Tidal
Period, Tide, Trade
Problems, and Exercises
(1). What is streamflow? How can it
be expressed in a mathematical model? Finally, describe
the effect of an intense 1 hour storm on streamflow
over 24 hours using a hydrograph.
(2). What factors control the rate
of evaporation on a soil surface?
(3). Discuss the movement of water
into soils. How and why does infiltration vary with
(4). Why does runoff occur?
(5). What forces influence the storage
of water in the soil matrix?
(6). What factors remove water stored
within the soil matrix?
(7). Describe the mathematical equation
used to model stream discharge.
(8). How do water droplets form in
clouds and fog?
(9). What is acid deposition? How does
it form? What chemicals are responsible and where
do they come from? How does acid deposition effect
(10). What is potential evapotranspiration
and how does it differ from actual evapotranspiration.
Finally, what factors control the rate at which water
leaves the Earth's surface by way of evaporation
(11). Fully explain the global distribution
patterns of precipitation.
(12). Explain how relative humidity
(13). On the map supplied draw the
appropriate currents in the Atlantic and Pacific
Oceans, N and S Hemisphere. Also be able to identify
the following currents: Brazil, Gulf Stream, Antarctic
Circumpolar (West Wind Drift), Peru, South and North
Equatorial, California, Canary, Equatorial Counter,
Benguela, Kuroshio, and N. Pacific.
(14). How is salinity measured? What
salts make up seawater?
(15). What is the relationship between
dissolved gases in the ocean and ocean temperature
(16). Discuss how tides form. What
is the difference between a Neap and Spring tide?
Explain diurnal, semidurinal, and mixed tides.