Chapter (1)
----------Hydrology
Hydrology is the study of the movement, distribution, and quality of
water throughout the Earth, and thus addresses both the hydrologic cycle
and water resources. A practitioner of hydrology is a hydrologist, working
within the fields of either earth or environmental science, physical
geography or civil and environmental engineering.
Domains of hydrology include hydrometeorology, surface hydrology,
hydrogeology, drainage basin management and water quality, where water
plays the central role. Oceanography and meteorology are not included
because water is only one of many important aspects.
Hydrological research is useful in that it allows us to better understand the
world in which we live, and also provides insight for environmental
engineering, policy and planning
History of hydrology
Hydrology has been a subject of investigation and engineering for
millennia. For example, in about 4000 B.C. the Nile was dammed to
improve agricultural productivity of previously barren lands.
Mesopotamia/Mesopotamian towns were protected from flooding with high
earthen walls. Aqueducts were built by the Ancient Greece/Greeks and
Ancient Romans, while the History of China built irrigation and flood
control works. The ancient Sinhalese used hydrology to build complex
Irrigation Works of Ancient Sri Lanka, known for invention of the Valve Pit
which allowed construction of large reservoirs, anicuts and canals which
still function.
Vitruvius/Marcus Vitruvius, in the first century B.C., described a
philosophical theory of the hydrologic cycle, in which precipitation falling
in the mountains infiltrated the earth's surface and led to streams and
springs in the lowlands. With adoption of a more scientific approach,
Leonardo da Vinci and Bernard Palissy independently reached an accurate
representation of the hydrologic cycle. It was not until the 17th century
that hydrologic variables began to be quantified.
Pioneers of the modern science of hydrology include Pierre Perrault, Edme
Mariotte and Edmund Halley. By measuring rainfall, runoff, and drainage
area, Perrault showed that rainfall was sufficient to account for flow of the
Seine. Marriotte combined velocity and river cross-section measurements
to obtain discharge, again in the Seine. Halley showed that the evaporation
from the Mediterranean Sea was sufficient to account for the outflow of
rivers flowing into the sea.
Advances in the 18th century included the Bernoulli piezometer and
Bernoulli's equation, by Daniel Bernoulli, the Pitot tube. The 19th century
saw development in groundwater hydrology, including Darcy's law, the
Dupuit-Thiem well formula, and Hagen-Poiseuille's capillary flow equation.
Rational analyses began to replace empiricism in the 20th century, while
governmental agencies began their own hydrological research programs.
Of particular importance were Leroy Sherman's unit hydrograph, the
infiltration theory of Robert E. Horton, and C.V. Theis's Aquifer
test/equation describing well hydraulics.
Since the 1950's, hydrology has been approached with a more theoretical
basis than in the past, facilitated by advances in the physical understanding
of hydrological processes and by the advent of computers and especially
Geographic Information Systems (GIS).
Hydrologic cycle
Precipitation is condensed water vapor that falls to the
Earth's surface. Most precipitation occurs as rain, but also
includes snow, hail, fog drip, graupel, and sleet.
Approximately 505,000 km³ of water fall as precipitation
each year, 398,000 km³ of it over the oceans.
Canopy interception is the precipitation that is
intercepted by plant foliage and eventually evaporates back to
the atmosphere rather than falling to the ground.
Snowmelt refers to the runoff produced by melting
snow.
Runoff includes the variety of ways by which water
moves across the land. This includes both surface runoff and
channel runoff. As it flows, the water may infiltrate into the
ground, evaporate into the air, become stored in lakes or
reservoirs, or be extracted for agricultural or other human
uses.
Infiltration is the flow of water from the ground surface
into the ground. Once infiltrated, the water becomes soil
moisture or groundwater.
Subsurface Flow is the flow of water underground, in
the vadose zone and aquifers. Subsurface water may return
to the surface (eg. as a spring or by being pumped) or
eventually seep into the oceans. Water returns to the land
surface at lower elevation than where it infiltrated, under the
force of gravity or gravity induced pressures. Groundwater
tends to move slowly, and is replenished slowly, so it can
remain in aquifers for thousands of years.
Evaporation is the transformation of water from liquid to
gas phases as it moves from the ground or bodies of water
into the overlying atmosphere. The source of energy for
evaporation is primarily solar radiation. Evaporation often
implicitly includes transpiration from plants, though
together they are specifically referred to as
evapotranspiration. Approximately 90% of atmospheric water
comes from evaporation, while the remaining 10% is from
transpiration Total annual evapotranspiration amounts to
approximately 505,000 km³ of water, 434,000 km³ of which
evaporates from the oceans.
Sublimation is the state change directly from solid water
(snow or ice) to water vapor.
Advection is the movement of water — in solid, liquid,
or vapor states — through the atmosphere. Without
advection, water that evaporated over the oceans could not
precipitate over land.
Condensation is the transformation of water vapor to
liquid water droplets in the air, producing clouds and fog.