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Hydrologic Cycle
› Hydrology as a science [3]
- Hydrology is the science that treats the waters of the Earth, their occurrence, circulation and distribution, their chemical and physical properties, and their reaction with their environment, including their relation to living things. The domain of hydrology embraces the full life history of water on the Earth.
› Hydrologic cycle as the most fundamental principle of hydrology [3]
- The global pattern of continuously circulating water between the ocean, the atmosphere, and land
- Water evaporates from the oceans and the land surface, is carried over the earth in atmospheric circulation as water vapor, precipitates as rain or snow, is intercepted and transpired by vegetation, evaporates from land, water and canopy surfaces, provides runoff on the land surface, infiltrates into soils, recharges groundwater, discharges into streams, and ultimately, flows out into the oceans from which it will eventually evaporate once again.
- This immense water engine, fueled by solar energy, driven by gravity, proceed endlessly in the presence or absence of human activity.
› Major characteristics of hydrologic cycle [4]
- A dynamic system powered by the solar radiation and embraced by constant flow
- A closed system to which no new water is added or lost in any significant amount
- A recycling system which enables water to remain clean
- A system in balance barring the generally adverse impacts of human activities
- An interactive system signified by water’s readily changing states and moving between the atmosphere, groundwater aquifers, and surface water bodies.
› Main processes of hydrologic cycle
In atmosphere
- Precipitation
- Evaporation and transpiration or Evapotranspiration (ET)
- Interception
In hydrosphere
In lithosphere (- 60 km, crust + partial upper mantle)
- Infiltration
- Interflow
- Groundwater flow (or base flow)
Q: The rates and relative significance of each process differ in different regions under different climatic and landuse conditions. Provide an example.
› The Water budget/balance [2]
- Describe the hydrologic cycle quantitatively by applying the principle of conservation of mass
where M = mass within the control volume (any well defined domain), t = time, = mass inflow rate, and = mass outflow rate.
If the density is constant, Eq. 1 can be written as
where V = volume of water within the control volume, = volume inflow rate, and = volume outflow rate.
- For any control volume at steady state , a completely general water budget equation is given by
where , , , , , , and are volume of water stored, precipitation rate, surface-water inflow rate, groundwater inflow rate, surface-water outflow rate, groundwater outflow rate, and evapotranspiration rate, all on average.
where and are average surface runoff and groundwater runoff rates, respectively. Neglecting the groundwater runoff, Eq. 4 becomes
or (5)
- The catchment/watershed/drainage basin water budget
› Origin of The Earth’s Hydrosphere [1]
“As suggested (by some scientists), the Earth originated when fragments of rock materials flying through space came together by gravitational attraction. It is unlikely that large volumes of free water existed on the Earth’s surface at that time. If the Earth originally had little or no water, and water now covers three-fourths of the Earth’s surface, what geological processes produced the gigantic volumes of water available today?”
“When Plate collisions began and magma formed in the subduction process, gasses such as carbon dioxide (CO 2 ), nitrogen (N 2 ), hydrogen (H 2 ), hydrogen sulfide (H 2 S), sulfur dioxide (SO 2 ), and carbon monoxide (CO) were produced in the accompanying volcanic eruptions. The principal gas released was water vapor, however, because hydrogen and oxygen exist in the chemical structure of many rock-forming minerals. When these rocks are melted, hydrogen and oxygen are released during volcanism and unite quickly in the atmosphere to form water vapor. Over geological time, large amounts of oxygen were also contributed by photosynthesis; this oxygen can readily combine with hydrogen liberated in volcanic eruptions. The water formed from the melting of rocks is called juvenile water, that is, water never before on Earth in a combined form. It should be noted that some of the water vapor released in volcanic eruptions today is groundwater that was incorporated into the magma before the eruption (see figure in the next page).”
“Another source of water comes from the destruction of rocks at the Earth’s surface by a process called weathering. Some rocks originate under high pressure and temperature at great depth in the Earth. Once exposed, these rocks are out of their original chemical and physical equilibrium, leading to gradual disintegration and the release of certain gases, including water vapor.”
“In addition to plate movements and rock weathering, what other processes control today’s global water supplies? Water is lost from the Earth’s surface in two ways: by atmospheric and subduction processes. Vaporized water in the atmosphere can, under certain conditions, escape the force of gravity and leave the Earth’s atmosphere. This occurs when water molecules are broken up by ultraviolet light from the sun. The lighter hydrogen atoms can easily escape, leaving oxygen behind in the atmosphere. On a global scale, this loss is not great.”
“Major water loss occurs through sea-floor spreading, sedimentation, and plate subduction. Although the Earth’s age is estimated at 4.6 billion years, the average of the sea floor is about 65 million years, or only 1. percent the age of Earth. Thus, the sea floor is constantly being recycled through the upper mantle. As the newly formed plates move or spread from the oceanic rift zones, the upper layers of the plate (basalt) absorb water from the sea. This occurs quite slowly and the actual volume of water absorbed per square foot of basalt is not great, but the total surface of the sea floor is so large that huge quantities of water are held in the upper layers of the basalt... Water trapped in the basalt and sediment is either released during the partial melting that occurs in subduction and expelled in volcanic activity, or carried down into the mantle where hydration of rock- forming materials takes place.”
“In light of ongoing geologic processes affecting water supply, is the Earth actually gaining or losing water today? Many scientists believe that the global water supply has remained stable for approximately the last 500 million years.”
References
[1] Driscoll, F.G., 1995. Groundwater and Wells , U.S. Filter/Johnson Screens, St. Paul. [2] Hornberger, G.M., J.P. Raffensperger, P.L. Wiberg, and K.N. Eshleman, 1998. Elements of Physical Hydrology , The Johns Hopkins University Press, Baltimore. [3] Maidment, D.R., 1993. Handbook of Hydrology , McGraw-Hill, Inc., New York. [4] Watson, I. and A.D. Burnett, 1995. Hydrology: An Environmental Approach. CRC Lewis Publishers, Baco Raton.
