4.1 Water Systems

In 1960, the Aral Sea, on the border of Kazakhstan and Uzbekistan, was the fourth-largest lake on Earth. Soviet planners diverted its two feeder rivers to irrigate cotton fields in the desert.

By the 2010s, roughly how much of the Aral Sea's original volume had been lost?

4.1.1

What drives water's movement

Movements of water in the hydrosphere are driven by two forces. Solar radiation provides the heat needed for evaporation, and heat is released again when water vapor condenses. Gravity causes water to drain through soil and flow in rivers toward the sea.

4.1.2

The hydrological cycle as a system

The global hydrological cycle operates as a system with stores and flows, the same systems language from Topic 1. In a water cycle diagram, stores are drawn as boxes and flows as arrows between them.

Illustrated diagram of the hydrological cycle showing evaporation, transpiration and condensation over land and water, transport of water vapor by wind, precipitation over mountains, snowmelt and surface run-off, infiltration into groundwater, and groundwater flow back toward the ocean. — The hydrological cycle, showing every store (ocean, atmosphere, groundwater, ice, living things) connected by named flows. The rest of this page works through each flow individually.

4.1.3

Where Earth's water actually is

The main stores in the hydrological cycle, and their approximate share of all water on Earth, are the oceans (96.5%), glaciers and ice caps (1.7%), groundwater (1.7%), surface freshwater (0.02%), the atmosphere (0.001%) and living organisms (0.0001%). These percentages are approximate, you do not need to memorize them exactly, but you should have a clear sense of the relative proportions: almost all of Earth's water is ocean water, and everything else, ice, groundwater, rivers, lakes, the atmosphere, every living thing, fits into the remaining 3.5%. Within that 3.5%, nearly all of it is locked up as ice or groundwater, not sitting in the rivers and lakes people actually draw from: surface freshwater, the atmosphere and every living organism combined add up to under 1% of that already-small slice.

⚠Common misconception

Students often picture most of Earth's freshwater sitting in the rivers and lakes they can actually see and draw from. It does not. Surface freshwater is only about 0.02% of all water on Earth, dwarfed by the 1.7% locked in ice caps and glaciers and the 1.7% held underground. The freshwater people and ecosystems rely on day to day is a tiny, uneven slice of an already-small 3.5% non-ocean share, which is exactly why groundwater depletion and glacial melt threaten supply even though "there is so much water on Earth."

4.1.4

Flows in the hydrological cycle

Water moves between stores through named flows: transpiration, sublimation, evaporation, condensation, advection, precipitation, melting, freezing, surface run-off, infiltration, percolation, streamflow and groundwater flow. Sublimation is ice transforming directly into water vapor. Advection is the wind-blown movement of water vapor or condensed and frozen water droplets, clouds. Infiltration is water entering the soil. Percolation is water moving within the soil.

Every one of these flows is either a transformation, a change of state (liquid, solid or vapor), or a transfer, water moving location without changing state. That distinction is exactly what the application-of-skills activity for this statement, in the Skills tab, asks you to practice: build and use a systems diagram of the hydrological cycle showing its transfers and transformations.

IB link

4.1.2 and 4.1.4's named application of skills is creating and using a systems diagram showing the transfers and transformations of the hydrological cycle. The Skills tab has the real practice activity for this.

4.1.5

Human activity alters flows and stores

Human activities such as agriculture, deforestation and urbanization can alter the flows and stores of the hydrological cycle. A change in land use, deforestation and urbanization typically reduce evapotranspiration while increasing surface run-off, which can produce flash floods downstream.

4.1.6

Steady state and sustainable withdrawal

The steady state of any water body, its inputs balancing its outputs, can be demonstrated with a flow diagram, and that same diagram can be used to calculate a sustainable rate of harvesting from a lake or aquifer: withdraw no faster than the store is naturally replenished, and the store stays in balance. Withdraw faster, and it depletes, exactly what happened to the Aral Sea.

🔗Linking across the ESS course

This same logic, a store stays stable only while withdrawal does not outpace replenishment, reappears as the definition of a renewable resource in Topic 7. A resource used faster than its natural regeneration rate stops behaving as renewable and starts being depleted like a finite store, whether that store is an aquifer here or a fish stock or forest there. The Aral Sea is the water-systems version of the same overexploitation pattern Topic 7 treats as its central sustainability test.

The Aral Sea: a store pushed out of steady state

1960 surface area: about 68,000 km², making it the world's fourth-largest lake
Today's combined surface area: around 8,000 km², a loss of roughly 88% of its area and over 90% of its volume
Cause: Soviet-era diversion of the Amu Darya and Syr Darya rivers, its main inflows, for cotton irrigation starting in the 1960s, while evaporation continued unchanged
Split into a North (Small) and South (Large) Aral Sea by the late 1980s
Since Kazakhstan completed the Kok-Aral Dam in 2005, the North Aral Sea has partly recovered: rising volume, falling salinity, and a fishing industry landing thousands of tons a year again. The South Aral Sea, without a comparable dam, remains largely a desiccated desert, the Aralkum

Figures: Astana Times, 2025; Global Voices, 2026; peer-reviewed hydrology literature on the Aral Sea basin.

Try it: set an inflow and outflow rate below and watch what happens to a water body's stored volume over 60 years. The three presets loosely track the Aral Sea's own three real phases, illustrative units, not exact historical flow data.

Inflow, relative units/year50

Outflow, relative units/year50

Net balance0 / year

Store statusStable

Sustainable withdrawal means outflow does not exceed inflow.

Sources: this tab

Global water store percentages: as specified in the IB ESS guide.
Hydrological cycle diagram: NASA/JPL-Caltech, Atmospheric Infrared Sounder, via Flickr (CC BY 2.0).
Aral Sea area, volume loss, timeline and North Aral Sea recovery: Astana Times, 2025; Global Voices, 2026; peer-reviewed hydrology literature on the Aral Sea basin.
IB Environmental Systems and Societies Guide, first assessment 2026, Topic 4, Subtopic 4.1, statements 4.1.1-4.1.6.

4.1.2 & 4.1.4 · skill

Build a systems diagram of the hydrological cycle

A hydrological cycle systems diagram draws stores as boxes and flows as arrows between them, exactly like the energy and matter diagrams from Topic 1. The named application of skills for these two statements is showing the cycle's transfers (movement, no change of state) and transformations (a change of state: liquid, solid or vapor) correctly. Tap a flow below, then tap whether it is a transfer or a transformation.

Transformation, a change of state

Transfer, a change of location

🛠Application of skills

On paper or in an exam, draw the six stores as labeled boxes (oceans, ice caps and glaciers, groundwater, surface freshwater, atmosphere, organisms), then connect them with labeled arrows using the 13 flow names above. An arrow's label tells you whether that connection is a transfer or a transformation, which is exactly what you just practiced.

Glossary

Every term introduced in this subtopic. Terms marked HL are only required at Higher Level.

Hydrosphere: All the water on and near Earth's surface, in every state, liquid, solid and vapor.
Hydrological cycle: The continuous movement of water between stores, driven by solar radiation and gravity.
Store: A location where water accumulates within the hydrological cycle, drawn as a box in a systems diagram; for example, oceans, ice caps, or groundwater.
Flow: A movement of water between stores, drawn as an arrow in a systems diagram; either a transfer or a transformation.
Transfer: A flow in which water changes location but not state, for example surface run-off or infiltration.
Transformation: A flow in which water changes state, liquid, solid or vapor, for example evaporation or freezing.
Evaporation: Liquid water turning into water vapor, requiring heat energy.
Transpiration: Water vapor released from plants, mainly through their leaves.
Sublimation: Ice transforming directly into water vapor, without passing through a liquid state.
Condensation: Water vapor turning into liquid droplets, releasing heat energy.
Advection: The wind-blown horizontal movement of water vapor or condensed and frozen water droplets, clouds.
Precipitation: Water falling from the atmosphere to the surface as rain, snow, sleet or hail.
Infiltration: Water entering the soil from the surface.
Percolation: Water moving downward or sideways within the soil.
Surface run-off: Water flowing across the land surface, rather than soaking in.
Streamflow: Water moving through a river or stream channel.
Groundwater flow: Water moving underground through saturated rock or soil.
Steady state: A condition in which a store's inputs balance its outputs, so its size stays roughly constant over time.
Sustainable yield: A withdrawal rate from a store, such as a lake or aquifer, that does not exceed the rate at which the store is naturally replenished.
Cohesion HL: The attraction between water molecules, caused by hydrogen bonding.
Adhesion HL: The attraction between water molecules and other substances.
Specific heat capacity HL: The energy needed to raise the temperature of a substance by one degree; water's is unusually high, which buffers large bodies of water against rapid temperature swings.
Density anomaly HL: Water is at its densest at about 4°C, so ice floats and water bodies freeze from the surface downward, insulating life below.
Carbon sink HL: A store that absorbs more carbon than it releases; the oceans are a major carbon sink, absorbing atmospheric carbon dioxide.
Ocean acidification HL: A fall in ocean pH caused by dissolved carbon dioxide reacting with seawater; surface ocean pH has dropped by about 0.1 units since pre-industrial times.
Stratification HL: The formation of distinct layers in a body of water, driven by differences in temperature and salinity that restrict mixing between layers.
Thermocline HL: The transition layer in a stratified water body between the warmer mixed surface layer and the colder water below.
Upwelling HL: The vertical movement of cold, nutrient-rich water from depth to the surface, often driven by wind displacing surface water.
Salinity HL: The concentration of dissolved salts in water; along with temperature, one of the two properties that drive thermohaline circulation.
Thermohaline circulation HL: A global ocean circulation system driven by differences in temperature and salinity, which affect water density; includes the North Atlantic conveyor belt.
AMOC HL: The Atlantic Meridional Overturning Circulation, the North Atlantic branch of thermohaline circulation, currently the subject of active research into whether and how fast it may weaken.

Test Yourself is coming soon for this subtopic

Practice questions and markschemes for this page are still being written. Check back once they have been added.

4.1.7

Water's unique properties

Water has unique physical and chemical properties that support and sustain life. These include polarity, which produces cohesion (attraction between water molecules from hydrogen bonding) and adhesion (attraction between water and other substances), solvent properties, transparency, a high specific heat capacity, density that varies with temperature, and gas solubility that varies with temperature and pressure.

Practice: match each property to the consequence it produces.

Water rises up through a plant's narrow xylem vessels, against gravity, without any pump.

A coastal city's daily temperature swings far less than a city at the same latitude deep inland.

A lake freezes over on the surface in winter, but fish and other organisms survive in liquid water underneath the ice all season.

Water's solvent properties matter just as much: dissolved nutrients and gases, oxygen and carbon dioxide included, only become available to aquatic organisms because water dissolves them, and both gases dissolve more readily in colder water than warmer water.

4.1.8

The oceans as a carbon sink

The oceans act as a carbon sink, absorbing carbon dioxide from the atmosphere and sequestering it. This has moderated the rise in atmospheric carbon dioxide from burning fossil fuels, though a saturation point may eventually be reached.

4.1.9

Two very different timescales

Carbon absorbed by the oceans has two very different fates. Over the short term, carbon dissolves as carbon dioxide, and this is what causes ocean acidification. Over the much longer term, carbon is taken up into living organisms as biomass, which can accumulate on the seabed as sediment, containing inorganic carbonates and undecomposed organic matter. Over millions of years, these sediments can become fossil fuels.

Short term: dissolved carbon dioxide causes ocean acidification

Timescale: years to decades

MechanismDissolved CO₂ reacts with seawater to form carbonic acid, lowering pH

Measured changeSurface ocean pH has fallen about 0.1 units since pre-industrial times, from roughly 8.2 to 8.1, a roughly 30% rise in acidity on the underlying logarithmic scale

Don't mix these up

Acidification is the ocean absorbing carbon faster than life and geology can lock it away. Sediment and fossil fuel formation is what eventually locks that same carbon away, just on a timescale of millions of years, far too slow to offset current emissions.

4.1.10

Temperature and density with depth

Water temperature varies with depth: cold water sits below, warmer water above. Differences in density between these layers restrict mixing, producing persistent stratification.

4.1.11

Where stratification occurs

Stratification occurs in deeper lakes, coastal areas, enclosed seas and the open ocean. A thermocline forms the transition layer between the warmer mixed layer at the surface and the colder water below. The warm and cold layers also differ in their dissolved oxygen and mineral nutrient concentrations.

Three line graphs of ocean depth (0-5000 m) against density, salinity and temperature, each showing a sharp transition zone in the upper few hundred meters: the pycnocline, halocline and thermocline respectively, with much more gradual change below. — Ocean depth profiles for density, salinity and temperature. Each property changes sharply in the same shallow band, the pycnocline, halocline and thermocline, then barely at all through the much larger volume of water beneath it.

In temperate lakes, this layering is seasonal rather than constant: in spring and fall, surface and deep water reach a similar temperature and density, the water column mixes freely from top to bottom (turnover), and stratification temporarily disappears. Summer stratification then re-forms as the sun warms the surface layer, and in winter, a lake can stratify again in reverse, with the coldest water (below 4°C, or ice) sitting on top of slightly warmer, denser water beneath.

Diagram of a temperate lake's seasonal thermal stratification cycle across four panels: spring, an isothermal 4 degree Celsius water column mixing freely; summer, a warm 20 degree Celsius surface layer sitting above a sharp thermocline over cold 4 degree Celsius deep water; fall, the water column mixing freely again at 4 degrees; and winter, a thin layer of ice and 1 degree Celsius water over a thermocline above 4 degree Celsius deep water. — A temperate lake's annual stratification cycle. Spring and fall turnover mix the whole water column; summer and winter each stratify, in opposite directions, around the same 4°C density maximum.

4.1.12

Stronger stratification under warming

Global warming and changing salinity have increased the intensity of ocean stratification, most pronounced in the upper 200 meters. Temperature increases have had global effects, and melting ice caps have reduced salinity in the Antarctic specifically.

IB link

4.1.12's named application of skills: extract data from a database and analyze water temperature alongside oxygen and salinity concentrations using an appropriate statistical test. In an exam, expect a real dataset and a request for a specific test, such as correlation, not just a verbal description of the trend.

4.1.13

Upwelling

Upwellings in oceans and freshwater bodies bring cold, nutrient-rich water up from depth to the surface, the mass vertical movement that happens when wind displaces surface water out of the way. Seasonal upwelling cycles occur in stratified lakes and are also associated with El Niño-Southern Oscillation (ENSO) events.

The Humboldt Current: upwelling at global scale

The Humboldt (Peru) Current, off the west coast of South America, covers roughly 0.1% of the world's ocean surface
Its upwelling supports about 10% of the world's entire fish catch from that 0.1% of ocean area, and its anchoveta fishery was the world's largest single-species fishery by catch weight in 2022, at roughly 4.9 million tonnes
El Niño events weaken the trade winds that drive the upwelling, deepening the thermocline and cutting off the nutrient supply, which has repeatedly collapsed anchoveta landings, most notably during the severe 1997–98 El Niño, when biomass fell from about 5.8 to 1.2 million tonnes

Figures: Chavez et al., "The northern Humboldt Current System," Progress in Oceanography, 2008; FAO, "The State of World Fisheries and Aquaculture," 2024; Fréon et al., "Interdecadal variability of anchoveta abundance," Progress in Oceanography, 2008.

4.1.14

Thermohaline circulation and the North Atlantic conveyor belt

Thermohaline circulation systems are driven by differences in temperature and salinity, which produce differences in water density. In the North Atlantic: rivers and melting ice caps add low-salinity, low-density water. Wind-driven currents carry cool surface water from the equator toward the North Atlantic, losing freshwater to evaporation along the way and becoming steadily more saline. That increased salinity, combined with the already-cool temperature, makes the water dense enough to sink, forming deep ocean currents that flow back toward the equator. This whole loop, the North Atlantic conveyor belt, distributes heat around the world and shapes regional climate.

✎Exam-safe wording

AMOC (the Atlantic Meridional Overturning Circulation, the North Atlantic branch of this system) is an active, contested research area, not a settled result. The IPCC's Sixth Assessment Report judges it "very likely" that AMOC weakens over the 21st century, with only medium confidence that this stops short of an abrupt collapse before 2100, and low confidence in exactly how much it has already weakened. Some 2023 studies proposed collapse could begin by mid-century; other, more recent modeling work argues collapse this century is unlikely. Treat any single confident date for AMOC collapse, in either direction, with skepticism: the honest position right now is uncertainty around an expected weakening, not an agreed timeline.

A weakened or collapsed AMOC would matter far beyond the ocean itself: it currently helps keep north-west Europe milder than its latitude would otherwise suggest, and a major slowdown would also disrupt monsoon systems and marine ecosystems that depend on today's circulation pattern.

Sources: this tab

Ocean acidification figures: NOAA, 2024.
Lake stratification seasonal-cycle diagram: Hydrated, English Wikipedia, public domain.
Ocean depth profiles diagram (density/pycnocline, salinity/halocline, temperature/thermocline): ReddKing, "Ocean Depth Profiles," via Wikimedia Commons (CC BY 4.0).
Humboldt Current area and fish-catch share: Chavez, F.P. et al., "The northern Humboldt Current System: Brief history, present status and a view towards the future," Progress in Oceanography, 79(2-4), 2008.
Anchoveta as the world's largest single-species fishery by catch weight: FAO, "The State of World Fisheries and Aquaculture" (SOFIA), 2024.
1997-98 El Niño impact on Peruvian anchoveta landings: Fréon, P. et al., "Interdecadal variability of anchoveta abundance and overcapacity of the fishery in Peru," Progress in Oceanography, 79(2-4), 2008.
AMOC weakening confidence levels: IPCC Sixth Assessment Report (2021); Ditlevsen & Ditlevsen, Nature Communications, 2023; subsequent 2024-2025 modeling studies showing a contested range of collapse-risk estimates.
IB Environmental Systems and Societies Guide, first assessment 2026, Topic 4, Subtopic 4.1, statements 4.1.7-4.1.14.