There is no less water on Earth than there was a thousand years ago.
And yet some populations do not have enough of it. This page explains why both statements are true at once.
Water is neither made nor destroyed: it circulates
Water evaporates from the oceans, lakes, soils and vegetation. It rises, cools, condenses and falls back as rain or snow. It runs off, seeps into the ground, joins rivers and streams, then returns to the sea.1
This cycle is perpetual and massive. Considerable volumes are constantly on the move through the atmosphere and the lithosphere.
The total quantity of water on Earth remains almost constant on a human timescale.2
A flow is not a stock
This conceptual distinction is fundamental:
- A flow refers to what circulates: a rate of flow, a movement, an amount produced per unit of time.
- A stock refers to what is present: a reserve, a total mass, a volume held in place.3
The water cycle is a colossal flow. The total quantity of water on Earth forms a stable stock.
These two notions do not contradict each other: they describe two distinct dimensions of the same system.
The bridge between the two: residence time
One quantity links flow and stock, and it is easy to grasp: how long does a water molecule stay in a reservoir before leaving it?4
Hydrologists call it the residence time (or turnover time). It is obtained by dividing the stock by the flow.5
The differences are staggering:
- In the atmosphere, the stock is tiny and the flow enormous. A molecule stays there for only a few days.
- In the ocean, the stock is immense and the exchanges slow. A molecule stays there on average for more than two thousand years.6
That is why the rain can change from one week to the next while the ocean does not budge. A single cycle contains reservoirs that respond at completely different speeds.
The formula is worth remembering: residence time = stock ÷ flow. It applies to water, but also to carbon, to the money in a budget, to the pupils in a school.
What does “being short of water” mean?
A shortage never results from a decrease in the planet’s overall volume of water.
Being short of water means not having enough of it:
- In the right place (geographical distribution);
- At the right time (seasonal variability);
- Of the required quality (fitness for drinking, salinity);
- Technically and financially accessible.7
If just one of these four conditions is missing, there is a shortage.
- An aquifer may be present more than a hundred metres down without being exploitable at reasonable cost.
- A region may endure torrential rainfall for two months, then go through ten months of drought.
- A mighty river may carry water unfit for human consumption without heavy treatment.8
Water shortage is a problem of distribution in space and time, of quality and of access, and not a question of overall volume.
“We drink the same water as the dinosaurs”: analysis of a popular saying
This claim is very widespread. It is instructive, but inaccurate if taken literally.
What is accurate
Water has been circulating in a closed loop for billions of years. There is no massive or continuous extraterrestrial input creating “new” water. On a global scale, the mass of water present on Earth is stable.
What is inaccurate
The saying suggests that a water molecule (H₂O) is an indestructible object passing through the geological eras without alteration.
That is false. Water molecules are continually broken apart and re-formed:
- In living things: photosynthesis splits water molecules, while cellular respiration synthesises new ones.
- In the Earth’s crust: water is incorporated into mineral structures through hydration and released from them through metamorphic dehydration.
- In the upper atmosphere: ultraviolet radiation breaks molecular bonds through photolysis.
The lesson
What persists through the ages is not the matter itself, but the cycle that organises it. Water is a dynamic flow whose structure is maintained through constant breakdown and recombination.
Water management and the history of technology
Irrigation without drainage leads to the salinisation of land: irrigation water contains dissolved salts which, as the water evaporates, become concentrated in the soil until it is rendered sterile.9
Ancient societies developed suitable techniques to prevent this. Historians have sometimes attributed these systems to outside or colonial interventions without any documentary basis. Critical analysis of the sources requires distinguishing between what is historically proven from what is mere conjecture.
Definition as the basis of measurement: the example of rivers
Determining the longest river in the world (Amazon or Nile) depends on the criteria chosen to define the source and the main course:
- The longest course?
- The greatest discharge?
- The branch that flows all year round?10
Changing the defining criterion changes the ranking. The measurement depends directly on the convention adopted.
Note 1
Driven mainly by solar heating, water is evaporated from ocean and land surfaces, transported by winds, and condensed to form clouds and precipitation that falls to land and oceans.
Trenberth, Kevin E., Smith, Lesley, Qian, Taotao, Dai, Aiguo and Fasullo, John, “Estimates of the Global Water Budget and Its Annual Cycle Using Observational and Model Data”, Journal of Hydrometeorology, vol. 8, 2007, p. 758-769
Notice how of the world's total water supply of about 332.5 million cubic miles of water, over 96 percent is saline. And, of the total freshwater, over 68 percent is locked up in ice and glaciers. Another 30 percent of freshwater is in the ground.
United States Geological Survey (USGS), Water Science School, “Where is Earth’s Water?”
Note 2
The total amount of water present on the Earth is fixed and does not change. Powered by the Sun, water is continually being circulated between the oceans, the atmosphere and the land. This circulation and conservation of the Earth's water, known as the water cycle, is a crucial component of our weather and climate.
Agence spatiale européenne, “Water Cycle”, image and caption, 2004
Note 3
In Earth systems science, the term reservoir is used for a distinctive kind of place where a certain kind of material is stored, or resides, for some period of time. […] Material moves into and out of reservoirs. The rate at which a given material moves between reservoirs is called a flux.
Southard, John, “1.3: Systems, Cycles, Reservoirs, and Fluxes”, BioGeoChemistry, Geosciences LibreTexts, after MIT OpenCourseWare, course 12.090 The Environment of the Earth’s Surface, spring 2007, CC BY-NC-SA licence
Note 4
Overall, the e-folding residence time of atmospheric moisture is just over 8 days.
Trenberth, Kevin E., “Atmospheric Moisture Residence Times and Cycling: Implications for Rainfall Rates and Climate Change”, Climatic Change, vol. 39, 1998, p. 667-694
Note 5
The residence time of a water molecule in the atmosphere (or any of the other reservoirs) can be estimated by dividing the amount that is there by the rate at which it is transferred in and out.
Earle, Steven, Physical Geology, 2e éd., BCcampus, 2019, section 13.1 “The Hydrological Cycle” et annexe 3, open access
Note 6
Overall, the e-folding residence time of atmospheric moisture is just over 8 days.
Trenberth, Kevin E., “Atmospheric Moisture Residence Times and Cycling: Implications for Rainfall Rates and Climate Change”, Climatic Change, vol. 39, 1998, p. 667-694
1,338,000,000/1,580 = 846,835 days average residence time for water in the ocean (or 2320 years)
Earle, Steven, Physical Geology, 2e éd., BCcampus, 2019, section 13.1 “The Hydrological Cycle” et annexe 3, open access
Note 7
Drinking water from an improved water source that is accessible on premises, available when needed and free from faecal and priority chemical contamination […] 'By 2030, achieve universal and equitable access to safe and affordable drinking water for all'
OMS and UNICEF, Progress on Household Drinking Water, Sanitation and Hygiene, Joint Monitoring Programme, Genève
But as water quality issues are prevalent worldwide, we need to rethink the concept of water scarcity to include also the quality of freshwater resources available for different water use sectors and ecosystems. […] By contrast, for irrigation and domestic applications, salinity, nutrient and other pollutant levels pose more important constraints on usability.
Wageningen University & Research, “Rethinking Water Scarcity: Quality Matters”, actualité du 10 October 2017 sur l’article de Michelle van Vliet et al. paru dans Nature Geoscience, open access
Note 8
Depth to groundwater is another important factor controlling accessibility and cost of accessing groundwater (figure 3). Water levels deeper than 50 m will not be able to be accessed easily by a hand pump. At depths >100 m the cost of borehole drilling increases significantly due to the requirement for more sophisticated drilling equipment.
MacDonald, Alan M., Bonsor, Helen C., Ó Dochartaigh, Brighid É. and Taylor, Richard G., “Quantitative Maps of Groundwater Resources in Africa”, Environmental Research Letters, vol. 7, no. 2, 2012, art. 024009
Note 9
Secondary salinization occurs when significant amounts of water are provided by irrigation, with no adequate provision of drainage for the leaching and removal of salts, resulting in the soils becoming salty and unproductive.
CISEAU, IPTRID and AGLL (FAO), Management of Irrigation-Induced Salt-Affected Soils, brochure, FAO, n.d.
Note 10
But whether geographers accept the claim of Mantaro as the Amazon's true origin depends on which definition of a river's source they choose to apply: Is it the farthest point upstream that provides the largest volume of water, or is it the most distant point up the longest tributary in the river's drainage basin? […] The current internationally accepted definition is the most distant point of a river's longest tributary that flows continuously.
Lee, Jane J., “Where Does the Amazon River Begin?”, National Geographic, 15 February 2014, open access
Notes and sources
- ↑
Passage Water evaporates from the oceans, lakes, soils and vegetation. It rises, cools, condenses and falls back
Driven mainly by solar heating, water is evaporated from ocean and land surfaces, transported by winds, and condensed to form clouds and precipitation that falls to land and oceans.
Source Trenberth, Kevin E., Smith, Lesley, Qian, Taotao, Dai, Aiguo and Fasullo, John, “Estimates of the Global Water Budget and Its Annual Cycle Using Observational and Model Data”, Journal of Hydrometeorology, vol. 8, 2007, p. 758-769, section 1 (“Introduction”), p. 758.
Open access open the source · checked on 9 September 2026
Passage the global volume of the oceans and the distribution of freshwater reserves
Notice how of the world's total water supply of about 332.5 million cubic miles of water, over 96 percent is saline. And, of the total freshwater, over 68 percent is locked up in ice and glaciers. Another 30 percent of freshwater is in the ground.
Source United States Geological Survey (USGS), Water Science School, “Where is Earth’s Water?”, body of the page, paragraph beginning “Notice how of the world’s total water supply”; figures from the table “One estimate of global water distribution” (oceans: 1,338,000,000 km³), after Shiklomanov 1993.
Open access open the source · checked on 27 October 2026
- ↑
Passage The total quantity of water on Earth remains almost constant on a human timescale.
The total amount of water present on the Earth is fixed and does not change. Powered by the Sun, water is continually being circulated between the oceans, the atmosphere and the land. This circulation and conservation of the Earth's water, known as the water cycle, is a crucial component of our weather and climate.
Source Agence spatiale européenne, “Water Cycle”, image and caption, 2004, image caption, 1st paragraph.
Open access open the source · checked on 27 October 2026
- ↑
Passage A flow refers to what circulates […] A stock refers to what is present
In Earth systems science, the term reservoir is used for a distinctive kind of place where a certain kind of material is stored, or resides, for some period of time. […] Material moves into and out of reservoirs. The rate at which a given material moves between reservoirs is called a flux.
Source Southard, John, “1.3: Systems, Cycles, Reservoirs, and Fluxes”, BioGeoChemistry, Geosciences LibreTexts, after MIT OpenCourseWare, course 12.090 The Environment of the Earth’s Surface, spring 2007, CC BY-NC-SA licence, section 1.3, 5th and 6th paragraphs.
Open access open the source · checked on 27 October 2026
- ↑
Passage how long does a water molecule stay in a reservoir before leaving it?
Overall, the e-folding residence time of atmospheric moisture is just over 8 days.
Source Trenberth, Kevin E., “Atmospheric Moisture Residence Times and Cycling: Implications for Rainfall Rates and Climate Change”, Climatic Change, vol. 39, 1998, p. 667-694, abstract.
Paywalled open the source · checked on 25 October 2026
- ↑
Passage It is obtained by dividing the stock by the flow.
The residence time of a water molecule in the atmosphere (or any of the other reservoirs) can be estimated by dividing the amount that is there by the rate at which it is transferred in and out.
Source Earle, Steven, Physical Geology, 2e éd., BCcampus, 2019, section 13.1 “The Hydrological Cycle” et annexe 3, open access, section 13.1 “The Hydrological Cycle”, paragraph on residence time.
Open access open the source · checked on 24 October 2026
- ↑
Passage A molecule stays there for only a few days.
Overall, the e-folding residence time of atmospheric moisture is just over 8 days.
Source Trenberth, Kevin E., “Atmospheric Moisture Residence Times and Cycling: Implications for Rainfall Rates and Climate Change”, Climatic Change, vol. 39, 1998, p. 667-694, abstract.
Paywalled open the source · checked on 9 September 2026
Passage In the ocean […] A molecule stays there on average for more than two thousand years.
1,338,000,000/1,580 = 846,835 days average residence time for water in the ocean (or 2320 years)
Source Earle, Steven, Physical Geology, 2e éd., BCcampus, 2019, section 13.1 “The Hydrological Cycle” et annexe 3, open access, appendix 3, answer to exercise 13.1 (the question, section 13.1: “The volume of the oceans is 1,338,000,000 km3 and the flux rate is approximately the same (1,580 km3 per day).”).
Open access open the source · checked on 24 October 2026
- ↑
Passage Of the required quality […] Technically and financially accessible
Drinking water from an improved water source that is accessible on premises, available when needed and free from faecal and priority chemical contamination […] 'By 2030, achieve universal and equitable access to safe and affordable drinking water for all'
Source OMS and UNICEF, Progress on Household Drinking Water, Sanitation and Hygiene, Joint Monitoring Programme, Genève, drinking water service ladder (“Drinking water ladder”), definition of “Safely managed”; section “Monitoring SDG targets related to drinking water” (SDG target 6.1).
Open access · checked on 24 October 2026
Passage Of the required quality (fitness for drinking, salinity)
But as water quality issues are prevalent worldwide, we need to rethink the concept of water scarcity to include also the quality of freshwater resources available for different water use sectors and ecosystems. […] By contrast, for irrigation and domestic applications, salinity, nutrient and other pollutant levels pose more important constraints on usability.
Source Wageningen University & Research, “Rethinking Water Scarcity: Quality Matters”, actualité du 10 October 2017 sur l’article de Michelle van Vliet et al. paru dans Nature Geoscience, open access, introduction to the news item; remarks by Michelle van Vliet.
Open access open the source · checked on 25 October 2026
- ↑
Passage An aquifer may be present more than a hundred metres down without being exploitable at reasonable cost.
Depth to groundwater is another important factor controlling accessibility and cost of accessing groundwater (figure 3). Water levels deeper than 50 m will not be able to be accessed easily by a hand pump. At depths >100 m the cost of borehole drilling increases significantly due to the requirement for more sophisticated drilling equipment.
Source MacDonald, Alan M., Bonsor, Helen C., Ó Dochartaigh, Brighid É. and Taylor, Richard G., “Quantitative Maps of Groundwater Resources in Africa”, Environmental Research Letters, vol. 7, no. 2, 2012, art. 024009, § 3.2 (“Aquifer productivity”), paragraph on depth to groundwater (figure 3).
Open access open the source · checked on 27 October 2026
- ↑
Passage Irrigation without drainage leads to the salinisation of land
Secondary salinization occurs when significant amounts of water are provided by irrigation, with no adequate provision of drainage for the leaching and removal of salts, resulting in the soils becoming salty and unproductive.
Source CISEAU, IPTRID and AGLL (FAO), Management of Irrigation-Induced Salt-Affected Soils, brochure, FAO, n.d., “Origins of irrigation-induced salt-affected soils” section.
Open access open the source · checked on 27 October 2026
- ↑
Passage The longest course? […] The greatest discharge? […] The branch that flows all year round?
But whether geographers accept the claim of Mantaro as the Amazon's true origin depends on which definition of a river's source they choose to apply: Is it the farthest point upstream that provides the largest volume of water, or is it the most distant point up the longest tributary in the river's drainage basin? […] The current internationally accepted definition is the most distant point of a river's longest tributary that flows continuously.
Source Lee, Jane J., “Where Does the Amazon River Begin?”, National Geographic, 15 February 2014, open access, two passages from the article (their order on the page has not been checked).
Open access open the source · checked on 24 October 2026
Bibliography
15
- Agence spatiale européenne, “Water Cycle”, image and caption, 2004, open access (the total amount of water on Earth, fixed; its circulation between oceans, atmosphere and land). https://www.esa.int/ESA_Multimedia/Images/2007/12/Water_Cycle
- CISEAU, IPTRID and AGLL (FAO), Management of Irrigation-Induced Salt-Affected Soils, brochure, FAO, n.d., open access (the salinisation of soils irrigated without adequate drainage). https://www.fao.org/fileadmin/templates/cpesap/C-RESAP_Info_package/Links/Module_4/salinity_brochure_eng.pdf
- Earle, Steven, Physical Geology, 2e éd., BCcampus, 2019, section 13.1 “The Hydrological Cycle” et annexe 3, open access (residence time, calculated by dividing the stock by the flow: just over eight days in the atmosphere, about 2,300 years on average in the ocean). https://opentextbc.ca/physicalgeology2ed/chapter/13-1-the-hydrological-cycle/
- Falkowski, Paul, Scholes, R. J. et al., “The Global Carbon Cycle: A Test of Our Knowledge of Earth as a System”, Science, vol. 290, 2000, p. 291-296, paywalled (the flow/stock analogy applied to the major cycles).
- Jacobsen, Thorkild and Adams, Robert M., “Salt and Silt in Ancient Mesopotamian Agriculture”, Science, vol. 128, 1958, p. 1251-1258, paywalled (salinisation through irrigation; see Entry 2.9).
- Konikow, Leonard F., “Long-Term Groundwater Depletion in the United States”, Groundwater, vol. 53, 2015, p. 2-9, paywalled (the irreversible depletion, on a human timescale, of fossil aquifers).
- Lee, Jane J., “Where Does the Amazon River Begin?”, National Geographic, 15 February 2014, open access (the source of a river, and therefore its length, changes according to the definition chosen: the point that supplies the largest volume of water, or the most distant point on the longest tributary that flows all year round). https://www.nationalgeographic.com/adventure/article/140213-amazon-river-length-source-maps-science
- MacDonald, Alan M., Bonsor, Helen C., Ó Dochartaigh, Brighid É. and Taylor, Richard G., “Quantitative Maps of Groundwater Resources in Africa”, Environmental Research Letters, vol. 7, no. 2, 2012, art. 024009, open access (depth to groundwater as a factor of access and cost: beyond 50 m, no hand pump; beyond 100 m, much costlier drilling; the large deep aquifers of northern Africa, costly to develop). https://doi.org/10.1088/1748-9326/7/2/024009
- OMS and UNICEF, Progress on Household Drinking Water, Sanitation and Hygiene, Joint Monitoring Programme, Genève, open access (access to drinking water; see Entry 3.14).
- Shiklomanov, Igor A., “World Fresh Water Resources”, dans Gleick, Peter H. (dir.), Water in Crisis, Oxford University Press, 1993, in libraries (the global volume of the oceans and the distribution of freshwater reserves).
- Southard, John, “1.3: Systems, Cycles, Reservoirs, and Fluxes”, BioGeoChemistry, Geosciences LibreTexts, after MIT OpenCourseWare, course 12.090 The Environment of the Earth’s Surface, spring 2007, CC BY-NC-SA licence, open access (the definitions of a reservoir, where material is stored, and of a flux, the rate at which it moves). https://geo.libretexts.org/Bookshelves/Geography_(Physical)/BioGeoChemistry_(LibreTexts)/01%3A_Introduction/1.03%3A_Systems_Cycles_Reservoirs_and_Fluxes
- Trenberth, Kevin E., “Atmospheric Moisture Residence Times and Cycling: Implications for Rainfall Rates and Climate Change”, Climatic Change, vol. 39, 1998, p. 667-694, paywalled (the residence time of water vapour in the atmosphere, of the order of a few days). https://doi.org/10.1023/A:1005319109110
- Trenberth, Kevin E., Smith, Lesley, Qian, Taotao, Dai, Aiguo and Fasullo, John, “Estimates of the Global Water Budget and Its Annual Cycle Using Observational and Model Data”, Journal of Hydrometeorology, vol. 8, 2007, p. 758-769, open access (the annual fluxes of evaporation and precipitation, and estimates of the main reservoirs). https://journals.ametsoc.org/view/journals/hydr/8/4/jhm600_1.xml
- United States Geological Survey (USGS), Water Science School, “Where is Earth’s Water?”, open access (the volume of the oceans and the distribution of fresh water, after Shiklomanov’s 1993 tables). https://www.usgs.gov/special-topics/water-science-school/science/where-earths-water
- Wageningen University & Research, “Rethinking Water Scarcity: Quality Matters”, actualité du 10 October 2017 sur l’article de Michelle van Vliet et al. paru dans Nature Geoscience, open access (water shortage also depends on its quality; salinity limits the use of water for irrigation and domestic purposes). https://www.wur.nl/nl/nieuws/Rethinking-water-scarcity-quality-matters.htm
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