Hydropower (from Greek: ὕδωρ, "water"), also known as water power, is the use of falling or fast-running water to produce electricity or to power machines. This is achieved by converting the water's kinetic energy into electrical or mechanical energy. Hydropower is a form of green energy production.
Since ancient times, hydropower from watermills has been used as a renewable energy source for irrigation and the operation of different mechanical devices, such as gristmills, sawmills, textile mills, trip hammers, dock cranes, domestic lifts, and ore mills. A trompe, which produces compressed air from falling water, is sometimes used to power other machinery at a distance.
Hydropower as a source of energy is beneficial in reducing the use of fossil fuels, thereby mitigating anthropogenic climate change. However, there are multiple economic, sociological, and environmental downsides to its use.
International institutions such as the World Bank view hydropower as a means for economic development without adding substantial amounts of carbon to the atmosphere.
A water piston from the Nongshu
by Wang Zhen
Directly water-powered ore mill, late nineteenth century
There are many evidence suggesting that the fundamentals of hydropower date back to ancient Greek civilization. According to separate evidence, the waterwheel emerged in China around the same period independently from the Greeks. Other evidence suggest that earliest evidence of water wheels and watermills date back to the ancient Near East in the 4th century BC. Moreover, evidence of the use of hydropower for human benefits using irrigation machines dates back to ancient civilizations such as Sumer and Babylonia in the region of Mesopotamia. Studies suggest that the water wheel was the initial form of water power and it was driven by either humans or animals.
In the Roman Empire water-powered mills were described by Vitruvius by the first century BC. The Barbegal mill had sixteen water wheels processing up to twenty-eight tons of grain per day. Roman waterwheels were also used for sawing marble such as the Hierapolis sawmill of the late 3rd century AD.  Such sawmills had a waterwheel which drove two crank-and-connecting rods to power two saws. It also appears in two 6th century Eastern Roman saw mills excavated at Ephesus and Gerasa respectively. The crank and connecting rod mechanism of these Roman watermills converted the rotary motion of the waterwheel into the linear movement of the saw blades.
The water-powered trip hammers and bellows in China, during the Han dynasty (202 BC - 220 AD), were initially thought to be powered by water scoops. However, some historians suggested that they were powered by waterwheels. This is since it was theorized that water scoops would not have had the motive force to operate their blast furnace bellows. There are many texts that describe the Hun waterwheel, some of the earliest ones are the Jijiupian dictionary of 40 BC, Yang Xiong's text known as the Fangyan of 15 BC, as well as the Xin Lun written by Huan Tan about 20 AD. It was also during this time that the engineer Du Shi (c. AD 31) applied the power of waterwheels to piston-bellows in forging cast iron.
Another example of early use of hydropower is seen in hushing. Hushing is the use of the power of a wave of water released from a tank in the extraction of metal ores. The method was first used at the Dolaucothi Gold Mines in Wales from 75 AD onwards. However, this method was further developed in Spain in mines such as Las Médulas. Hushing was also widely used in Britain in the Medieval and later periods to extract lead and tin ores. It later evolved into hydraulic mining when used during the California Gold Rush.
The Islamic Empire spanned a large region, mainly in Asia and Africa - along with other surrounding areas. During the Islamic Golden Age and the Arab Agricultural Revolution (8th–13th centuries) hydropower was widely used and developed and early uses of tidal power emerged along with large hydraulic factory complexes. There was a wide range of water-powered industrial mills used in the region including fulling mills, gristmills, paper mills, hullers, sawmills, ship mills, stamp mills, steel mills, sugar mills, and tide mills. By the 11th century, every province throughout the Islamic Empire had these industrial mills in operation, from Al-Andalus and North Africa to the Middle East and Central Asia. Muslim engineers also used water turbines, employed gears in watermills and water-raising machines, and pioneered the use of dams as a source of water power, used to provide additional power to watermills and water-raising machines.
Furthermore, in his book, The Book of Knowledge of Ingenious Mechanical Devices, the Muslim mechanical engineer, Al-Jazari (1136–1206) described designs for 50 devices. Many of these devices were water powered including clocks, a device to serve wine, and five devices to lift water from rivers or pools, where three of them are animal-powered and one can be powered by animal or water. Moreover, they included an endless belt with jugs attached, a cow-powered shadoof, and a reciprocating device with hinged valves.
Benoit Fourneyron the British Engineer who developed the first hydropower turbine
In the 19th century the Fourneyron turbine was developed by Benoit Fourneyron which was the first device using hydropower turbine. This device was implemented in the commercial plant of Niagara Falls and it is still operating today. In the early 20th century specifically 1978, the English Engineer William Armstrong made a significant achievement in the development of hydropower when he built and operated the first private electrical power station which was located in his house in Cragside in Northumberland, England.  In 1753, French engineer Bernard Forest de Bélidor published Architecture Hydraulique which described vertical- and horizontal-axis hydraulic machines. The growing demand for the Industrial Revolution would drive development as well.
At the beginning of the Industrial Revolution in Britain, water was the main source of power for new inventions such as Richard Arkwright's water frame. 
Although the use of water power gave way to steam power in many of the larger mills and factories, it was still used during the 18th and 19th centuries for many smaller operations, such as driving the bellows in small blast furnaces (e.g. the Dyfi Furnace) and gristmills, such as those built at Saint Anthony Falls, which uses the 50-foot (15 m) drop in the Mississippi River. 
Technological advances had moved the open water wheel into an enclosed turbine or water motor. In 1848 James B. Francis, while working as head engineer of Lowell's Locks and Canals company, improved on these designs to create a turbine with 90% efficiency. He applied scientific principles and testing methods to the problem of turbine design. His mathematical and graphical calculation methods allowed the confident design of high-efficiency turbines to exactly match a site's specific flow conditions. The Francis reaction turbine is still in wide use today. In the 1870s, deriving from uses in the California mining industry, Lester Allan Pelton developed the high efficiency Pelton wheel impulse turbine, which used hydropower from the high head streams characteristic of the mountainous California interior.
Calculating the amount of available power
A hydropower resource can be evaluated by its available power. Power is a function of the hydraulic head and volumetric flow rate. The head is the energy per unit weight (or unit mass) of water. The static head is proportional to the difference in height through which the water falls. Dynamic head is related to the velocity of moving water. Each unit of water can do an amount of work equal to its weight times the head.
The power available from falling water can be calculated from the flow rate and density of water, the height of fall, and the local acceleration due to gravity:
- (work flow rate out) is the useful power output (in watts)
- ("eta") is the efficiency of the turbine (dimensionless)
- is the mass flow rate (in kilograms per second)
- ("rho") is the density of water (in kilograms per cubic metre)
- is the volumetric flow rate (in cubic metres per second)
- is the acceleration due to gravity (in metres per second per second)
- ("Delta h") is the difference in height between the outlet and inlet (in metres)
To illustrate, the power output of a turbine that is 85% efficient, with a flow rate of 80 cubic metres per second (2800 cubic feet per second) and a head of 145 metres (480 feet), is 97 Megawatts:[note 1]
Operators of hydroelectric stations will compare the total electrical energy produced with the theoretical potential energy of the water passing through the turbine to calculate efficiency. Procedures and definitions for calculation of efficiency are given in test codes such as ASME PTC 18 and IEC 60041. Field testing of turbines is used to validate the manufacturer's guaranteed efficiency. Detailed calculation of the efficiency of a hydropower turbine will account for the head lost due to flow friction in the power canal or penstock, rise in tail water level due to flow, the location of the station and effect of varying gravity, the temperature and barometric pressure of the air, the density of the water at ambient temperature, and the altitudes above sea level of the forebay and tailbay. For precise calculations, errors due to rounding and the number of significant digits of constants must be considered.
Some hydropower systems such as water wheels can draw power from the flow of a body of water without necessarily changing its height. In this case, the available power is the kinetic energy of the flowing water. Over-shot water wheels can efficiently capture both types of energy.
The water flow in a stream can vary widely from season to season. Development of a hydropower site requires analysis of flow records, sometimes spanning decades, to assess the reliable annual energy supply. Dams and reservoirs provide a more dependable source of power by smoothing seasonal changes in water flow. However reservoirs have significant environmental impact, as does alteration of naturally occurring stream flow. The design of dams must also account for the worst-case, "probable maximum flood" that can be expected at the site; a spillway is often included to bypass flood flows around the dam. A computer model of the hydraulic basin and rainfall and snowfall records are used to predict the maximum flood.
Disadvantages and limitations
Refer to greenhouse gases for more information regarding greenhouse gasses impacts
There are a few disadvantages of hydropower that have been identified in the past years. Those disadvantages vary in severity and impact with some being outweighed by the benefits.  One of the most prominent sociological disadvantages is the displacement of people that live in the areas surrounding a hydro plant construction site, or when the reservoir banks become unstable.  Another identified sociological disadvantage of hydropower constructions is the occasional requirement to demolish cultural or religious sites.his has been identified as a reason for hydropower projects to be refused. This is since the extend of the impact of demolishing such important sites is deemed to be unethical by international standards, like that published by WCD and will result in a public outrage. Furthermore, the resettlement of the displaced communities needs to be factored into the cost of hydropower projects. Writers note that some companies understate the cost of resettlements in project assessments to cut on costs.  Furthermore, the presence of large dams in an area poses as a major hazard for people living in the communities surrounding it. There are several reason that correspond to this hazard, one of which being floods during rainy seasons due to dams overflow.
As per the impact on the ecosystem, it was found that dams and reservoirs have major negative impacts on river ecosystems. Large and deep dam and reservoir plants cover large areas of land which causes greenhouse gas emissions from underwater rotting vegetation. Furthermore, although at lower levels than other renewable energy sources, it was found that hydropower produces methane gas which is a greenhouse gas. This occurs when organic matters accumulate at the bottom of the reservoir because of the deoxygenation of water which triggers anaerobic digestion.  Furthermore, studies found that the construction of dams and reservoirs can result in habitat loss for some aquatic species. 
Use of hydropower
A hydropower scheme which harnesses the power of the water which pours down from the Brecon Beacons mountains, Wales
powered by falling water breaks the quietness of a Japanese garden with the sound of a bamboo rocker arm hitting a rock.
Interior of the Lyme Regis
watermill, UK (14th century)
A watermill or water mill is a mill that uses hydropower. It is a structure that uses a water wheel or water turbine to drive a mechanical process such as milling (grinding), rolling, or hammering. Such processes are needed in the production of many material goods, including flour, lumber, paper, textiles, and many metal products. These watermills may comprise gristmills, sawmills, paper mills, textile mills, hammermills, trip hammering mills, rolling mills, wire drawing mills.
One major way to classify watermills is by wheel orientation (vertical or horizontal), one powered by a vertical waterwheel through a gear mechanism, and the other equipped with a horizontal waterwheel without such a mechanism. The former type can be further divided, depending on where the water hits the wheel paddles, into undershot, overshot, breastshot and pitchback (backshot or reverse shot) waterwheel mills. Another way to classify water mills is by an essential trait about their location: tide mills use the movement of the tide; ship mills are water mills onboard (and constituting) a ship.
Watermills impact the river dynamics of the watercourses where they are installed. During the time watermills operate channels tend to sedimentate
, particularly backwater.
Also in the backwater area, inundation events
of adjacent floodplains
increase. Over time however these effects are cancelled by river banks becoming higher.
Where mills have been removed, river incision
increases and channels deepen.
Compressed air hydro
Where there is a plentiful head of water it can be made to generate compressed air directly without moving parts. In these designs, a falling column of water is purposely mixed with air bubbles generated through turbulence or a venturi pressure reducer at the high-level intake. This is allowed to fall down a shaft into a subterranean, high-roofed chamber where the now-compressed air separates from the water and becomes trapped. The height of the falling water column maintains compression of the air in the top of the chamber, while an outlet, submerged below the water level in the chamber allows water to flow back to the surface at a lower level than the intake. A separate outlet in the roof of the chamber supplies the compressed air. A facility on this principle was built on the Montreal River at Ragged Shutes near Cobalt, Ontario in 1910 and supplied 5,000 horsepower to nearby mines.
Hydropower is currently the world's first renewable energy source of electricity since it generates about 15% of the global electricity.  Furthermore, hydroelectricity is the number one application of hydropower. The generation of electricity through hydropower starts with the process of converting either the potential energy of water that is present due to the site's elevation or the kinetic energy of falling water into electrical energy. 
There are a few types of Hydroelectric power plants in terms of the way the water is harvested and used to generate energy. One of these types is one involving building a dam and a reservoir that collects a certain amount of water. In this case the water in the reservoir will be available upon demand and it will be used to generate electricity by passing through specialty channels that connect the dam to the reservoir. This is then passed to turbines that derive a generator which as the name implies generates electricity. The other type of hydroelectric power plants is called the run-of-river project. In this case, to control the flow of water a barrage is built and instead of collecting water in a reservoir. Also, the kinetic energy of flowing water is the main source of energy that is converted to electrical energy. 
Both of these types of hydroelectricity plants designs have some limitations. For example, the construction of dams can be very loud and will result in discomfort to the people living around it, also the presence of the dam and reservoirs will occupy a relatively large amount of space which is not usually favoured by communities living in the area.  Moreover, the reduction of the amount of water of the downstream due to presence of a reservoir can potentially have major environmental consequences such as harming habitats living in the downstream of the site. On the other hand, the limitation of the run-of-river project is the decreased efficiency of electricity generation because the process depends on the speed of the river flow which is seasonal. This means that when the rainy season begins in the area, electricity generation will be maximized while it will decrease during the dry season.
The size of hydroelectricity water plants can vary small community sized plants called micro hydro, to very large plants supplying power to a whole country. As of 2019, the five largest power stations in the world are conventional hydroelectric power stations with dams. 
Hydroelectricity can also be used to store energy in the form of potential energy between two reservoirs at different heights with pumped-storage hydroelectricity. Water is pumped uphill into reservoirs during periods of low demand to be released for generation when demand is high or system generation is low. Other forms of electricity generation with hydropower include tidal stream generators using energy from tidal power generated from oceans, rivers, and human-made canal systems to generating electricity.
- ^ Taking the density of water to be 1000 kilograms per cubic metre (62.5 pounds per cubic foot) and the acceleration due to gravity to be 9.81 metres per second per second.
- ^ a b Egré, D., & Milewski, J. 2002. The diversity of hydropower projects. Energy Policy, 30(14), 1225–1230. https://doi.org/10.1016/S0301-4215(02)00083-6
- ^ a b Hill, Donald (2013). A History of Engineering in Classical and Medieval Times. Routledge. pp. 163–164. ISBN 9781317761570.
- ^ Bartle, A. 2002. Hydropower potential and development activities. Energy Policy, 30(14), 1231–1239. https://doi.org/10.1016/S0301-4215(02)00084-8
- ^ Howard Schneider (8 May 2013). "World Bank turns to hydropower to square development with climate change". The Washington Post. Retrieved 9 May 2013.
- ^ a b Munoz-Hernandez, G. A. et al. 2013. Modelling and Controlling Hydropower Plants. 1st ed. 2013. [Online]. London: Springer London.
- ^ Terry S. Reynolds, Stronger than a Hundred Men: A History of the Vertical Water Wheel, JHU Press, 2002 ISBN 0-8018-7248-0, p. 14
- ^ a b c d e f g h i Breeze, P. 2018. Hydropower. Elsevier Science & Technology.
- ^ Oleson, John Peter (30 June 1984). Greek and Roman mechanical water-lifting devices: the history of a technology. Springer. p. 373. ISBN 90-277-1693-5. ASIN 9027716935.
- ^ GREENE, K.1990. PERSPECTIVES ON ROMAN TECHNOLOGY. Oxford journal of archaeology. [Online] 9 (2), 209–219.
- ^ Magnusson, R. J. 2001. Water technology in the Middle Ages cities, monasteries, and waterworks after the Roman Empire . Baltimore: The Johns Hopkins University Press.
- ^ Terry Reynolds: Stronger Than a Hundred Men. A History of the Vertical Water Wheel, The Johns Hopkins University Press, 1983, pp. 26-30
- ^ Adam Lucas: Wind, Water, Work: Ancient And Medieval Milling Technology, Brill Academic Publishers, 2006, pg 55
- ^ a b Needham, Joseph (1986), Science and Civilisation in China, Volume 4: Physics and Physical Technology, Part 2, Mechanical Engineering, Taipei: Cambridge University Press, p. 370, ISBN 0-521-05803-1
- ^ Environmental History. The Industrial Alchemy of Hydraulic Mining: Law, Technology, and Resource-Intensive Industrialization. vol. 10, no. 3, Oxford University Press, 2005, p. 589–.
- ^ Nakamura, T. K. et al. 2018. Remains of the 19th Century: Deep storage of contaminated hydraulic mining sediment along the Lower Yuba River, California. Elementa (Washington, D.C.). [Online] 6 (1), 70–.
- ^ Hoyland, R. G. 2015. In God’s path the Arab conquests and the creation of an Islamic empire . Oxford. England. Oxford University Press.
- ^ Ahmad Y. al-Hassan (1976). Taqi al-Din and Arabic Mechanical Engineering, pp. 34–35. Institute for the History of Arabic Science, University of Aleppo.
- ^ Adam Robert Lucas (2005), "Industrial Milling in the Ancient and Medieval Worlds: A Survey of the Evidence for an Industrial Revolution in Medieval Europe", Technology and Culture 46 (1), pp. 1–30 .
- ^ Ahmad Y. al-Hassan, Transfer Of Islamic Technology To The West, Part II: Transmission Of Islamic Engineering Archived 18 February 2008 at the Wayback Machine
- ^ Jones, R. V. 1974. The Book of Knowledge of Ingenious Mechanical Devices. Physics bulletin. [Online] 25 (10), 474–474.
- ^ "History of Hydropower". US Department of Energy. Archived from the original on 26 January 2010.
- ^ "Hydroelectric Power". Water Encyclopedia.
- ^ BELIDOR (1751) Architecture Hydraulique. Journal des Savants. 157442–.
- ^ a b Perkin, H. J. 1969. The origins of modern English society 1780-1880 . London: Routledge & K. Paul.
- ^ Lewis, B J; Cimbala; Wouden (2014). "Major historical developments in the design of water wheels and Francis hydroturbines". Iop Conference Series: Earth and Environmental Science. IOP. 22 (1): 5–7. Bibcode:2014E&ES...22a2020L. doi:10.1088/1755-1315/22/1/012020.
- ^ S. K., Sahdev. Basic Electrical Engineering. Pearson Education India. p. 418. ISBN 978-93-325-7679-7.
- ^ a b c d e f Breeze, P. 2019. Power generation technologies (Third edition.). Newnes.
- ^ a b c Maaß, Anna-Lisa; Schüttrumpf, Holger (2019). "Elevated floodplains and net channel incision as a result of the construction and removal of water mills". Geografiska Annaler: Series A, Physical Geography. 101 (2): 157–176. doi:10.1080/04353676.2019.1574209.
- ^ Maynard, Frank (November 1910). "Five thousand horsepower from air bubbles". Popular Mechanics: 633.
- ^ Kaygusuz. K. 2016. Hydropower as clean and renewable energy source for electricity generation. Karadeniz Technical University, Chemistry, Trabzon, Turkey. Volume 5(1). pp 359-369
- ^ Brian F. Towler. 2014. Chapter 10 - Hydroelectricity. in The Future of Energy. [Online]. Elsevier Inc. pp. 215–235.
- ^ Førsund, F. R. 2014. Pumped-storage hydroelectricity, in Hydropower Economics. [Online]. Boston, MA: Springer US. pp. 183–206.
- ^ Davis, S. 2003. Microhydro clean power from water . New Society Publishers.