Water systems — core idea
· Water systems support life on Earth because water moves through the hydrosphere and links with the atmosphere, biosphere, geosphere and carbon cycle.
· Solar radiation drives evaporation by providing heat energy.
· Gravity drives water movement through soil, rivers, groundwater flow and drainage to the sea.
· Condensation releases heat, which helps transfer energy within the Earth system.
· Exam focus: describe water systems using stores, flows, inputs, outputs, transfers and transformations.
This diagram shows how water is stored and transferred between the atmosphere, land, ice and oceans. Use it to practise identifying stores as boxes/areas and flows as arrows/processes. It is especially useful for distinguishing evaporation, precipitation, infiltration and runoff. Source
Global hydrological cycle as a system
· The global hydrological cycle is a system with stores and flows.
· In exam diagrams, show stores as boxes and flows as arrows.
· The cycle is largely a closed system globally for matter, but water moves between stores through many flows.
· Key skill: create and use a systems diagram showing transfers and transformations in the hydrological cycle.
· Transfers = water changes location, e.g. surface run-off, streamflow, groundwater flow.
· Transformations = water changes state, e.g. evaporation, condensation, freezing, melting, sublimation.
Main stores in the hydrological cycle
· Oceans are by far the largest store: about 96.5% of Earth’s water.
· Glaciers and ice caps store about 1.7%.
· Groundwater stores about 1.7%.
· Surface freshwater is tiny: about 0.02%.
· Atmosphere stores about 0.001%.
· Organisms store about 0.0001%.
· Do not memorize exact percentages; know the relative proportions: oceans dominate, accessible freshwater is very limited.
Flows in the hydrological cycle
· Evaporation = liquid water becomes water vapour, mainly from oceans/lakes/soil.
· Transpiration = water vapour is released from plants; together with evaporation this forms evapotranspiration.
· Sublimation = ice changes directly into water vapour.
· Condensation = water vapour becomes liquid droplets or ice crystals, forming clouds.
· Advection = wind-blown movement of water vapour or cloud droplets/ice crystals.
· Precipitation = water falls as rain, snow, sleet or hail.
· Melting and freezing transform water between solid and liquid states.
· Surface run-off = water flows over land into rivers/lakes/oceans.
· Infiltration = water enters the soil.
· Percolation = water moves downward through soil/rock.
· Streamflow = water moves through rivers and streams.
· Groundwater flow = underground movement of water through permeable rock/soil.
Human impacts on hydrological flows and stores
· Agriculture, deforestation and urbanization can alter water flows and stores.
· Deforestation reduces transpiration and evapotranspiration because fewer plants release water vapour.
· Urbanization increases impermeable surfaces, reducing infiltration and increasing surface run-off.
· Reduced evapotranspiration + increased run-off can lead to flash floods.
· Agriculture can alter infiltration, groundwater storage and run-off through irrigation, soil compaction, drainage and vegetation removal.
· Strong exam answers link human activity → changed flow/store → environmental consequence.
Steady state and sustainable water harvesting
· A water body can be shown in steady state using a flow diagram of inputs and outputs.
· Steady state means inputs and outputs are balanced over time, so the store remains relatively stable.
· Inputs may include precipitation, stream inflow, groundwater input and run-off.
· Outputs may include evaporation, stream outflow, groundwater flow and human abstraction.
· Sustainable harvesting from lakes or aquifers must not exceed the net renewable input.
· Exam logic: if outputs > inputs, the water store declines and use is unsustainable.
HL only — properties of water that support life
· Polarity: water molecules have uneven charge distribution, making water a powerful polar molecule.
· Hydrogen bonding causes cohesion, meaning water molecules attract each other.
· Adhesion means water is attracted to other substances, helping water move through soils, plant tissues and small spaces.
· Solvent properties allow water to dissolve and transport ions, minerals, gases and biological molecules.
· Transparency allows light penetration, supporting photosynthesis in aquatic ecosystems.
· High specific heat capacity means water resists rapid temperature change, stabilizing aquatic habitats and climate.
· Density anomaly: freshwater is densest at 4°C, so ice floats and insulates water below, helping aquatic life survive freezing conditions.
· Gas solubility varies with temperature and pressure; warmer water holds less dissolved oxygen, which can stress aquatic organisms.
This diagram shows how polarity allows water molecules to form hydrogen bonds. These bonds explain cohesion, adhesion and several life-supporting properties of water. It is useful for connecting molecular structure to ecosystem-level effects. Source
HL only — oceans as a carbon sink
· Oceans absorb carbon dioxide from the atmosphere and act as a major carbon sink.
· This absorption has moderated increases in atmospheric carbon dioxide from fossil fuel burning.
· A saturation point may be reached if oceans become less able to absorb additional carbon dioxide.
· Short term: carbon is stored as dissolved carbon dioxide, which contributes to ocean acidification.
· Long term: carbon is taken up by organisms as biomass and can accumulate in seabed sediments.
· Seabed sediments contain inorganic carbonates and organic carbon compounds that may eventually contribute to fossil fuel formation over millions of years.
· Exam link: water systems interact with the carbon cycle, climate change and ocean acidification.
HL only — stratification, thermoclines and mixing
· Stratification occurs when water layers differ in temperature, salinity and therefore density, restricting mixing.
· In many water bodies, warmer, less dense water stays above cooler, denser water.
· A thermocline is the transition layer between the warmer mixed surface layer and cooler deep water.
· Stratification occurs in deep lakes, coastal areas, enclosed seas and the open ocean.
· Surface and deep water layers often differ in dissolved oxygen and mineral nutrient concentrations.
· Stronger stratification can reduce nutrient mixing, lowering productivity in surface waters.
· Global warming and salinity changes have increased ocean stratification, especially in the upper 200 m.
· Melting ice can reduce salinity, especially around Antarctica, altering density-driven mixing.
HL only — upwelling and productivity
· Upwelling = vertical movement of cold, nutrient-rich deep water to the surface.
· It occurs when wind-blown surface water is displaced and deeper water rises to replace it.
· Upwelling supplies mineral nutrients to surface waters, increasing phytoplankton productivity.
· High productivity supports food webs and important fisheries.
· Seasonal upwelling can occur in stratified lakes and during conditions associated with ENSO.
· Exam link: upwelling connects water systems, nutrient availability, aquatic productivity and food production systems.
This diagram shows the mechanism of upwelling. Cold deep water rises when surface water is displaced by wind. This brings nutrients to the surface and can increase productivity in aquatic ecosystems. Source
HL only — thermohaline circulation and climate
· Thermohaline circulation is driven by differences in temperature and salinity.
· Temperature and salinity affect water density, which drives sinking and deep-water movement.
· The ocean conveyor belt distributes heat around the world and affects global climate.
· Warm surface waters such as the Gulf Stream move towards the North Atlantic.
· These waters lose freshwater through evaporation, become more saline, cool, become denser and sink.
· Sinking forms deep ocean currents that move water back towards the equator.
· Inputs of low-salinity water from rivers and melting ice caps can affect density and circulation.
· Exam link: thermohaline circulation connects water systems, climate regulation, global warming and ice melt.
This map shows the global pattern of density-driven ocean circulation. Surface and deep currents redistribute heat, helping regulate climate. Use it to explain how temperature, salinity and density interact. Source
Checklist: can you do this?
· Draw a hydrological cycle systems diagram with stores as boxes and flows as arrows.
· Distinguish transfers from transformations in the water cycle.
· Explain how deforestation, agriculture and urbanization alter water flows and stores.
· Use input–output logic to explain steady state and sustainable harvesting from a lake or aquifer.
· HL only: explain how water properties, ocean carbon storage, stratification, upwelling and thermohaline circulation support life and affect climate.
Exam tips for high-scoring answers
· Always use precise system vocabulary: store, flow, input, output, transfer, transformation, steady state.
· For diagrams, label flows clearly and show direction with arrows.
· Link causes to consequences: e.g. urbanization → reduced infiltration → increased run-off → flash flooding.
· For HL, connect physical processes to impacts: warming → stronger stratification → reduced mixing → lower nutrient supply/productivity.
· Use percentages only comparatively: oceans dominate global water storage, while accessible freshwater is very limited.
Shubhi is a seasoned educational specialist with a sharp focus on IB, A-level, GCSE, AP, and MCAT sciences. With 6+ years of expertise, she excels in advanced curriculum guidance and creating precise educational resources, ensuring expert instruction and deep student comprehension of complex science concepts.
Shubhi is a seasoned educational specialist with a sharp focus on IB, A-level, GCSE, AP, and MCAT sciences. With 6+ years of expertise, she excels in advanced curriculum guidance and creating precise educational resources, ensuring expert instruction and deep student comprehension of complex science concepts.