The science encompassing the behavior of water as it occurs in the atmosphere, on the
surface of the ground, and underground. (Am. Soc. Civil Engineers, 1949, p. 1.)
The science that relates to the water of the earth. (Meinzer, 1923, p. 9.)
The science treating of the waters of the earth, their occurrence, distribution, and
movements. (Jarvis and others, 1936, p. 464.)
[Source: http://water.usgs.gov/wsc/glossary.html#H ]
The applied science concerned with the waters of the earth, their occurrences,
distribution, and circulation through the unending hydrologic cycle of: precipitation,
consequent runoff, infiltration, and storage; eventual evaporation; and so forth. It is
concerned with the physical and chemical reaction of water with the rest of the earth,
and its relation to the life of the earth.
[Source: http://www.nws.noaa.gov/om/hod/SHManual/SHMan014_glossary.htm ]
Definition and Scope:
The word hydrology is derived from the Greek Words “Hydor” which means water and “logos”
which means science. Thus in this broad sense hydrology is the science which is concerned with
all waters on Earth, its occurrence, distribution and circulation, its physical and chemical
properties, its effects on the environment and on life of all forms. However, in customary
usage, the word hydrology is rather used in a restricted sense in many respects. For example, it
may not cover all studies of ocean waters and it is not concerned with medical uses of water.
There are various elaborate definitions of hydrology. But from the view point of practical
applications, Wisler and Brater have aptly defined hydrology as the sciences that deal with
processes governing the depletion and replenishment of water resources of the land areas of
the earth. It is concerned with the transportation of water through the air, over the ground
surface and through the strata of the earth. It is the science that treats of the various phases
of the hydrologic cycle. Hydrology finds many applications in Hydraulics, Agricultural water
resources and other branches of engineering. Therefore the names like Engineering Hydrology
and applied Hydrology are also often used.
The discussion the ensuing chapter would indicates that hydrology is highly interdisciplinary
science. For its investigations it draws many principles from other branched of science like
physics, chemistry, biology, geology, fluid mechanics, mathematics, probability and statistics
and operation research. As water is transported into air, hydrology interacts with science like
hydrometeorology, meteorology and climatology. When dealing with surface water, the
supporting sciences required are potamology (science of surface streams), Limnology (science
of lakes), cryology (science of snow and ice), Glaciology and oceanlogy. Similarly when dealing
with ground water hydrology encompasses the domains of Agronomy, Hydrogeology,
Since humans first settled along the banks of lakes and rivers, there has been great interest in
the appropriate management of fresh water resources both as a necessity for life as well as to
avoid potential health hazards.
It was along the Indus in Pakistan, the Tigris and Euphrates in Mesopotamia, the Hwang
Ho in China, and the Nile in Egypt that the first hydraulic engineers created canals,
levees, dams, subsurface water conduits, and wells as early as 5000-6000 years ago.
Hydrologic information became vital to these early civilizations.
The flow rates and yields of rivers were monitored by the Egyptians as early as 3800
years ago, and rainfall measuring instruments were first utilized approximately 2400
years ago by Kautilya of India.
The idea of a global hydrologic cycle dates back at least 3000 years when early Greek
philosophers including Thales, Anaxagoras, Herodotus, Hippocrates, Plato, and Aristotle
conceptualized the basic ideas governing the process. Many initial ideas established by
the Greeks about the hydrologic cycle were reasonable. However, many of the initial
mechanisms concerning the routes by which water returned from the sea and entered
rivers were devoid of as much logic.
Despite the apparent gaps in hydrologic mechanisms, the Romans developed aqueduct
systems reflecting an extensive practical understanding of hydrology and hydraulics, and
did so utilizing the basic hydrologic ideas established and passed along by the Greeks
During the Renaissance, Leonardo da Vinci (1500) in France proclaimed on the basis of
field observations that the waters in rivers come from precipitation. It was during that
time that any unrealistic mechanisms proclaimed by the Greek philosophers concerning
the hydrologic cycle were either refuted or modified.
In the seventeenth century, the modern scientific approach to studying the hydrologic
cycle was initiated by the Frenchmen Pierre Perault and Edme Marriotte.
By the 1670’s and 1680’s, they had published data and calculations that supported the
contention that precipitation was the precursor to stream flow. By 1700, Edmun Halley, an
English scientist added to the work of Perault and Marriotte by estimating the quantity of water
involved in the hydrologic cycle of the Mediterranean Sea and surrounding lands.
Substantial progress was made during the eighteenth century in applications of
mathematics, fluid mechanics, and hydraulics by scientists like Pitot, Bernoulli, Euler,
Chezy, and other European professionals. The term “hydrology” arrived in its current
meaning around 1750, and by 1800 the work of English physicist and chemist John
Dalton solidified the current understanding of the global hydrologic cycle.
Until the 1800s, the physical processes governing groundwater flow had confounded
scientists and created barriers to understanding the hydrologic cycle. These barriers
were eliminated in 1856, when the French engineer Henry Darcy introduced his law
describing flow through porous media.
Other advances in the hydrological sciences were made throughout the 1800s.
Poiseuille, DuPuit, DuBoys, Stokes, Manning, Reynolds, and others made substantial
contributions to fluid mechanics, hydraulics, and sediment transport during this
period. Also during the 1800s, literary publications began to surface, with increasing
frequency in the last half of the century.
Many works examined relationships between precipitation and stream flow out of
necessity for engineers designing bridges and other structures. It was during this time
that the close association between hydrology and civil engineering was established.
Daniel Mead published the first English-language text in hydrology in 1904 and Adolf
Meyer followed with his text in 1919. Both texts were written for civil engineers. The
association of hydrology and civil engineering established during this time has been
argued as having both enhanced as well as possibly inhibited the development of
hydrology as a science.
The first half of the twentieth century saw great advancements in the hydrological
sciences starting with the addition of the Section of Scientific Hydrology in the
International Union of Geodesy and Geophysics in 1922. This was followed by the
addition of the Hydrology Section of the American Geophysical Union in 1930. These
were the first formal recognitions of the scientific status of hydrology. Many
individuals contributed substantially in their areas of hydrologic expertise during the
early and middle decades of the 20th century, including: A. Hazen, E. J. Gumbel, H. E.
Hurst, and W. B. Langbein with regard to statistical applications of hydrologic data; O. E.
Meinzer, C. V. Theis, C. S. Slichter, and M. K. Hubbert who contributed to the
development of theoretical and practical aspects of groundwater hydraulics; L. Prandtl,
T. Von Karman, H. Rouse, V. T. Chow, G. K. Gilbert, and H. A. Einstein in sediment
transport and stream hydraulics; R. E. Horton and L. B. Leopold who contributed greatly
to runoff processes and quantitative geomorphology; W. Thornthwaite and H. E.
Penman in furthering the understanding of hydroclimatalogical processes and
modeling evapotranspiration; and A. Wolman and R. S. Garrels who contributed greatly
to the understanding and modeling of water quality. It was not until the 1960s that
detailed field studies attempting to understand the physical processes by which water
enters streams began to emerge.
With the emergence of the twenty-first century, many new breakthroughs in the hydrological
sciences are eminent. In the forthcoming years, breakthroughs will describe the relationships
between hydrological regimes to current and future climate change, and the effects of
hydrologic processes on landform development. New findings will also include modeling of
regional evapotranspiration rates and geomorphologic water transport.