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Rain

Rain is a form of precipitation where water droplets that have condensed from atmospheric water vapor fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water for hydroelectric power plants, crop irrigation, and suitable conditions for many types of ecosystems.

The major cause of rain production is moisture moving along three-dimensional zones of temperature and moisture contrasts known as weather fronts. If enough moisture and upward motion is present, precipitation falls from convective clouds (those with strong upward vertical motion) such as cumulonimbus (thunder clouds) which can organize into narrow rainbands. In mountainous areas, heavy precipitation is possible where upslope flow is maximized within windward sides of the terrain at elevation which forces moist air to condense and fall out as rainfall along the sides of mountains. On the leeward side of mountains, desert climates can exist due to the dry air caused by downslope flow which causes heating and drying of the air mass. The movement of the monsoon trough, or Intertropical Convergence Zone, brings rainy seasons to savannah climes.

Heavy rainfall on a roof

The urban heat island effect leads to increased rainfall, both in amounts and intensity, downwind of cities. Global warming is also causing changes in the precipitation pattern, including wetter conditions across eastern North America and drier conditions in the tropics. Antarctica is the driest continent. The globally averaged annual precipitation over land is 715 mm (28.1 in), but over the whole Earth, it is much higher at 990 mm (39 in).^[1] Climate classification systems such as the Köppen classification system use average annual rainfall to help differentiate between differing climate regimes. Rainfall is measured using rain gauges. Rainfall amounts can be estimated by weather radar.

Formation

Water-saturated air

Air contains water vapor, and the amount of water in a given mass of dry air, known as the mixing ratio, is measured in grams of water per kilogram of dry air (g/kg).^[2]^[3] The amount of moisture in the air is also commonly reported as relative humidity; which is the percentage of the total water vapor air can hold at a particular air temperature.^[4] How much water vapor a parcel of air can contain before it becomes saturated (100% relative humidity) and forms into a cloud (a group of visible tiny water or ice particles suspended above the Earth’s surface)^[5] depends on its temperature. Warmer air can contain more water vapor than cooler air before becoming saturated. Therefore, one way to saturate a parcel of air is to cool it. The dew point is the temperature to which a parcel must be cooled in order to become saturated.^[6]

There are four main mechanisms for cooling the air to its dew point: adiabatic cooling, conductive cooling, radiational cooling, and evaporative cooling. Adiabatic cooling occurs when air rises and expands.^[7] The air can rise due to convection, large-scale atmospheric motions, or a physical barrier such as a mountain (orographic lift). Conductive cooling occurs when the air comes into contact with a colder surface,^[8] usually by being blown from one surface to another, for example from a liquid water surface to colder land. Radiational cooling occurs due to the emission of infrared radiation, either by the air or by the surface underneath.^[9] Evaporative cooling occurs when moisture is added to the air through evaporation, which forces the air temperature to cool to its wet-bulb temperature, or until it reaches saturation.^[10]

The main ways water vapor is added to the air are wind convergence into areas of upward motion,^[11] precipitation or virga falling from above,^[12] daytime heating evaporating water from the surface of oceans, water bodies or wet land,^[13] transpiration from plants,^[14] cool or dry air moving over warmer water,^[15] and lifting air over mountains.^[16] Water vapor normally begins to condense on condensation nuclei such as dust, ice, and salt in order to form clouds. Elevated portions of weather fronts (which are three-dimensional in nature)^[17] force broad areas of upward motion within the Earth’s atmosphere which form clouds decks such as altostratus or cirrostratus.^[18] Stratus is a stable cloud deck which tends to form when a cool, stable air mass is trapped underneath a warm air mass. It can also form due to the lifting of advection fog during breezy conditions.^[19]

Coalescence and fragmentation

Coalescence occurs when water droplets fuse to create larger water droplets. Air resistance typically causes the water droplets in a cloud to remain stationary. When air turbulence occurs, water droplets collide, producing larger droplets.

As these larger water droplets descend, coalescence continues, so that drops become heavy enough to overcome air resistance and fall as rain. Coalescence generally happens most often in clouds above freezing (in their top) and is also known as the warm rain process.^[20] In clouds below freezing, when ice crystals gain enough mass they begin to fall. This generally requires more mass than coalescence when occurring between the crystal and neighboring water droplets. This process is temperature dependent, as supercooled water droplets only exist in a cloud that is below freezing. In addition, because of the great temperature difference between cloud and ground level, these ice crystals may melt as they fall and become rain.^[21]

Raindrops have sizes ranging from 0.1 to 9 mm (0.0039 to 0.3543 in) mean diameter but develop a tendency to break up at larger sizes. Smaller drops are called cloud droplets, and their shape is spherical. As a raindrop increases in size, its shape becomes more oblate, with its largest cross-section facing the oncoming airflow. Large rain drops become increasingly flattened on the bottom, like hamburger buns; very large ones are shaped like parachutes.^[22]^[23] Contrary to popular belief, their shape does not resemble a teardrop.^[24] The biggest raindrops on Earth were recorded over Brazil and the Marshall Islands in 2004 — some of them were as large as 10 mm (0.39 in). The large size is explained by condensation on large smoke particles or by collisions between drops in small regions with particularly high content of liquid water.^[25]

Raindrops associated with melting hail tend to be larger than other raindrops.^[26]

Intensity and duration of rainfall are usually inversely related, i.e., high-intensity storms are likely to be of short duration and low-intensity storms can have a long duration.^[27]^[28]

Droplet size distribution

Main article: Raindrop size distribution

The final droplet size distribution is an exponential distribution. The number of droplets with diameter between d $d$ and D+dD $D+dD$ per unit volume of space is n(d)=n0e−d/⟨d⟩dD $n(d)=n_{0}e^{-d/\langle d\rangle }dD$ . This is commonly referred to as the Marshall–Palmer law after the researchers who first characterized it.^[23]^[29] The parameters are somewhat temperature-dependent,^[30] and the slope also scales with the rate of rainfall ⟨d⟩−1=41R−0.21 $\langle d\rangle ^{-1}=41R^{-0.21}$ (d in centimeters and R in millimeters per hour).^[23]

Deviations can occur for small droplets and during different rainfall conditions. The distribution tends to fit averaged rainfall, while instantaneous size spectra often deviate and have been modeled as gamma distributions.^[31] The distribution has an upper limit due to droplet fragmentation.^[23]

Raindrop impacts

Raindrops impact at their terminal velocity, which is greater for larger drops due to their larger mass-to-drag ratio. At sea level and without wind, 0.5 mm (0.020 in) drizzle impacts at 2 m/s (6.6 ft/s) or 7.2 km/h (4.5 mph), while large 5 mm (0.20 in) drops impact at around 9 m/s (30 ft/s) or 32 km/h (20 mph).^[32]

Rain falling on loosely packed material such as newly fallen ash can produce dimples that can be fossilized, called raindrop impressions.^[33] The air density dependence of the maximum raindrop diameter together with fossil raindrop imprints has been used to constrain the density of the air 2.7 billion years ago.^[34]

The sound of raindrops hitting water is caused by bubbles of air oscillating underwater.^[35]^[36]

The METAR code for rain is RA, while the coding for rain showers is SHRA.^[37]

Virga

Main article: Virga

In certain conditions, precipitation may fall from a cloud but then evaporate or sublime before reaching the ground. This is termed virga and is more often seen in hot and dry climates.

Causes

Frontal activity

Main article: Weather fronts

Stratiform (a broad shield of precipitation with a relatively similar intensity) and dynamic precipitation (convective precipitation which is showery in nature with large changes in intensity over short distances) occur as a consequence of slow ascent of air in synoptic systems (on the order of cm/s), such as in the vicinity of cold fronts and near and poleward of surface warm fronts. Similar ascent is seen around tropical cyclones outside the eyewall, and in comma-head precipitation patterns around mid-latitude cyclones.^[38]

A wide variety of weather can be found along an occluded front, with thunderstorms possible, but usually, their passage is associated with a drying of the air mass. Occluded fronts usually form around mature low-pressure areas.^[39] What separates rainfall from other precipitation types, such as ice pellets and snow, is the presence of a thick layer of air aloft which is above the melting point of water, which melts the frozen precipitation well before it reaches the ground. If there is a shallow near-surface layer that is below freezing, freezing rain (rain which freezes on contact with surfaces in subfreezing environments) will result.^[40] Hail becomes an increasingly infrequent occurrence when the freezing level within the atmosphere exceeds 3,400 m (11,000 ft) above ground level.^[41]

Convection

Diagram showing that as moist air becomes heated more than its surroundings, it moves upward, resulting in brief rain showers. — Convective precipitation

Diagram showing how moist air over the ocean rises and flows over the land, causing cooling and rain as it hits mountain ridges. — Orographic precipitation

Convective rain, or showery precipitation, occurs from convective clouds (e.g., cumulonimbus or cumulus congestus). It falls as showers with rapidly changing intensity. Convective precipitation falls over a certain area for a relatively short time, as convective clouds have limited horizontal extent. Most precipitation in the tropics appears to be convective; however, it has been suggested that stratiform precipitation also occurs.^[38]^[42] Graupel and hail indicate convection.^[43] In mid-latitudes, convective precipitation is intermittent and often associated with baroclinic boundaries such as cold fronts, squall lines, and warm fronts.^[44]

Orographic effects

Main articles: Orographic lift, Precipitation types (meteorology), and United States rainfall climatology

Orographic precipitation occurs on the windward side of mountains and is caused by the rising air motion of a large-scale flow of moist air across the mountain ridge, resulting in adiabatic cooling and condensation. In mountainous parts of the world subjected to relatively consistent winds (for example, the trade winds), a more moist climate usually prevails on the windward side of a mountain than on the leeward or downwind side. Moisture is removed by orographic lift, leaving drier air (see katabatic wind) on the descending and generally warming, leeward side where a rain shadow is observed.^[16]

In Hawaii, Mount Waiʻaleʻale, on the island of Kauai, is notable for its extreme rainfall, as it is amongst the places in the world with the highest levels of rainfall, with 9,500 mm (373 in).^[45] Systems known as Kona storms affect the state with heavy rains between October and April.^[46] Local climates vary considerably on each island due to their topography, divisible into windward (Koʻolau) and leeward (Kona) regions based upon location relative to the higher mountains. Windward sides face the east to northeast trade winds and receive much more rainfall; leeward sides are drier and sunnier, with less rain and less cloud cover.^[47]

In South America, the Andes mountain range blocks Pacific moisture that arrives in that continent, resulting in a desert-like climate just downwind across western Argentina.^[48] The Sierra Nevada range creates the same effect in North America forming the Great Basin and Mojave Deserts.^[49]^[50]

Within the tropics

Human influence

The fine particulate matter produced by car exhaust and other human sources of pollution forms cloud condensation nuclei leads to the production of clouds and increases the likelihood of rain. As commuters and commercial traffic cause pollution to build up over the course of the week, the likelihood of rain increases: it peaks by Saturday, after five days of weekday pollution has been built up. In heavily populated areas that are near the coast, such as the United States’ Eastern Seaboard, the effect can be dramatic: there is a 22% higher chance of rain on Saturdays than on Mondays.^[62] The urban heat island effect warms cities 0.6 to 5.6 °C (33.1 to 42.1 °F) above surrounding suburbs and rural areas. This extra heat leads to greater upward motion, which can induce additional shower and thunderstorm activity. Rainfall rates downwind of cities are increased between 48% and 116%. Partly as a result of this warming, monthly rainfall is about 28% greater between 32 and 64 km (20 and 40 mi) downwind of cities, compared with upwind.^[63] Some cities induce a total precipitation increase of 51%.^[64]

Increasing temperatures tend to increase evaporation which can lead to more precipitation. Precipitation generally increased over land north of 30°N from 1900 through 2005 but has declined over the tropics since the 1970s. Globally there has been no statistically significant overall trend in precipitation over the past century, although trends have varied widely by region and over time. Eastern portions of North and South America, northern Europe, and northern and central Asia have become wetter. The Sahel, the Mediterranean, southern Africa and parts of southern Asia have become drier. There has been an increase in the number of heavy precipitation events over many areas during the past century, as well as an increase since the 1970s in the prevalence of droughts—especially in the tropics and subtropics. Changes in precipitation and evaporation over the oceans are suggested by the decreased salinity of mid- and high-latitude waters (implying more precipitation), along with increased salinity in lower latitudes (implying less precipitation and/or more evaporation). Over the contiguous United States, total annual precipitation increased at an average rate of 6.1 percent since 1900, with the greatest increases within the East North Central climate region (11.6 percent per century) and the South (11.1 percent). Hawaii was the only region to show a decrease (−9.25 percent).^[65]

Analysis of 65 years of United States of America rainfall records show the lower 48 states have an increase in heavy downpours since 1950. The largest increases are in the Northeast and Midwest, which in the past decade, have seen 31 and 16 percent more heavy downpours compared to the 1950s. Rhode Island is the state with the largest increase, 104%. McAllen, Texas is the city with the largest increase, 700%. Heavy downpour in the analysis are the days where total precipitation exceeded the top one percent of all rain and snow days during the years 1950–2014.^[66]^[67]

The most successful attempts at influencing weather involve cloud seeding, which include techniques used to increase winter precipitation over mountains and suppress hail.^[68]

Characteristics

Patterns

Main article: Rainband

Rainbands are cloud and precipitation areas which are significantly elongated. Rainbands can be stratiform or convective,^[69] and are generated by differences in temperature. When noted on weather radar imagery, this precipitation elongation is referred to as banded structure.^[70] Rainbands in advance of warm occluded fronts and warm fronts are associated with weak upward motion,^[71] and tend to be wide and stratiform in nature.^[72]

Rainbands spawned near and ahead of cold fronts can be squall lines which are able to produce tornadoes.^[73] Rainbands associated with cold fronts can be warped by mountain barriers perpendicular to the front’s orientation due to the formation of a low-level barrier jet.^[74] Bands of thunderstorms can form with sea breeze and land breeze boundaries if enough moisture is present. If sea breeze rainbands become active enough just ahead of a cold front, they can mask the location of the cold front itself.^[75]

Once a cyclone occludes an occluded front (a trough of warm air aloft) will be caused by strong southerly winds on its eastern periphery rotating aloft around its northeast, and ultimately northwestern, periphery (also termed the warm conveyor belt), forcing a surface trough to continue into the cold sector on a similar curve to the occluded front. The front creates the portion of an occluded cyclone known as its comma head, due to the comma-like shape of the mid-tropospheric cloudiness that accompanies the feature. It can also be the focus of locally heavy precipitation, with thunderstorms possible if the atmosphere along the front is unstable enough for convection.^[76] Banding within the comma head precipitation pattern of an extratropical cyclone can yield significant amounts of rain.^[77] Behind extratropical cyclones during fall and winter, rainbands can form downwind of relative warm bodies of water such as the Great Lakes. Downwind of islands, bands of showers and thunderstorms can develop due to low-level wind convergence downwind of the island edges. Offshore California, this has been noted in the wake of cold fronts.^[78]

Rainbands within tropical cyclones are curved in orientation. Tropical cyclone rainbands contain showers and thunderstorms that, together with the eyewall and the eye, constitute a hurricane or tropical storm. The extent of rainbands around a tropical cyclone can help determine the cyclone’s intensity.^[79]

Acidity

Köppen climate classification

Main article: Köppen climate classification

The Köppen classification depends on average monthly values of temperature and precipitation. The most commonly used form of the Köppen classification has five primary types labeled A through E. Specifically, the primary types are A, tropical; B, dry; C, mild mid-latitude; D, cold mid-latitude; and E, polar. The five primary classifications can be further divided into secondary classifications such as rain forest, monsoon, tropical savanna, humid subtropical, humid continental, oceanic climate, Mediterranean climate, steppe, subarctic climate, tundra, polar ice cap, and desert.

Rain forests are characterized by high rainfall, with definitions setting minimum normal annual rainfall between 1,750 and 2,000 mm (69 and 79 in).^[84] A tropical savanna is a grassland biome located in semi-arid to semi-humid climate regions of subtropical and tropical latitudes, with rainfall between 750 and 1,270 mm (30 and 50 in) a year. They are widespread on Africa, and are also found in India, the northern parts of South America, Malaysia, and Australia.^[85] The humid subtropical climate zone is where winter rainfall is associated with large storms that the westerlies steer from west to east. Most summer rainfall occurs during thunderstorms and from occasional tropical cyclones.^[86] Humid subtropical climates lie on the east side continents, roughly between latitudes 20° and 40° degrees away from the equator.^[87]

An oceanic (or maritime) climate is typically found along the west coasts at the middle latitudes of all the world’s continents, bordering cool oceans, as well as southeastern Australia, and is accompanied by plentiful precipitation year-round.^[88] The Mediterranean climate regime resembles the climate of the lands in the Mediterranean Basin, parts of western North America, parts of Western and South Australia, in southwestern South Africa and in parts of central Chile. The climate is characterized by hot, dry summers and cool, wet winters.^[89] A steppe is a dry grassland.^[90] Subarctic climates are cold with continuous permafrost and little precipitation.^[91]

Pollution

This section needs expansion. You can help by making an edit request. (October 2022)

This section is an excerpt from Per- and polyfluoroalkyl substances § Prevalence in rain, soil, water bodies, and air.[edit]

In 2022, levels of at least four perfluoroalkyl acids (PFAAs) in rain water worldwide greatly exceeded the EPA’s lifetime drinking water health advisories as well as comparable Danish, Dutch, and European Union safety standards, leading to the conclusion that “the global spread of these four PFAAs in the atmosphere has led to the planetary boundary for chemical pollution being exceeded”.^[92]

It had been thought that PFAAs would eventually end up in the oceans, where they would be diluted over decades, but a field study published in 2021 by researchers at Stockholm University found that they are often transferred from water to air when waves reach land, are a significant source of air pollution, and eventually get into rain. The researchers concluded that pollution may impact large areas.^[93]^[94]^[95]

In 2024, a worldwide study of 45,000 groundwater samples found that 31% of samples contained levels of PFAS that were harmful to human health; these samples were taken from areas not near any obvious source of contamination.^[96]Soil is also contaminated and the chemicals have been found in remote areas such as Antarctica.^[97] Soil contamination can result in higher levels of PFAs found in foods such as white rice, coffee, and animals reared on contaminated ground.^[98]^[99]^[100]

Measurement

Gauges

See also: Rain gauge, Disdrometer, and Snow gauge

Rain is measured in units of length per unit time, typically in millimeters per hour,^[101] or in countries where imperial units are more common, inches per hour.^[102] The “length”, or more accurately, “depth” being measured is the depth of rain water that would accumulate on a flat, horizontal and impermeable surface during a given amount of time, typically an hour.^[103] One millimeter of rainfall is the equivalent of one liter of water per square meter.^[104]

The standard way of measuring rainfall or snowfall is the standard rain gauge, which can be found in 100-mm (4-in) plastic and 200-mm (8-in) metal varieties.^[105] The inner cylinder is filled by 25 mm (0.98 in) of rain, with overflow flowing into the outer cylinder. Plastic gauges have markings on the inner cylinder down to 0.25 mm (0.0098 in) resolution, while metal gauges require use of a stick designed with the appropriate 0.25 mm (0.0098 in) markings. After the inner cylinder is filled, the amount inside it is discarded, then filled with the remaining rainfall in the outer cylinder until all the fluid in the outer cylinder is gone, adding to the overall total until the outer cylinder is empty.^[106] Other types of gauges include the popular wedge gauge (the cheapest rain gauge and most fragile), the tipping bucket rain gauge, and the weighing rain gauge.^[107] For those looking to measure rainfall the most inexpensively, a can that is cylindrical with straight sides will act as a rain gauge if left out in the open, but its accuracy will depend on what ruler is used to measure the rain with. Any of the above rain gauges can be made at home, with enough know-how.^[108]

When a precipitation measurement is made, various networks exist across the United States and elsewhere where rainfall measurements can be submitted through the Internet, such as CoCoRAHS or GLOBE.^[109]^[110] If a network is not available in the area where one lives, the nearest local weather or met office will likely be interested in the measurement.^[111]

Remote sensing

Intensity

Heavy rain in Zapopan

Heavy rain in Glenshaw, Pennsylvania

Duration: 5 minutes and 37 seconds.5:37

The sound of heavy rainfall in a suburban neighborhood

Problems playing this file? See media help.

Rainfall intensity is classified according to the rate of precipitation, which depends on the considered time.^[115] The following categories are used to classify rainfall intensity:

Light rain — when the precipitation rate is < 2.5 mm (0.098 in) per hour
Moderate rain — when the precipitation rate is between 2.5–7.6 mm (0.098–0.299 in) or 10 mm (0.39 in) per hour^[116]^[117]
Heavy rain — when the precipitation rate is > 7.6 mm (0.30 in) per hour,^[116] or between 10 and 50 mm (0.39 and 1.97 in) per hour^[117]
Violent rain — when the precipitation rate is > 50 mm (2.0 in) per hour^[117]

Terms used for a heavy or violent rain include gully washer, trash-mover and toad-strangler.^[118] The intensity can also be expressed by rainfall erosivity R-factor^[119] or in terms of the rainfall time-structure n-index.^[115]

Return period

Forecasting

Main article: Quantitative precipitation forecast

Example of a five-day rainfall forecast from the Hydrometeorological Prediction Center

The Quantitative Precipitation Forecast (abbreviated QPF) is the expected amount of liquid precipitation accumulated over a specified time period over a specified area.^[123] A QPF will be specified when a measurable precipitation type reaching a minimum threshold is forecast for any hour during a QPF valid period. Precipitation forecasts tend to be bound by synoptic hours such as 0000, 0600, 1200 and 1800 GMT. Terrain is considered in QPFs by use of topography or based upon climatological precipitation patterns from observations with fine detail.^[124] Starting in the mid to late 1990s, QPFs were used within hydrologic forecast models to simulate impact to rivers throughout the United States.^[125]

Forecast models show significant sensitivity to humidity levels within the planetary boundary layer, or in the lowest levels of the atmosphere, which decreases with height.^[126] QPF can be generated on a quantitative, forecasting amounts, or a qualitative, forecasting the probability of a specific amount, basis.^[127] Radar imagery forecasting techniques show higher skill than model forecasts within 6 to 7 hours of the time of the radar image. The forecasts can be verified through use of rain gauge measurements, weather radar estimates, or a combination of both. Various skill scores can be determined to measure the value of the rainfall forecast.^[128]

Impact

Agricultural

Precipitation, especially rain, has a dramatic effect on agriculture. All plants need at least some water to survive, therefore rain (being the most effective means of watering) is important to agriculture. While a regular rain pattern is usually vital to healthy plants, too much or too little rainfall can be harmful, even devastating to crops. Drought can kill crops and increase erosion,^[129] while overly wet weather can cause harmful fungus growth.^[130] Plants need varying amounts of rainfall to survive. For example, certain cacti require small amounts of water,^[131] while tropical plants may need up to hundreds of inches of rain per year to survive.

In areas with wet and dry seasons, soil nutrients diminish and erosion increases during the wet season.^[27] Animals have adaptation and survival strategies for the wetter regime. The previous dry season leads to food shortages into the wet season, as the crops have yet to mature.^[132] Developing countries have noted that their populations show seasonal weight fluctuations due to food shortages seen before the first harvest, which occurs late in the wet season.^[133] Rain may be harvested through the use of rainwater tanks; treated to potable use or for non-potable use indoors or for irrigation.^[134] Excessive rain during short periods of time can cause flash floods.^[135]

Culture and religion

Global climatology

Deserts

Main article: Desert

The northern half of Africa is dominated by the world’s most extensive hot, dry region, the Sahara Desert. Some deserts also occupy much of southern Africa: the Namib and the Kalahari. Across Asia, a large annual rainfall minimum, composed primarily of deserts, stretches from the Gobi Desert in Mongolia west-southwest through western Pakistan (Balochistan) and Iran into the Arabian Desert in Saudi Arabia. Most of Australia is semi-arid or desert,^[150] making it the world’s driest inhabited continent. In South America, the Andes mountain range blocks Pacific moisture that arrives in that continent, resulting in a desert-like climate just downwind across western Argentina.^[48] The drier areas of the United States are regions where the Sonoran Desert overspreads the Desert Southwest, the Great Basin, and central Wyoming.^[151]

Polar deserts

Main articles: Polar desert and Polar climate

Since rain only falls as liquid, it rarely falls when surface temperatures are below freezing unless there is a layer of warm air aloft, in which case it becomes freezing rain. Due to the entire atmosphere being below freezing, frigid climates usually see very little rainfall and are often known as polar deserts. A common biome in this area is the tundra, which has a short summer thaw and a long frozen winter. Ice caps see no rain at all, making Antarctica the world’s driest continent.

Rainforests

Monsoons

Impact of the Westerlies

Wettest known locations

Cherrapunji, situated on the southern slopes of the Eastern Himalaya in Shillong, India is the confirmed wettest place on Earth, with an average annual rainfall of 11,430 mm (450 in). The highest recorded rainfall in a single year was 22,987 mm (905.0 in) in 1861. The 38-year average at nearby Mawsynram, Meghalaya, India is 11,873 mm (467.4 in).^[168] The wettest spot in Australia is Mount Bellenden Ker in the north-east of the country which records an average of 8,000 mm (310 in) per year, with over 12,200 mm (480.3 in) of rain recorded during 2000.^[169] The Big Bog on the island of Maui has the highest average annual rainfall in the Hawaiian Islands, at 10,300 mm (404 in).^[170] Mount Waiʻaleʻale on the island of Kauaʻi achieves similar torrential rains, while slightly lower than that of the Big Bog, at 9,500 mm (373 in)^[171] of rain per year over the last 32 years, with a record 17,340 mm (683 in) in 1982. Its summit is considered one of the rainiest spots on earth, with a reported 350 days of rain per year.

Lloró, a town situated in Chocó, Colombia, is probably the place with the largest rainfall in the world, averaging 13,300 mm (523.6 in) per year.^[172] The Department of Chocó is extraordinarily humid. Tutunendaó, a small town situated in the same department, is one of the wettest estimated places on Earth, averaging 11,394 mm (448.6 in) per year; in 1974 the town received 26,303 mm (86 ft 3.6 in), the largest annual rainfall measured in Colombia. Unlike Cherrapunji, which receives most of its rainfall between April and September, Tutunendaó receives rain almost uniformly distributed throughout the year.^[173] Quibdó, the capital of Chocó, receives the most rain in the world among cities with over 100,000 inhabitants: 9,000 mm (354 in) per year.^[172] Storms in Chocó can drop 500 mm (20 in) of rainfall in a day. This amount is more than what falls in many cities in a year.

Continent	Highest average		Place	Elevation		Years of record
Continent	in	mm	Place	ft	m	Years of record
South America	523.6	13,299	Lloró, Colombia (estimated)^[a]^[b]	520	158^[c]	29
Asia	467.4	11,872	Mawsynram, India^[a]^[d]	4,597	1,401	39
Africa	405.0	10,287	Debundscha, Cameroon	30	9.1	32
Oceania	404.3	10,269	Big Bog, Maui, Hawaii (US)^[a]	5,148	1,569	30
South America	354.0	8,992	Quibdo, Colombia	120	36.6	16
Australia	340.0	8,636	Mount Bellenden Ker, Queensland	5,102	1,555	9
North America	256.0	6,502	Hucuktlis Lake, British Columbia	12	3.66	14
Europe	183.0	4,648	Crkvice, Montenegro	3,337	1,017	22
Source (without conversions): Global Measured Extremes of Temperature and Precipitation, National Climatic Data Center. 9 August 2004.^[174]

	Continent	Place	Highest rainfall
	Continent	Place	in	mm
Highest average annual rainfall^[175]	Asia	Mawsynram, India	467.4	11,870
Highest in one year^[175]	Asia	Cherrapunji, India	1,042	26,470
Highest in one calendar month^[176]	Asia	Cherrapunji, India	366	9,296
Highest in 24 hours^[175]	Indian Ocean	Foc Foc, La Réunion	71.8	1,820
Highest in 12 hours^[175]	Indian Ocean	Foc Foc, La Réunion	45.0	1,140
Highest in one minute^[175]	North America	Unionville, Maryland, US	1.23	31.2

March 13, 2025

Drinking Water
Drinking water or potable water is water that is safe for ingestion, either when drunk directly in liquid form or consumed indirectly through food preparation. It is often (but not always) supplied through taps, in which case it is also called tap water.

The amount of drinking water required to maintain good health varies, and depends on physical activity level, age, health-related issues, and environmental conditions.^[1]^[2] For those who work in a hot climate, up to 16 litres (4.2 US gal) a day may be required.^[1]

About 1 to 2 billion people lack safe drinking water.^[3] Water can carry vectors of disease and is a major cause of death and illness worldwide.^[4] Developing countries are most affected by unsafe drinking water.

Sources

[edit]

Further information: Water resources and Water security

Drinking water vending machines in Thailand. One litre of potable water is sold (into the customer’s own bottle) for 1 baht.

Potable water is available in almost all populated areas of the world, although it may be expensive, and the supply may not always be sustainable. Sources where drinking water is commonly obtained include springs, hyporheic zones and aquifers (groundwater), from rainwater harvesting, surface water (from rivers, streams, glaciers), or desalinated seawater.

For these water sources to be consumed safely, they must receive adequate water treatment and meet drinking water quality standards.^[5]

An experimental source is atmospheric water generators.^[6]

Springs are often used as sources for bottled waters.^[7]

Supply

[edit]

Main articles: Water supply and Water supply network

The most efficient and convenient way to transport and deliver potable water is through pipes. Plumbing can require significant capital investment. Some systems suffer high operating costs. The cost to replace the deteriorating water and sanitation infrastructure of industrialized countries may be as high as $200 billion a year. Leakage of untreated and treated water from pipes reduces access to water. Leakage rates of 50% are not uncommon in urban systems.^[8]

Tap water, delivered by domestic water systems refers to water piped to homes and delivered to a tap or spigot.

Quantity

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Usage for general household use

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Further information: Water use and Residential water use in the U.S. and Canada

In the United States, the typical water consumption per capita, at home, is 69.3 US gallons (262 L; 57.7 imp gal) of water per day.^[9]^[10] Of this, only 1% of the water provided by public water suppliers is for drinking and cooking.^[11] Uses include (in decreasing order) toilets, washing machines, showers, baths, faucets, and leaks.

Total renewable water resources per capita in 2020

Usage for drinking

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This section is an excerpt from Daily consumption of drinking water.[edit]

The recommended daily amount of drinking water for humans varies.^[12] It depends on activity, age, health, and environment. In the United States, the Adequate Intake for total water, based on median intakes, is 4.0 litres (141 imp fl oz; 135 US fl oz) per day for males older than 18, and 3.0 litres (106 imp fl oz; 101 US fl oz) per day for females over 18; it assumes about 80% from drink and 20% from food.^[13] The European Food Safety Authority recommends 2.0 litres (70 imp fl oz; 68 US fl oz) of total water per day for women and 2.5 litres (88 imp fl oz; 85 US fl oz) per day for men.^[14]

Animals

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The qualitative and quantitative aspects of drinking water requirements on domesticated animals are studied and described within the context of animal husbandry. For example, a farmer might plan for 35 U.S. gallons (130 L) per day for a dairy cow, a third of that for a horse, and a tenth of that for a hog.^[15]

However, relatively few studies have been focused on the drinking behavior of wild animals.

Quality

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Countries where tap water is safe to drink (blue)

Main article: Water quality

Further information: Water pollution and Hard water

According to the World Health Organization’s 2017 report, safe drinking water is water that “does not represent any significant risk to health over a lifetime of consumption, including different sensitivities that may occur between life stages”.^[16]^: 2

According to a report by UNICEF and UNESCO, Finland has the best drinking water quality in the world.^[17]^[18]

Parameters to monitor quality

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Parameters for drinking water quality typically fall within three categories: microbiological, chemical, physical.

Microbiological parameters include coliform bacteria, E. coli, and specific pathogenic species of bacteria (such as cholera-causing Vibrio cholerae), viruses, and protozoan parasites. Originally, fecal contamination was determined with the presence of coliform bacteria, a convenient marker for a class of harmful fecal pathogens. The presence of fecal coliforms (like E. Coli) serves as an indication of contamination by sewage. Additional contaminants include protozoan oocysts such as Cryptosporidium sp., Giardia lamblia, Legionella, and viruses (enteric).^[19] Microbial pathogenic parameters are typically of greatest concern because of their immediate health risk.

Example for physical and chemical parameters measured in drinking water samples in Kenya and Ethiopia as part of a systematic review of published literature^[20]

Physical and chemical parameters include heavy metals, trace organic compounds, total suspended solids, and turbidity. Chemical parameters tend to pose more of a chronic health risk through buildup of heavy metals although some components like nitrates/nitrites and arsenic can have a more immediate impact. Physical parameters affect the aesthetics and taste of the drinking water and may complicate the removal of microbial pathogens.

Pesticides are also potential drinking water contaminants of the category chemical contaminants. Pesticides may be present in drinking water in low concentrations, but the toxicity of the chemical and the extent of human exposure are factors that are used to determine the specific health risk.^[21]

Perfluorinated alkylated substances (PFAS) are a group of synthetic compounds used in a large variety of consumer products, such as food packaging, waterproof fabrics, carpeting and cookware. PFAS are known to persist in the environment and are commonly described as persistent organic pollutants. PFAS chemicals have been detected in blood, both humans and animals, worldwide, as well as in food products, water, air and soil.^[22] Animal testing studies with PFAS have shown effects on growth and development, and possibly effects on reproduction, thyroid, the immune system and liver.^[23] As of 2022 the health impacts of many PFAS compounds are not understood. Scientists are conducting research to determine the extent and severity of impacts from PFAS on human health.^[24] PFAS have been widely detected in drinking water worldwide and regulations have been developed, or are under development, in many countries.^[25]

Drinking water quality standards

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This section is an excerpt from Drinking water quality standards.[edit]

Drinking water quality standards describes the quality parameters set for drinking water. Water may contain many harmful constituents, yet there are no universally recognized and accepted international standards for drinking water. Even where standards do exist, the permitted concentration of individual constituents may vary by as much as ten times from one set of standards to another. Many countries specify standards to be applied in their own country. In Europe, this includes the European Drinking Water Directive^[26] and in the United States, the United States Environmental Protection Agency (EPA) establishes standards as required by the Safe Drinking Water Act. China adopted its own drinking water standard GB3838-2002 (Type II) enacted by Ministry of Environmental Protection in 2002.^[27] For countries without a legislative or administrative framework for such standards, the World Health Organization publishes guidelines on the standards that should be achieved.^[28]Where drinking water quality standards do exist, most are expressed as guidelines or targets rather than requirements, and very few water standards have any legal basis or, are subject to enforcement.^[29] Two exceptions are the European Drinking Water Directive and the Safe Drinking Water Act in the United States,^[30] which require legal compliance with specific standards. In Europe, this includes a requirement for member states to enact appropriate local legislation to mandate the directive in each country. Routine inspection and, where required, enforcement is enacted by means of penalties imposed by the European Commission on non-compliant nations.

Health issues due to low quality

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Further information: WASH § Health aspects, and Waterborne diseases

Mortality rate attributable to unsafe water, sanitation, and hygiene (WASH)^[31]

The “F-diagram” (feces, fingers, flies, fields, fluids, food), showing pathways of fecal–oral disease transmission. The vertical blue lines show barriers: toilets, safe water, hygiene and handwashing.

The World Health Organization considers access to safe drinking-water a basic human right. Contaminated water is estimated to result in more than half a million deaths per year.^[32] More people die from unsafe water than from war, then-U.N. secretary-general Ban Ki-moon said in 2010.^[4] Contaminated water together with the lack of sanitation was estimated to cause about one percent of disability adjusted life years worldwide in 2010.^[33] According to the WHO, the most common diseases linked with poor water quality are cholera, diarrhea, dysentery, hepatitis A, typhoid, and polio.^[34]

One of the main causes for contaminated drinking water in developing countries is lack of sanitation and poor hygiene. For this reason, the quantification of the burden of disease from consuming contaminated drinking water usually looks at water, sanitation and hygiene aspects together. The acronym for this is WASH – standing for water, sanitation and hygiene.

This section is an excerpt from WASH § WASH-attributable burden of diseases and injuries.[edit]

The WHO has investigated which proportion of death and disease worldwide can be attributed to insufficient WASH services. In their analysis they focus on the following four health outcomes: diarrhea, acute respiratory infections, malnutrition, and soil-transmitted Helminthiasis (STHs).^[35]^: vi These health outcomes are also included as an indicator for achieving Sustainable Development Goal 3 (“Good Health and Well-being”): Indicator 3.9.2 reports on the “mortality rate attributed to unsafe water, sanitation, and lack of hygiene”.In 2023, WHO summarized the available data with the following key findings: “In 2019, use of safe WASH services could have prevented the loss of at least 1.4 million lives and 74 million disability-adjusted life years (DALYs) from four health outcomes. This represents 2.5% of all deaths and 2.9% of all DALYs globally.”^[35]^: vi Of the four health outcomes studied, it was diarrheal disease that had the most striking correlation, namely the highest number of “attributable burden of disease“: over 1 million deaths and 55 million DALYs from diarrheal diseases were linked with lack of WASH. Of these deaths, 564,000 deaths were linked to unsafe sanitation in particular.

Diarrhea, malnutrition and stunting

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Poverty often leads to unhygienic living conditions, as in this community in the Indian Himalayas. Such conditions promote contraction of diarrheal diseases, as a result of contaminated drinking water, poor sanitation and hygiene.

This section is an excerpt from WASH § Diarrhea, malnutrition and stunting.[edit]

Diarrhea is primarily transmitted through fecal–oral routes. In 2011, infectious diarrhea resulted in about 0.7 million deaths in children under five years old and 250 million lost school days.^[36]^[37] This equates to about 2000 child deaths per day.^[38] Children suffering from diarrhea are more vulnerable to become underweight (due to stunted growth).^[39]^[40] This makes them more vulnerable to other diseases such as acute respiratory infections and malaria. Chronic diarrhea can have a negative effect on child development (both physical and cognitive).^[41]Numerous studies have shown that improvements in drinking water and sanitation (WASH) lead to decreased risks of diarrhea.^[42] Such improvements might include for example, the use of water filters, provision of high-quality piped water and sewer connections.^[42] Diarrhea can be prevented – and the lives of 525,000 children annually be saved (estimate for 2017) – by improved sanitation, clean drinking water, and hand washing with soap.^[43] In 2008 the same figure was estimated as 1.5 million children.^[44]

Consumption of contaminated groundwater

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Main articles: Groundwater pollution and Arsenic contamination of groundwater

Sixty million people are estimated to have been poisoned by well water contaminated by excessive fluoride, which dissolved from granite rocks. The effects are particularly evident in the bone deformations of children. Similar or larger problems are anticipated in other countries including China, Uzbekistan, and Ethiopia. Although helpful for dental health in low dosage, fluoride in large amounts interferes with bone formation.^[45]

Diagram of water well types

Long-term consumption of water with high fluoride concentration (> 1.5 ppm F) can have serious undesirable consequences such as dental fluorosis, enamel mottle and skeletal fluorosis, bone deformities in children. Fluorosis severity depends on how much fluoride is present in the water, as well as people’s diet and physical activity. Defluoridation methods include membrane-based methods, precipitation, absorption, and electrocoagulation.^[46]

Natural arsenic contamination of groundwater is a global threat with 140 million people affected in 70 countries globally.^[47]

Examples of poor drinking water quality incidents

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Some well-known examples of water quality problems with drinking water supplies include:^[48]
- In 1854, a cholera outbreak in London’s Soho district was identified by John Snow as originating from contaminated water from the Broad Street pump. This can be regarded as a founding event of the science of epidemiology.
- Contamination of groundwater with arsenic in Bangladesh; considered to be “the largest-scale mass poisoning of a population” (began in the 1970s–1980s, officially recognized in 1993)
- Hinkley groundwater contamination (Chromium-6 contamination of private groundwater wells in Hinkley, California, United States (settled in 1996).
- Walkerton E. coli outbreak (Escherichia coli O157:H7 and Campylobacter contamination of water supply in Walkerton, Ontario, Canada in 2000). In 2000, an E. coli outbreak occurred in Walkerton, Ontario, Canada. Seven people died from drinking contaminated water. Hundreds suffered from the symptoms of the disease, not knowing if they too would die.^[49]
- Flint water crisis (lead and Legionella contamination of water supply in Flint, Michigan, United States, started in 2014)
- Jackson, Mississippi water crisis, United States (2022)
Water supply can get contaminated by pathogens which may originate from human excreta, for example due to a breakdown or design fault in the sanitation system, or by chemical contaminants.

Further examples of contamination include:
- In 1987, a cryptosporidiosis outbreak is caused by the public water supply of which the filtration was contaminated, in western Georgia^[50]
- In 1993, Milwaukee Cryptosporidium outbreak
- In 1998, an outbreak of typhoid fever in northern Israel, which was associated with the contaminated municipal water supply^[51]
- In 1997, 369 cases of cryptosporidiosis occurred, caused by a contaminated fountain in the Minnesota zoo. Most of the sufferers were children^[52]
- In 1998, a non-chlorinated municipal water supply was blamed for a campylobacteriosis outbreak in northern Finland^[53]
- In 2000, a gastroenteritis outbreak that was brought by a non-chlorinated community water supply, in southern Finland^[54]
- In 2004, contamination of the community water supply, serving the Bergen city centre of Norway, was later reported after the outbreak of waterborne giardiasis^[55]
- In 2007, contaminated drinking water was pinpointed which had led to the outbreak of gastroenteritis with multiple aetiologies in Denmark^[56]
Examples of chemical contamination include:
- In 1988, many people were poisoned in Camelford, when a worker put 20 tonnes of aluminium sulphate coagulant in the wrong tank.
- In 1993, a fluoride poisoning outbreak resulting from overfeeding of fluoride, in Mississippi^[57]
- In 2019 oil for an electric transformer oil entered the water supply for the city of Uummannaq in Greenland. A cargo ship in harbour was able to maintain a minimum supply to the city for two days until the mains supply was restored and flushing of all the pipework was started.^[58]
Treatment

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Main articles: Water purification and Water treatment

Water treatment plant

Most water requires some treatment before use; even water from deep wells or springs. The extent of treatment depends on the source of the water. Appropriate technology options in water treatment include both community-scale and household-scale point-of-use (POU) designs.^[59] Only a few large urban areas such as Christchurch, New Zealand have access to sufficiently pure water of sufficient volume that no treatment of the raw water is required.^[60]

In emergency situations when conventional treatment systems have been compromised, waterborne pathogens may be killed or inactivated by boiling^[61] but this requires abundant sources of fuel, and can be very onerous on consumers, especially where it is difficult to store boiled water in sterile conditions. Other techniques, such as filtration, chemical disinfection, and exposure to ultraviolet radiation (including solar UV) have been demonstrated in an array of randomized control trials to significantly reduce levels of water-borne disease among users in low-income countries,^[62] but these suffer from the same problems as boiling methods.

Another type of water treatment is called desalination and is used mainly in dry areas with access to large bodies of saltwater.

Publicly available treated water has historically been associated with major increases in life expectancy and improved public health. Water disinfection can greatly reduce the risks of waterborne diseases such as typhoid and cholera. Chlorination is currently the most widely used water disinfection method, although chlorine compounds can react with substances in water and produce disinfection by-products (DBP) that pose problems to human health.^[63] Local geological conditions affecting groundwater are determining factors for the presence of various metal ions, often rendering the water “soft” or “hard“.^{[citation needed]}

In the event of contamination of drinking water, government officials typically issue an advisory regarding water consumption. In the case of biological contamination, residents are usually advised to boil their water before consumption or to use bottled water as an alternative. In the case of chemical contamination, residents may be advised to refrain from consuming tap water entirely until the matter is resolved.

Point of use methods

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Main articles: Portable water purification and Self-supply of water and sanitation

The ability of point of use (POU) options to reduce disease is a function of both their ability to remove microbial pathogens if properly applied and such social factors as ease of use and cultural appropriateness. Technologies may generate more (or less) health benefit than their lab-based microbial removal performance would suggest.

The current priority of the proponents of POU treatment is to reach large numbers of low-income households on a sustainable basis. Few POU measures have reached significant scale thus far, but efforts to promote and commercially distribute these products to the world’s poor have only been under way for a few years.

Solar water disinfection is a low-cost method of purifying water that can often be implemented with locally available materials.^[64]^[65]^[66]^[67] Unlike methods that rely on firewood, it has low impact on the environment.

Addition of fluoride

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In many areas, low concentration of fluoride (< 1.0 ppm F) is intentionally added to tap water to improve dental health, although in some communities water fluoridation remains a controversial issue. (See water fluoridation controversy).

This section is an excerpt from Water fluoridation.[edit]

Water fluoridation is the controlled addition of fluoride to public water supplies to reduce tooth decay. Fluoridated water maintains fluoride levels effective for cavity prevention, achieved naturally or through supplementation.^[68] In the mouth, fluoride slows tooth enamel demineralization and enhances remineralization in early-stage cavities.^[69] Defluoridation is necessary when natural fluoride exceeds recommended limits.^[70] The World Health Organization (WHO) recommends fluoride levels of 0.5–1.5 mg/L, depending on climate and other factors.^[71] In the U.S., the recommended level has been 0.7 mg/L since 2015, lowered from 1.2 mg/L.^[72]^[73] Bottled water often has unknown fluoride levels.^[74]

Global access

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Further information: WASH and List of countries by access to clean water

World map for SDG 6 Indicator 6.1.1 in 2015: “Proportion of population using safely managed drinking water services”

Population in survey regions living without safely managed drinking water as reported by the WHO/UNICEF JMP^[6]

According to the World Health Organization (WHO), “access to safe drinking-water is essential to health, a basic human right and a component of effective policy for health protection.”^[16]^: 2 In 1990, only 76 percent of the global population had access to drinking water. By 2015 that number had increased to 91 percent.^[75] In 1990, most countries in Latin America, East and South Asia, and Sub-Saharan Africa were well below 90%. In Sub-Saharan Africa, where the rates are lowest, household access ranges from 40 to 80 percent.^[75] Countries that experience violent conflict can have reductions in drinking water access: One study found that a conflict with about 2,500 battle deaths deprives 1.8% of the population of potable water.^[76]

Typically in developed countries, tap water meets drinking water quality standards, even though only a small proportion is actually consumed or used in food preparation. Other typical uses for tap water include washing, toilets, and irrigation. Greywater may also be used for toilets or irrigation. Its use for irrigation however may be associated with risks.^[32]

Globally, by 2015, 89% of people had access to water from a source that is suitable for drinking – called improved water sources.^[32] In sub-Saharan Africa, access to potable water ranged from 40% to 80% of the population. Nearly 4.2 billion people worldwide had access to tap water, while another 2.4 billion had access to wells or public taps.^[32]

By 2015, 5.2 billion people representing 71% of the global population used safely managed drinking water services.^[77] As of 2017, 90% of people having access to water from a source that is suitable for drinking – called improved water source – and 71% of the world could access safely managed drinking water that is clean and available on-demand.^[32] Estimates suggest that at least 25% of improved sources contain fecal contamination.^[78] 1.8 billion people still use an unsafe drinking water source which may be contaminated by feces.^[32] This can result in infectious diseases, such as gastroenteritis, cholera, and typhoid, among others.^[32] Reduction of waterborne diseases and development of safe water resources is a major public health goal in developing countries. In 2017, almost 22 million Americans drank from water systems that were in violation of public health standards, which could contribute to citizens developing water-borne illnesses.^[79]^{[full citation needed]} Safe drinking water is an environmental health concern. Bottled water is sold for public consumption in most parts of the world.

Improved sources are also monitored based on whether water is available when needed (5.8 billion people), located on premises (5.4 billion), free from contamination (5.4 billion), and within a 30-minute round trip.^[77]^: 3 While improved water sources such as protected piped water are more likely to provide safe and adequate water as they may prevent contact with human excreta, for example, this is not always the case.^[75] According to a 2014 study, approximately 25% of improved sources contained fecal contamination.^[78]

The population in Australia, New Zealand, North America and Europe have achieved nearly universal basic drinking water services.^[77]^: 3

Because of the high initial investments, many less wealthy nations cannot afford to develop or sustain appropriate infrastructure, and as a consequence people in these areas may spend a correspondingly higher fraction of their income on water.^[80] 2003 statistics from El Salvador, for example, indicate that the poorest 20% of households spend more than 10% of their total income on water. In the United Kingdom, authorities define spending of more than 3% of one’s income on water as a hardship.^[81]

Global monitoring of access

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The WHO/UNICEF Joint Monitoring Program (JMP) for Water Supply and Sanitation^[82] is the official United Nations mechanism tasked with monitoring progress towards the Millennium Development Goal (MDG) relating to drinking-water and sanitation (MDG 7, Target 7c), which is to: “Halve, by 2015, the proportion of people without sustainable access to safe drinking-water and basic sanitation”.^[83]

Access to safe drinking water is indicated by safe water sources. These improved drinking water sources include household connection, public standpipe, borehole condition, protected dug well, protected spring, and rain water collection. Sources that do not encourage improved drinking water to the same extent as previously mentioned include: unprotected wells, unprotected springs, rivers or ponds, vender-provided water, bottled water (consequential of limitations in quantity, not quality of water), and tanker truck water. Access to sanitary water comes hand in hand with access to improved sanitation facilities for excreta, such as connection to public sewer, connection to septic system, or a pit latrine with a slab or water seal.^[84]

According to this indicator on improved water sources, the MDG was met in 2010, five years ahead of schedule. Over 2 billion more people used improved drinking water sources in 2010 than did in 1990. However, the job is far from finished. 780 million people are still without improved sources of drinking water, and many more people still lack safe drinking water. Estimates suggest that at least 25% of improved sources contain fecal contamination^[78] and an estimated 1.8 billion people globally use a source of drinking water that suffers from fecal contamination.^[85] The quality of these sources varies over time and often gets worse during the wet season.^[86] Continued efforts are needed to reduce urban-rural disparities and inequities associated with poverty; to dramatically increase safe drinking water coverage in countries in sub-Saharan Africa and Oceania; to promote global monitoring of drinking water quality; and to look beyond the MDG target towards universal coverage.^[87]

Regulations

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Main articles: Drinking water quality standards and List of water supply and sanitation by country

Guidelines for the assessment and improvement of service activities relating to drinking water have been published in the form of drinking water quality standards such as ISO 24510.^[88]

European Union

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See also: Water Framework Directive and Water supply and sanitation in the European Union

For example, the EU sets legislation on water quality. Directive 2000/60/EC of the European Parliament and of the Council of 23 October 2000 establishing a framework for Community action in the field of water policy, known as the water framework directive, is the primary piece of legislation governing water.^[89] This drinking water directive relates specifically to water intended for human consumption. Each member state is responsible for establishing the required policing measures to ensure that the legislation is implemented. For example, in the UK the Water Quality Regulations prescribe maximum values for substances that affect wholesomeness and the Drinking Water Inspectorate polices the water companies.

Japan

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See also: Water supply and sanitation in Japan

To improve water quality, Japan’s Ministry of Health revised its water quality standards, which were implemented in April 2004.^[90] Numerous professionals developed the drinking water standards.^[90] They also determined ways to manage the high quality water system. In 2008, improved regulations were conducted to improve the water quality and reduce the risk of water contamination.^[90]

New Zealand

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See also: Water supply and sanitation in New Zealand

The Water Services Act 2021 brought Taumata Arowai’ into existence as the new regulator of drinking water and waste water treatment in New Zealand. Initial activities including the establishment of a national register of water suppliers and establishing a network of accredited laboratories for drinking water and waste water analysis^[91]

Singapore

Simplified diagram of a water supply network

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See also: Water supply and sanitation in Singapore

Singapore is a significant importer of water from neighbouring Malaysia but also has made great efforts to reclaim as much used water as possible to ensure adequate provision for the very crowded city-state. Their reclaimed water is marketed as NEWater. Singapore updated its water quality regulation in 2019, setting standards consistent with the WHO recommended standards. Monitoring is undertaken by the Environmental Public Health Department of the Singaporean Government^[92]

United Kingdom

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See also: Water supply and sanitation in the United Kingdom

In the United Kingdom regulation of water supplies is a devolved matter to the Welsh and Scottish Parliaments and the Northern Ireland Assembly.

In England and Wales there are two water industry regulatory authorities.
- Water Services Regulation Authority (Ofwat) is the economic regulator of the water sector; it protects the interests of consumers by promoting effective competition and ensuring that water companies carry out their statutory functions. Ofwat has a management board comprising a chairman, Chief Executive and Executive and Non-Executive members. There is a staff of about 240.^[93]
- The Drinking Water Inspectorate (DWI) provides independent assurance that the privatised water industry delivers safe, clean drinking water to consumers. The DWI was established in 1990 and comprises a Chief Inspector of Drinking Water and a team of about 40 people.^[94] The current standards of water quality are defined in Statutory Instrument 2016 No. 614 the Water Supply (Water Quality) Regulations 2016.^[95]
The functions and duties of the bodies are formally defined in the Water Industry Act 1991 (1991 c. 56) as amended by the Water Act 2003 (2003 c. 37) and the Water Act 2014 (2014 c. 21).^[96]

In Scotland water quality is the responsibility of independent Drinking Water Quality Regulator (DWQR).^[97]

In Northern Ireland the Drinking Water Inspectorate (DWI) regulates drinking water quality of public and private supplies.^[98] The current standards of water quality are defined in the Water Supply (Water Quality) Regulations (Northern Ireland) 2017.^[99]

United States

[edit]

Further information: Water supply and sanitation in the United States

This section is an excerpt from Drinking water quality in the United States.[edit]

Drinking water quality in the United States is generally safe. In 2016, over 90 percent of the nation’s community water systems were in compliance with all published U.S. Environmental Protection Agency (US EPA) standards.^[100] Over 286 million Americans get their tap water from a community water system. Eight percent of the community water systems—large municipal water systems—provide water to 82 percent of the US population.^[101] The Safe Drinking Water Act requires the US EPA to set standards for drinking water quality in public water systems (entities that provide water for human consumption to at least 25 people for at least 60 days a year).^[102] Enforcement of the standards is mostly carried out by state health agencies.^[103] States may set standards that are more stringent than the federal standards.^[104]

Despite improvements in water quality regulations, disparities in access to clean drinking water persist in marginalized communities. A 2017 study by the Natural Resources Defense Council (NRDC) highlighted that rural areas and low-income neighborhoods are disproportionately affected by water contamination, often due to aging infrastructure and inadequate funding for water systems.^[105] These inequities underscore the need for more targeted investment and stronger enforcement of the Safe Drinking Water Act in vulnerable regions.Drinking water quality in the U.S. is regulated by state and federal laws and codes, which set maximum contaminant levels (MCLs) and Treatment Technique requirements for some pollutants and naturally occurring constituents, determine various operational requirements, require public notification for violation of standards, provide guidance to state primacy agencies, and require utilities to publish Consumer Confidence Reports.^[106]

History

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Main article: History of water supply and sanitation

In drinking water access, quality and quantity are both important parameters but the quantity is often prioritized.^[48] Throughout human history, water quality has been a constant and ongoing challenge. Certain crises have led to major changes in knowledge, policy, and regulatory structures. The drivers of change can vary: the cholera epidemic in the 1850s in London led John Snow to further our understanding of waterborne diseases. However, London’s sanitary revolution was driven by political motivations and social priorities before the science was accepted.^[48]
March 13, 2025