Hydrology is the scientific study of the movement, distribution and management of water on Earth and other planets, including the water cycle, water resources and watersheddrainage basin area contributing water to a network of stream channels, a lake or other areas where water can collect sustainability.^[1]

Chemical wastes improperly discarded in the past are appearing in today's water supplies, but hydrology knowledge and application help scientists and the public respond to changes in complex water systems and solve water-related problems. Hydrologists play a vital role in finding those solutions.^[1]

Branches of hydrology include: hydrogeology,geology that deals with the distribution and movement of groundwater in the soil and rocks of the Earth's crust hydrogeology, chemical hydrology, ecohydrology,the study of the interactions between water and ecological systems hydroinformatics,application of information and communications technologies to address problems in the equitable and efficient use of water for many different purposes isotopeone of two or more species of atoms of a chemical element with the same atomic number and position in the periodic table and nearly identical chemical behaviour but with different atomic masses and physical properties hydrology, surface hydrology, hydrometeorology,the study of the transfer of water and energy between the land surface and the lower atmosphere drainage basinan area of land where precipitation collects and drains into a common outlet, such as a river, bay or other body of water management and water quality.^[2]

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Water Properties

Water Salinity[3]
classification	salinity in pptparts per trillion	location
fresh water	0 - 0.5	ponds, lakes, rivers, streams, aquifers
brackish water	0.5 - 30	estuaries, mangrove swamps
saline water	50+	seawater, salt lakes
briny water	10,000 ppm to 35,000 ppm	brine ponds

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Hydro Power and Dams

The first U.S. hydroelectric powerthe amount of energy transferred or converted per unit time plant was built on the Fox River in 1882 in Appleton, Wisconsin. It powered two paper mills and one home.^[1]

Water mills, used before the Industrial Revolution, are large wheels on flowing river banks that generated energy for grain grinding, timber cutting and fires for steel production.^[1]

Hydroelectricity, used today, is a clean, renewable energy source, produced by moving water. It improves hygiene and education and provides community employment opportunities.^[1]

Billions of people depend on it, and it is often one of the first energy sources used by developing countries to provide energy to rural areas. As their populations have increased China and India built dozens of dams to produced energy.^[1]

Egypt began construction of the Aswan Dam power plant Aswan Dam power plant
Orlova-tpe
Feb. 28, 2012
Wikipedia Aswan dam
https://en.wikipedia.org/
wiki/Aswan_Dam#/media/
File:Aswan_Dam.jpg
CC BY-SA 3.0 complex on the Nile River in 1960. Engineers realized that ancient cliff temples of Abu Simbel were going to be flooded by the Lake Nasser reservoir. To save the cultural relics, the Egyptian government moved the entire mountainside to artificial hills above the dam.^[1]

The U.S. also depends on hydroelectric energy. During the 1930s Great Depression dams, including the Columbia River Bonneville Dam, Bonneville Dam
Visitor7
Jun. 20, 2013
Wikipedia Bonneville dam
https://en.wikipedia.org/
wiki/Bonneville_Dam#/media/
File:Spillway,_Bonneville_Dam-2.jpg
CC BY-SA 3.0 the Sacramento River Shasta Dam, Shasta dam
Bureau of Reclamation
Feb. 12, 2017
Wikipedia Shasta dam
https://en.wikipedia.org/
wiki/Shasta_Dam#/media/
File:Water_released_
from_Shasta_Dam_(2017).jpg
CC BY-SA 2.0 and the Colorado River Hoover Dam, were built as part of the New Deal, a series of government programs that provided jobs.^[1]

The most famous hydroelectric New Deal power project was the Tennessee Valley Authority (TVA) which constructed several dams on the Tennessee River and its tributaries. TVA is now the largest public U.S. power company, providing electricity to Alabama, Georgia, Kentucky, Mississippi, North Carolina, Tennessee and Virginia.^[1]

One of the most famous hydroelectric plants, the Robert Moses hydroelectric power plant,, Robert Moses
hydroelectric power plant
Busfahrer
Apr. 18, 2008
Wikipedia Niagra Falls
https://en.wikipedia.org/
wiki/Niagara_Falls#/media/
File:Robert_moses_niagara_
power_plant_01.jpg
CC BY 3.0 borders the U.S. and Ontario, Canada at Niagara Falls. While engineers can't turn the falls off, they can limit the plant's water intake.^[1]

To produce hydroelectric power, water needs to be retained in a reservoir usually created by a dammed river or artificial lake. A controlled water flow of millions of gallons is channeled through dam tunnels to rotate turbines that move generators.^[1]

Hydroelectricity sometimes has a human cost, requiring residents to permanently relocate before dam construction begins. And these structures may prevent fish from swimming upstream. The Bonneville Dam installed fish ladders, wide steps built on the side of the river and dam, to enable fish to slowly swim upstream.^[1]

Aquatic birds and plants are sometimes at risk when dams flood river banks, destroying wetland habitat for thousands of organisms. Hydroelectric plants may change reservoir water temperatures, forcing animal migrations.^[1]

Hydropower plant energy is measured in gigawatt hours (GWh),a unit of energy equal to billion watt hours terawatt hours (TWh),a unit of energy equal to one trillion watt hours and petajoules (PJ)a unit of energy equal to one million billion joules.

10 Largest Hydroelectric Power Plants[2]
dam	river	location	annual power generation	construction dates	information	image
Three Gorges	Yangtze River	Yichang, Hubei province, China	101.6 terawatt hours/ 366 petajoules	1993-2012	world's biggest hydropower station 181 meter tall and 2,335 meter-long gravity dam 32 turbine generator units rated 700 megawatts each and two 50 megawatt power generators
Itaipu	Parana River	Brazil and Paraguay border, Brazil and Paraguay	76.382 terawatt hours/ 274.98 petajoules	1975-1982	built by a U.S.-Italy consortium power production began in May, 1984 supplied 15% of Brazil's energy consumption and 90% of energy consumed in Paraguay in 2018 consists of 20 generating units with a capacity of 700 megawatts each in 2016 it was the world's largest hydropower plant
Xiluodu	Jinsha River	Sichuan Province, China	55.2 terawatt hours/ 198.72 petajoules	2005-2013	world's first ultra-high concrete double-curvature arch dam at an elevation of 610 meters maximum dam height is 285.05 meters power transmitted through the State Grid and China Southern power grid
Simón Bolívar	Caroni River	Bolívar state, Venezuela	47,000 gigawatt hours/ 169.2 petajoules	1963-1986	also called the Guri Dam consists of 20 generating units of different capacities ranging between 130 megawatts and 770 megawatts was refurbished in 2007 and 2009
Belo Monte	Xingu River	Pará, Brazil	39.5 terawatt hours/ 142.2 petajoules	2011-2016	owned and operated by Norte Energia, a consortium led by the Brazilian electric utility company Eletrobas comprises two dams and two powerhouses
Tucuruí	Tocantins River	Tucuruí, Pará, Brazil	21.4 terawatt hours/ 77.04 petajoules	1975-1984	concrete gravity dam 78 meters tall and 12,500 meters long consortium of Alstom, GE Hydro, Inepar-Fem, and Odebrecht supplied equipment delivers electricity to the town of Belém and the surrounding area
Grand Coulee	Columbia River	Washington, U.S.	20.24 terawatt hours/ 72.86 petajoules	1933-1942	owned and operated by the U.S. Bureau of Reclamation consists of three power plants and a concrete gravity dam 168 m tall and 1,592 m long overhaul began in 2013, 2016 and 2019
Xiangjiaba	Jinsha River	Sichuan, Yibin and Shuifu County, Yunnan, China	30.7 terawatt hours/ 110.52 petajoules	2006-2012	dam is 162 meters high with a crest elevation of 384 meters eight units of 800 megawatts each consists of various structures for flood discharge, diversion, power generation and ship lift
Sayano-Shushenskaya	Yenisei River	Sayanogorsk, Khakassia, Russia	23.5 terawatt hours/ 84.6 petajoules	1963-1978	arch-gravity dam 242 meters tall and 1,066 meters long was constructed 70% is delivered to four aluminium smelters in Siberia in 2009 the plant was shut down temporarily following an accident, which damaged the turbines it reopened in 2010, after repair ten new units with 96.6% efficiency are planned for installation
Longtan	Hongshui River	Tian'e County, Guangxi, China	18,700 gigawatt hours/ 67.32 petajoules	2007-2009	consists of nine 700 megawatt generating units concrete gravity dam 216.5 meters high and 832 meters wide turbine generators supplied by Voith, Dongfang, Harbin, and Tianjin

• to meet the water need evenly all through the year
• to control flooding
• to generate electricity
• to divert the water flow from one channel to another^[3]

There are more than 90,000 large and small dams in the United States.^[3]

Dams have three structural elements:

• water retaining structure
• water releasing structure
• auxiliary spillwaybuilt for emergency release of water to prevent damage to a dam or ogee spillwayan overflow spillway constructed in cross-section the crest^[4]

During flood conditions, excess water is directed into a spillwaya structure that allows water to flow directly into the river or other body of water below a dam, bypassing all tunnels, turbines, and generators to prevent damage to surrounding lands. Dams can only provide limited energy, determined by hundreds of meters of siltationprocess of blocking something with sand or soil built up on reservoir bottoms.^[1]

The O'Shaughnessy Dam on the Tuolumne River in California drew criticism for its environmental impact. The dam, constructed in 1913, created the Hetch Hetchy reservoir and flooded the Hetch Hetchy Valley, part of Yosemite National Park.^[1]

Environmentalists opposed the dam, but the plant provided affordable hydroelectric energy to San Francisco and the surrounding area. The plant's existence is still controversial, and some environmentalists believe the plant should be destroyed so the valley can return to its original state.^[1]

Greatest Dams in the U.S.[3]
dam	river	location	feature	construction dates	information	image
Oroville	Feather	Oroville region, California	tallest	1961-1969	constructed by the California Department of Water Resources more than 770 feet high met the need for water for San Joaquin Valley the Edward Hyatt Pump-Generating Plant generates electricity of 28,000-megawatts
Cochiti	Rio Grande	Cochiti Pueblo, New Mexico	longest	1965-1973	5.5 miles/29,040 feet one of the world's shortest in height has a permanent recreation pool
Grand Coulee	Columbia	Okanogan and Grant Counties of Washington State	highest power production	1933-1942	primary objective of the dam was to control flooding and meet the need of irrigation later used for power generation of 21 billion kilowatt-hours
Hoover	Colorado	Clark County of Nevada and Mohave County of Arizona	largest water retention	1931-1936	1244 feet long, 726 feet high, wall width 16 feet The wall of the dam encompasses a volume of 3.25 million cubic yards

 
Phase Changes

The energy required to convert ice into water is called the latent heat of fusion.amount of heat required to convert a specific amount of a substance from a solid to a liquid It takes about 80 calories of heat to convert one gram of ice into water. The energy required to convert water into steam is called the latent heat of vaporization.amount of heat required to convert a specific amount of a substance from a liquid to a gas It takes about 600 calories of heat to convert one gram of water into steam.^[1]

Most of us are familiar with freezing, when liquid water undergoes a phase change and converts to solid ice, and melting, when water also undergoes a phase change from solid ice to liquid water. When heat is added to liquid water it changes to water vapor, dissipating in the atmosphere. When water vapor in the atmosphere cools, it undergoes condendensation.

The two phases we seldom see in daily life are sublimation,transition of a substance directly from the solid to the gas phase, without passing through the intermediate liquid phase when ice converts directly into water vapor in the atmosphere, and deposition,transition of a substance directly from the gas to the solid phase, without passing through the intermediate liquid phase when water vapor converts directly into solid ice. Sublimation can be observed by blowing across the tops of ice cubes in an ice cube tray.

There are 19 known forms of ice.^[2] Only a few appear in nature, while others can be created in laboratories, found in diamonds or identified on Jupiter's moon Europa. The forms differ in their arrangement of hydrogen atoms.^[3]

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Hydrologic Cycle

About 97.5% of Earth's water is contained in the oceans, 2.5% in polar ice caps and ground water, and 0.0001% in the atmosphere.^[1]

A water molecule will stay in the ocean for about 3,000 years before it is evaporated and added back into the water cycle. It will remain in the atmosphere for about 10 days before falling as rain, snow or condensation back into the oceans or onto land. Once on land, water can seep into the ground, flow across the surface and enter streams, rivers, or lakes, or become part of a glacier in colder environments.^[1]

Plants use a lot of water and return it to the atmosphere by transpiration.exhalation of water vapor through the stomata of a plant or leaf There is no net water loss in the atmosphere, biosphere, and cryosphere,portions of a planet`s surface where water is solid, including sea ice, lake ice, river ice, snow cover, glaciers, ice caps, ice sheets, and frozen ground but overall global distribution changes due to climatic variations and global warming.^[1]

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Water Balance

The simplest form of water balance equation is:

P = Q + E ± ΔS

P = precipitation, Q = runoff, E = evaporation and ΔS = storage in soil, aquifers or reservoirs.^[1]

A water balance analysis has two main applications. It can be used to assess current status and trends in water resource availability in an area over a specific period of time, and strengthen water management decision-making, by using data to assess validity of scenarios and strategies.^[1]

Water balance analysis divides water into "green" and "blue" water. "Blue" water, having been used for irrigation, is the surface and groundwater available for irrigation urban and industrial use and environmental flows. "Green" water, from rainfall, is water that has been stored in the soil and that evaporates into the atmosphere.^[1]

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Water Reuse^[*]

Recycling municipal water can help meet growing population needs. It is often less expensive than water from other sources and is more readily available. Sometimes the water is stored in basins, creating wetlands that attract and benefit local birds and animals.^[26]

In 1976, California's Orange County Water District began including 15 million gallons per day of highly treated municipal wastewater into its groundwater. In 2008 the system was expanded to 70 million gallons per day. The advanced treatment groundwater replenishment system uses municipal effluent from a nearby wastewater treatment plant, microfiltration,contaminated fluid is passed through a filter to remove microorganisms and suspended particles reverse osmosiswhen a solvent passes through a porous membrane in the direction opposite to that for natural osmosis when subjected to a hydrostatic pressure greater than the osmotic pressure and an advanced oxidation process to further treat the water, which is pumped into recharge basins and injected into wells, mixing with groundwater. The system provides drinking water for more than 2 million people and serves as a model for other potable use projects.^[3]

Indirect potable reuse (IPR)uses an environmental buffer, such as a lake, river or a groundwater aquifer before the water is treated at a drinking water treatment plant releases water from a wastewater treatment plant into an environmental buffer.a groundwater aquifer, surface water reservoir, lake or river in which advanced treated water is introduced before being used for potable reuse The water is mixed with water from other sources and is then removed from the buffer and purified to drinking water standards before delivery to water customers.^[5] In the past, IPR has been accepted because the public found it difficult to accept treated reclaimed water directly, called "toilet to tap," from water reclamation facilities even when that water meets EPA drinking water standards.^[8]

IPR can be stored during low demand times for use during high demand periods. Because stored water can become polluted by natural or chemical contaminants IPR often needs to be repurified.^[9]

The removal of IPR from a riparianrelating to wetlands adjacent to rivers and streams area can also result in long-term habitat loss and destruction of native species, which are then replaced by invasive, non-native species that resist removal.^[9]

In 2009 San Diego started the Water Purification Demonstration Project to show residents that IPR was safe and that a large-scale project could provide a reliable drinking water supply. The project, renamed Pure Water San Diego, explained to the public why alternative drinking water sources were necessary, demonstrated the water's purity and enlisted the support of academics and medical professionals to work with the media to ensure that water messages were factual and clear. The public can tour the facility to learn about IPR technology and taste purified drinking water.^[20]

Surveys conducted by the San Diego County Water Authority demonstrated a substantial shift in public opinion of IPR between 2004 and 2011. In 2004, 45% of residents opposed using advanced treated recycled water, but by 2011, that number fell to 11%,^[4] with many respondents requesting that highly purified water be used directly, rather than being released into the ground or a storage facility before use.^[21]

Hyperion 2035 is led by Los Angeles Sanitation and Environment. The project will upgrade the city's oldest wastewater treatment facility, the Hyperion Reclamation plant, with an advanced purification process enabling it to produce 100% recyclable wastewater by 2035.^[28]

The International Space Station (ISS) has provided a live lab for testing direct potable reuse (DPR)involves the treatment and distribution of water without an environmental buffer that returns purified water directly into a water system after wastes have been removed and the water has been purified through the station's wastewater treatment system. Because each crew member is allocated only two liters of water per day the ISS relies on a 2008 NASA DPR water recovery system that collects humidity and distills about 85% of water in urine. The system uses physical and chemical processes to remove contaminants from wastewater, stores it in a tank for reuse and checks water quality.^[4],[5]

There are two applications of DPR. The first uses advanced treated water (ATW) produced in an advanced water treatment facility (AWTF) and adds it to the water upstream of a drinking water treatment facility (DWTF). This option is used in Texas' Colorado River Municipal Water District's Big Spring Raw Water Production facility and the City of Wichita Falls DPR Project.^[6]

The second uses water produced in an AWTF permitted as a drinking water treatment facility (DWTF), adding it directly into a drinking water distribution system. This process is referred to as treated drinking water augmentation. This method is currently used in Windhoek, Namibia.^[7]

There are no federal DPR regulations. California and Texas, however, have been proactive in drafting them.

The California DPR initiative began in 2012 as a partnership between the WateReuse Research Foundation and WateReuse California.^[11] The state established a water recycling goal of 2.5 million acre-feet by 2030, more than four times its current water recycling effort, impossible with only non-potable reuse and IPR.^[12]

California Water Code, Division 7, Chapter 7.3 required adoption of DPR criteria by December 31, 2023.^[13] In 2017 the California State Water Board's Division of Drinking Water drafted a single criterion for DPR to streamline system development.^[14]

In 2022 the Texas Water Development Board published Final Report: Direct Potable Reuse Resource Document establishing source control, monitoring framework, water quality goals, treatment and testing strategies, risk assessment strategies, regulatory and legal considerations and public outreach plans for DPR implementation.^[6]

In 1970, Denver built an AWTF pilot plant. For five years, the plant used secondary effluent from the Metropolitan Denver Sewage Disposal plant to demonstrate safety and reliability.^[19] Research and design data were collected during the pilot.

Before the study, a University of Colorado survey showed that only 38% of participants favored DPR, but after public education efforts, another public opinion survey showed that 84% of Denver customers would accept DPR if water quality met or exceeded current drinking standards.^[19]

Formed in 1949, the Colorado River Municipal Water District (CRMWD) supplies water for arid west Texas communities, including Big Spring. Between 1950 and 1990, CRMWD built three dams to create surface water reservoirs storing water from Texas' Colorado River and developed four large groundwater well fields. Although CRMWD's surface water reservoirs have a combined storage capacity of over 1.2 million acre-feet, drought resulted in water levels below intake levels and dry reservoirs.^[14]

Sixty inches of annual evaporation, several decades of drought, an increasing population related to the oil and gas industry, lack of space for an additional reservoir and no suitable aquifers caused CRMWD to reject both IPR and more expensive desalination.^{[18], [22]}

Construction of the Big Spring reclamation facility began in 2010. As the first DPR facility in the U.S., Big Spring also made national headlines.^[22]

In 2012, reservoirs in Wichita Falls, Texas, were at less than 20% capacity and groundwater was unavailable. Water managers recognized that DPR was the only option remaining. The city had already installed an AWTF system to treat a brackish lake for IPR.^[22]

Turning to Big Spring for advice, the city began the 27-month permitting process with TCEQ. Anticipating approval, a 13-mile above-ground pipeline was built to transport effluent from the wastewater treatment plant to 37 the AWTF system at a cost of $13 million. The system came online in July 2014, providing 18.9 million liters of potable water per day, one-third of the city's daily demand.^[22]

The drought ended a year later and the system was converted back to IPR, delivering treated wastewater to Lake Arrowhead.^[22]

The Wichita Falls DPR program encountered almost no opposition because the water utility had an excellent reputation and a 40-year history of operating an IPR.^[20] Before the process began, city officials received support from professors, local doctors and the media and held meetings to educate the public about the project.^[22]

Windhoek, Namibia's water supply consists of dams fed by seasonal rivers and borehole water.^[15] As the most arid country in sub-Saharan Africa, its water sources rely on infrequent and inconsistent rainfall. The city used up all conventional water supplies within 500 kilometers, considered transporting water from the Okavango River, more than 800 kilometers from the city or using desalinated seawater pumped up 1,700 meters, but those scenarios were too expensive.^[16]

The fully automated plant, run by three trained operators per shift, continuously monitors water purity. The plant is producing 21,000 cubic meters (5.5 million gallons) per day^[15], 28% of city demand, at a cost about 37% lower than the cost of potable water from surface water sources, providing blended water to customers.^[16],[17]

In 1985 El Paso Water (EPWater) began using IPR, gaining the trust of its customers. In 2014 EPWater surveyed its customers about using highly treated wastewater into its drinking water system and found that 84% of its customers approved. After outreach efforts, another survey showed that 90% of respondents favored a DPR project.^[24]

EPWater conducted a feasibility study, operated a nine-month pilot program and hired an engineering firm to implement an innovative four-step membrane technology using reverse osmosis, ultraviolet light, disinfection with advanced oxidation and granular activated carbon filtration technology.^[24]

This is the second DPR plant in the world that will provide customers with a permanent DPR source. The plant is being designed with an auditorium and hallway where residents can learn about and see the treatment process, and a room where they can sample the purified water.^[24]

The first large-scale desalination plants were built in the 1960s. There are now about 20,000 facilities globally that convert sea water into fresh water.^[1]

Because of low average rainfall during the last two decades, in 2017 the city of Perth in Melbourne, Australia built a $3.5 billion desalination plant that provides a third of the city's water supply.^[1]

Desalinated seawater makes up more than 90% of Israel's domestic water supply. The Sorek Desalination Plant is one of the largest reverse osmosis desalination plants in the world. It provides drinking water for more than 1.5 million people.^[23]

Since January 2016 the San Antonio Water System (SAWS) Brackish Groundwater Desalination Facility, the H2Oaks Center, has been delivering 12 million-gallons-per-day of water to its customers. In the future it is expected to deliver 30 million gallons per day.^[25]

In April 2023 the California Department of Water Resources announced three desalination projects. The city of Torrence in Los Angeles County is constructing a pipeline to connect an existing well to the Goldsworthy Desalter system, increasing the supply of desalinated water by 1,120 acre-feet per year, enough to supply 2,200 homes.^[29]

The city of Fort Bragg Design Pilot Project will use a wave-powered seawater desalination iceberg buoy to provide drinkable water to the community.^[29]

There are two desalination methods and both create environmental problems. Thermal desalination heats water and captures the condensation. Reverse osmosis desalination forces sea water through a membrane that traps salt molecules, allowing smaller water molecules to pass. Both require a lot of energy, producing greenhouse gas emissions contributing to global warming.^[1]

It takes two gallons of sea water to produce one gallon of fresh water. The remaining gallon is very salty and returned to the ocean. If this water is not spread over a large area it can remove ocean oxygen, negatively affecting sea life.^[1]

The cost of desalinated water has dropped by more than half during the last three decades.^[1]

 
Conversions

 
Resources

 
[*] Some of the Hydrology section materials posted in this site originally posted at the Science → Hydrology section of
Journalism 455/555 Environmental Journalism and John Wesley Powell, 1834-1902, http://denisemeeks.com/journalism/jour_555/powell/ by this author

Denise Meeks, dmeeks@arizona.edu