The aqueducts required very careful planning before building, especially to determine the water source to be used, the length of aqueduct needed and its size. Great skill was needed to ensure a regular grade, so that the water would flow smoothly from its source without the flow damaging the walls of the channel. As the need for water grew, extra sources would be utilised, very often making use of existing structures as with the Aqua Claudia and Anio Novus in Rome. The problems of aqueduct building and use are described by Vitruvius and Frontinus, the latter producing a long report on the state of the Aqueducts of Rome in the last years of the first century AD.

Several surveying tools were used in the construction of Roman aqueducts, one example being the chorobates. The chorobates was used to level terrain before construction. It was a wooden frame supported by four legs with a flat board fitted with a water level and wooden arches to support the vaults. Another tool used in the construction of the aqueduct was the groma. Gromas were used to measure right angles. A groma consisted of stones hanging off four strings perpendicular to one another. The instrument which is the forerunner of the theodolite was known as the dioptra, and was used to measure vertical angles.

It has long been thought that the Pont du Gard was built by Augustus’ son-in-law and aide, Marcus Vipsanius Agrippa, around the year 19 BC. Newer excavations, however, suggest the construction may have taken place in the middle of the first century A.D, consequently, opinion is now somewhat divided on the matter. Designed to carry the water across the small Gardon river valley, it was part of a nearly 50 km (31 mi) aqueduct that brought water from the Fontaines d’Eure springs near Uzès to the Castellum in the Roman city of Nemausus (Nîmes). The full aqueduct had a gradient of 34 cm/km (1/3000), descending only 17 m vertically in its entire length and delivering 20,000 cubic meters (5 million gallons) of water daily.

It was constructed entirely without the use of mortar. The aqueduct’s stones – some of which weigh up to 6 tons – were precisely cut to fit perfectly together eliminating the need for mortar. The masonry was lifted into place by block and tackle with a massive human-powered treadmill providing the power for the winch. A complex scaffold was erected to support the aqueduct as it was being built. The face of the aqueduct still bears the mark of its construction, in the form of protruding scaffolding supports and ridges on the piers which supported the semicircular wooden frames on which the arches were constructed. It is believed to have taken about three years to build, employing between 800 and 1,000 workers.

In Ancient Rome, the Aqua Alsietina (sometimes called Aqua Augusta) was the earliest of the two western aqueducts, erected somewhere around 2 BC, during the reign of emperor Augustus. It was the only water supply for the Transtiberine region (the right bank of the river Tiber).

This aqueduct acquired water mainly from a lake just north of Rome called Lacus Alsietinus (a small lake in southern Etruria, currently known as Lago di Martignano) and some from lacus Sabatinus (Lago di Bracciano). The length of this mainly subterranean aqueduct was 22,172 paces (about 32.8 km) and had 358 arches. Its water supply had a diameter of 392 quinariae (about 9 m).

 
An example of a ancient Roman naumachiumThis water was not suitable for drinking, however, and emperor Augustus used it to fill his naumachia in Trastevere. This water supply allowed emperor Augustus and the public to enjoy sham naval battles. The water surplus was used for the irrigation of Caesar’s horti (gardens) and for the irrigation of fields. Such an abundant supply of water gives an idea how much water Rome had at its disposal.

In his chief work (written in 97 CE) De aquis urbis Romae (published in two books), containing a history and description of the water-supply of Rome, [[Sextus Julius Frontinus] as describes only a meager volume to the Aqua Alsietina. This makes sense, if the naumachia was no longer in use in his time (second half of the first century CE).

Some traces of this aqueduct were discovered in 1720. An inscribed stone slab was found inmknn 1887 near the Via Claudia. It is the only written record of the Aqua Alsietina.

The fountain of the Acqua Paola in Rome, built under Pope Paul V announces wrongly on its triumphal arch that “Paul V restored the ancient ducts of the Aqua Alsietina. “. Actually the engineers had rebuilt the old Aqua Traiana, which had run close to the Aqua Alsietina.

Hydroelectricity is electricity generated by hydropower, i.e., the production of power through use of the gravitational force of falling or flowing water. It is the most widely used form of renewable energy. Once a hydroelectric complex is constructed, the project produces no direct waste, and has a considerably lower output level of the greenhouse gas carbon dioxide (CO2) than fossil fuel powered energy plants. Worldwide, hydroelectricity supplied an estimated 816 GWe in 2005. This was approximately 20% of the world’s electricity, and accounted for about 88% of electricity from renewable sources.

Advantages

Economics
The major advantage of hydroelectricity is elimination of the cost of fuel. The cost of operating a hydroelectric plant is nearly immune to increases in the cost of fossil fuels such as oil, natural gas or coal, and no imports are needed.

Hydroelectric plants also tend to have longer economic lives than fuel-fired generation, with some plants now in service which were built 50 to 100 years ago. Operating labor cost is also usually low, as plants are automated and have few personnel on site during normal operation.

Where a dam serves multiple purposes, a hydroelectric plant may be added with relatively low construction cost, providing a useful revenue stream to offset the costs of dam operation. It has been calculated that the sale of electricity from the Three Gorges Dam will cover the construction costs after 5 to 8 years of full generation.

Greenhouse gas emissions
Since hydroelectric dams do not burn fossil fuels, they do not directly produce carbon dioxide (a greenhouse gas). While some carbon dioxide is produced during manufacture and construction of the project, this is a tiny fraction of the operating emissions of equivalent fossil-fuel electricity generation.

Related activities
Reservoirs created by hydroelectric schemes often provide facilities for water sports, and become tourist attractions in themselves. In some countries, aquaculture in reservoirs is common. Multi-use dams installed for irrigation support agriculture with a relatively constant water supply. Large hydro dams can control floods, which would otherwise affect people living downstream of the project.

Water politics is politics affected by water and water resources. Because of overpopulation, mass consumption, misuse, and water pollution, the availability of drinking water per capita is inadequate and shrinking as of the year 2006. For this reason, water is a strategic resource in the globe and an important element in many political conflicts. It causes health impacts and damage to biodiversity. The serious worldwide water situation is called water crisis.

UNESCO’s World Water Development Report (WWDR, 2003) from its World Water Assessment Program indicates that, in the next 20 years, the quantity of water available to everyone is predicted to decrease by 30%. 40% of the world’s inhabitants currently have insufficient fresh water for minimal hygiene. More than 2.2 million people died in 2000 from waterborne diseases (related to the consumption of contaminated water) or drought. In 2004, the UK charity WaterAid reported that a child dies every 15 seconds from easily preventable water-related diseases; often this means lack of sewage disposal; see toilet.

To halve, by 2015, the proportion of people without sustainable access to safe drinking water is one of the Millennium Development Goals.

Fresh water — now more precious than ever in our history for its extensive use in agriculture, high-tech manufacturing, and energy production — is increasingly receiving attention as a resource requiring better water management and sustainable use.

Organizations concerned in water protection include International Water Association (IWA), WaterAid, Water 1st, American Water Resources Association. Water related conventions are United Nations Convention to Combat Desertification (UNCCD), International Convention for the Prevention of Pollution from Ships, United Nations Convention on the Law of the Sea and Ramsar Convention. World Day for Water takes place at March 22 and World Ocean Day at June 8.

Water used in the production of a good or service is virtual water.

Water is considered a purifier in most religions. Major faiths that incorporate ritual washing (ablution) include Christianity, Hinduism, Rastafarianism, Islam, Shinto, Taoism, and Judaism. Immersion (or aspersion or affusion) of a person in water is a central sacrament of Christianity (where it is called baptism); it is also a part of the practice of other religions, including Judaism (mikvah) and Sikhism (Amrit Sanskar). In addition, a ritual bath in pure water is performed for the dead in many religions including Judaism and Islam. In Islam, the five daily prayers can be done in most cases after completing washing certain parts of the body using clean water (wudu). In Shinto, water is used in almost all rituals to cleanse a person or an area (e.g., in the ritual of misogi). Water is mentioned in the Bible 442 times in the New International Version and 363 times in the King James Version: 2 Peter 3:5(b) states, “The earth was formed out of water and by water” (NIV).

Some faiths use water especially prepared for religious purposes (holy water in some Christian denominations, Amrita in Sikhism and Hinduism). Many religions also consider particular sources or bodies of water to be sacred or at least auspicious; examples include Lourdes in Roman Catholicism, the Jordan River (at least symbolically) in some Christian churches, the Zamzam Well in Islam and the River Ganges (among many others) in Hinduism.

Water is often believed to have spiritual powers. In Celtic mythology, Sulis is the local goddess of thermal springs; in Hinduism, the Ganges is also personified as a goddess, while Saraswati have been referred to as goddess in Vedas. Also water is one of the “panch-tatva”s (basic 5 elements, others including fire, earth, space, air). Alternatively, gods can be patrons of particular springs, rivers, or lakes: for example in Greek and Roman mythology, Peneus was a river god, one of the three thousand Oceanids. In Islam, not only does water give life, but every life is itself made of water: “We made from water every living thing”.

The Ancient Greek philosopher Empedocles held that water is one of the four classical elements along with fire, earth and air, and was regarded as the ylem, or basic substance of the universe. Water was considered cold and moist. In the theory of the four bodily humors, water was associated with phlegm. Water was also one of the five elements in traditional Chinese philosophy, along with earth, fire, wood, and metal.

Water also plays an important role in literature as a symbol of purification. Examples include the critical importance of a river in As I Lay Dying by William Faulkner and the drowning of Ophelia in Hamlet.

Sherlock Holmes held that “From a drop of water, a logician could infer the possibility of an Atlantic or a Niagara without having seen or heard of one or the other.”

Water plays many critical roles within the field of food science. It is important for a food scientist to understand the roles that water plays within food processing to ensure the success of their products.

Solutes such as salts and sugars found in water affect the physical properties of water. The boiling and freezing points of water is affected by solutes. One mole of sucrose (sugar) raises the boiling point of water by 0.52 °C, and one mole of salt raises the boiling point by 1.04 °C while lowering the freezing point of water in a similar way.[26] Solutes in water also affect water activity which affects many chemical reactions and the growth of microbes in food.[27] Water activity can be described as a ratio of the vapor pressure of water in a solution to the vapor pressure of pure water.[26] Solutes in water lower water activity. This is important to know because most bacterial growth ceases at low levels of water activity.[27] Not only does microbial growth affect the safety of food but also the preservation and shelf life of food.

Water hardness is also a critical factor in food processing. It can dramatically affect the quality of a product as well as playing a role in sanitation. Water hardness is classified based on the amounts of removable calcium carbonate salt it contains per gallon. Water hardness is measured in grains; 0.064 g calcium carbonate is equivalent to one grain of hardness.[26] Water is classified as soft if it contains 1 to 4 grains, medium if it contains 5 to 10 grains and hard if it contains 11 to 20 grains.[26] The hardness of water may be altered or treated by using a chemical ion exchange system. The hardness of water also affects its pH balance which plays a critical role in food processing. For example, hard water prevents successful production of clear beverages. Water hardness also affects sanitation; with increasing hardness, there is a loss of effectiveness for its use as a sanitizer.[26]

Boiling, steaming, and simmering are popular cooking methods that often require immersing food in water or its gaseous state, steam. While cooking water is used for dishwashing too.

Water is used in power generation. Hydroelectricity is electricity obtained from hydropower. Hydroelectric power comes from water driving a water turbine connected to a generator. Hydroelectricity is a low-cost, non-polluting, renewable energy source. The energy is supplied by the sun. Heat from the sun evaporates water, which condenses as rain in higher altitudes, from where it flows down.

Pressurized water is used in water blasting and water jet cutters. Also, very high pressure water guns are used for precise cutting. It works very well, is relatively safe, and is not harmful to the environment. It is also used in the cooling of machinery to prevent over-heating, or prevent saw blades from over-heating.

Water is also used in many industrial processes and machines, such as the steam turbine and heat exchanger, in addition to its use as a chemical solvent. Discharge of untreated water from industrial uses is pollution. Pollution includes discharged solutes (chemical pollution) and discharged coolant water (thermal pollution). Industry requires pure water for many applications and utilizes a variety of purification techniques both in water supply and discharge.

Water supply facilities includes for example water wells cisterns for rainwater harvesting, water supply network, water purification facilities, water tanks, water towers, water pipes including old aqueducts. Atmospheric water generator is in development.

Drinking water is often collected at springs, extracted from artificial borings in the ground, or wells. Building more wells in adequate places is thus a possible way to produce more water, assuming the aquifers can supply an adequate flow. Other water sources are rainwater and river or lake water. This surface water, however, must be purified for human consumption. This may involve removal of undissolved substances, dissolved substances and harmful microbes. Popular methods are filtering with sand which only removes undissolved material, while chlorination and boiling kill harmful microbes. Distillation does all three functions. More advanced techniques exist, such as reverse osmosis. Desalination of abundant ocean or seawater is a more expensive solution used in coastal arid climates.

The distribution of drinking water is done through municipal water systems or as bottled water. Governments in many countries have programs to distribute water to the needy at no charge. Others argue that the market mechanism and free enterprise are best to manage this rare resource and to finance the boring of wells or the construction of dams and reservoirs.

Reducing waste by using drinking water only for human consumption is another option. In some cities such as Hong Kong, sea water is extensively used for flushing toilets citywide in order to conserve fresh water resources.

Polluting water may be the biggest single misuse of water; to the extent that a pollutant limits other uses of the water, it becomes a waste of the resource, regardless of benefits to the polluter. Like other types of pollution, this does not enter standard accounting of market costs, being conceived as externalities for which the market cannot account. Thus other people pay the price of water pollution, while the private firms’ profits are not redistributed to the local population victim of this pollution. Pharmaceuticals consumed by humans often end up in the waterways and can have detrimental effects on aquatic life if they bioaccumulate and if they are not biodegradable.

Wastewater facilities are sewers and wastewater treatment plants. Another way to remove pollution from surface runoff water is bioswale.

Water has a high heat of vaporization and is relatively inert, which makes it a good fire extinguishing fluid. The evaporation of water carries heat away from the fire. However, water cannot be used to fight fires of electric equipment, because impure water is electrically conductive, or of oils and organic solvents, because they float on water and the explosive boiling of water tends to spread the burning liquid.

Decomposition of water may have played a role in the Chernobyl disaster. Initially, cooling of the incandescent reactor was attempted, but the result was an explosion, when the extreme heat caused water to flash into steam, thus leading to a steam explosion; it may also have decomposed water into hydrogen and oxygen, which subsequently exploded.