IB ESS HLTopic 4 — Water SystemsPaper 1 & 2Core idea~9 min read
How Water Moves Around the Planet
Every drop of water you drank today has been here since the Earth was young. Nothing new arrives and almost nothing leaves — the same water just keeps moving between the sea, the sky, the ice and the ground. That constant reshuffling is the hydrological cycle, and it is the backbone of the whole Water topic.
📘 What you need to know
The hydrosphere is all the water on Earth — oceans, ice, groundwater, rivers, lakes and water vapour in the air.
About 97.5% of it is salty. Only about 2.5% is fresh, and most of that fresh water is locked in ice or buried underground.
The global water cycle is a closed system for matter but an open system for energy.
Water sits in stores and moves between them through flows.
Flows are either transformations (the water changes state) or transfers (the water changes place).
Two things drive the whole cycle: energy from the Sun and gravity.
Where Earth’s water actually is
People picture the planet’s water as rivers and rain because that is the water we see. In reality rivers and rain are the leftovers. Start with all the water on Earth and cut it down step by step, and you end up with a very small usable slice.
Notice the pattern: every time you zoom in, the water we can actually reach gets smaller. That is why water scarcity is possible on a planet covered in water.
If an exam asks why fresh water is a limited resource, do not just say “there is not much of it”. Say where it is: frozen in ice sheets or sitting deep underground, so it is not available where and when people need it.
One cycle, nothing in and nothing out
The global water cycle is a closed system. Matter is recycled inside it — water leaves the ocean, spends a week or so in the air, falls as rain, sits in a lake, and eventually returns to the sea. The total amount barely changes.
But energy behaves differently. Sunlight pours in, and heat radiates back out to space. So the same system is closed for matter and open for energy. Examiners like that distinction because students usually get half of it right.
The idea in one line
water is recycled • energy passes through
The main stores
Oceans — by far the biggest store, and salty.
Glaciers, ice sheets and snow — the biggest store of fresh water.
Groundwater and aquifers — water held in the pores and cracks of soil and rock.
Surface fresh water — rivers, lakes, ponds, wetlands.
The atmosphere — water vapour and cloud droplets. A tiny store, but the fastest moving one.
A store can be big and slow or small and fast. Deep groundwater might sit still for thousands of years. Water vapour in the air is replaced roughly every nine days. That difference in residence time matters: damage a slow store, like an ancient aquifer, and it will not refill in your lifetime.
Stores and flows: how the cycle fits together
Flows are the arrows that join the stores. Split them into two groups and the whole topic gets easier to remember.
Transformations — the water changes state (liquid, gas or solid). Evaporation, condensation, melting, freezing, sublimation.
Transfers — the water changes place but stays in the same state. Precipitation, run-off, infiltration, streamflow.
Drawing the cycle as boxes and arrows, rather than as a landscape, makes it obvious that a flow always has a start store and an end store. Exam answers should name both.
The flows you should be able to name
Flow
Type
What is happening
Evaporation
Transformation
Liquid water at the surface of a sea, lake or river gains heat and turns into vapour.
Transpiration
Transformation
Plants pull water up from the soil and lose it as vapour through pores in their leaves.
Evapotranspiration
Transformation
Evaporation and transpiration counted together — the usual way it is measured in the field.
Condensation
Transformation
Rising vapour cools, turns back into tiny droplets and forms cloud.
Sublimation
Transformation
Ice or snow turns straight into vapour without melting first.
Melting and freezing
Transformation
Ice becomes liquid when it warms; liquid becomes ice when it cools.
Advection
Transfer
Wind carries vapour or cloud sideways, often from sea to land.
Precipitation
Transfer
Water falls out of the atmosphere as rain, snow, sleet or hail.
Surface run-off
Transfer
Water flows over the ground because the soil is full, frozen or sealed.
Infiltration
Transfer
Water soaks from the surface down into the soil.
Percolation
Transfer
Water already in the soil keeps moving down into the rock below.
Streamflow
Transfer
Water travels downhill in channels towards a lake or the sea.
Groundwater flow
Transfer
Water creeps sideways through pores in rock, often ending up in a river or the ocean.
Watch the pair that trips everyone up.Infiltration is water getting into the soil. Percolation is what happens next, as that water keeps sinking through soil and rock towards the aquifer. Infiltration first, percolation second — they are two stages of the same journey, not two names for it.
What keeps it all moving
Only two things drive the cycle, and both are worth a mark.
Energy from the Sun. Heat gives surface water molecules enough energy to escape as vapour. No sunlight, no evaporation, no cycle. The Sun also powers the winds that carry that vapour to land.
Gravity. Once water condenses and falls, gravity does the rest — pulling rain down, dragging rivers downhill, and pushing water through soil into the rock beneath.
Chain the two together and you have the answer to almost any “explain the cycle” question: the Sun lifts water up, gravity brings it back down, and the loop repeats.
Worked examples
WORKED EXAMPLE 1
Estimating how much surface fresh water there is
Earth holds roughly 1 400 000 000 km3 of water. Fresh water is about 2.5% of that, and surface stores (lakes, rivers, wetlands, soil and the air) make up about 1.2% of the fresh water. Estimate the volume held in surface stores.
Step 1: find the fresh water1 400 000 000 × 0.025 = 35 000 000 km3Step 2: take the surface share of that35 000 000 × 0.012 = 420 000 km3About 4.2 × 105 km3that is roughly 0.03% of all the water on the planet — and it is the part almost every ecosystem and city relies on
WORKED EXAMPLE 2
Transformation or transfer?
Label each of these as a transformation or a transfer, and give a reason: (a) snow on a mountain turning straight into vapour on a dry sunny day, (b) rain soaking into a field, (c) wind blowing a rain cloud inland from the sea.
(a) Sublimation
Solid changes to gas, so the state changes → transformation.
(b) Infiltration
Still liquid water, just now below the surface → transfer.
(c) Advection
Cloud droplets move sideways without changing state → transfer.
Ask one question: did the state change?if yes it is a transformation, if no it is a transfer — nothing else to decide
WORKED EXAMPLE 3
Tracing one molecule from the sea to an aquifer
Describe the route a water molecule could take from the ocean surface to an underground aquifer, naming each flow. [4]
Build it as a chain of named flows
Ocean → evaporation → atmosphere
Then get it over the landadvection carries the vapour inland, then condensation forms cloud
Then get it into the groundprecipitation → infiltration into soil → percolation into rock
Ends up stored as groundwatermarks come from the named flows in the right order, not from long sentences
💡 Exam tip
Name flows, do not describe them. “Water goes into the soil” scores nothing; “infiltration” scores the mark.
Say which store the water came from and which it went to. A flow always joins two stores.
Closed for matter, open for energy. Learn that phrase word for word — it appears in mark schemes.
If a question says explain, give the cause as well as the process: the Sun supplies the energy, gravity supplies the pull.
Quote figures with the word about (about 2.5% fresh, about 97.5% saline). Examiners accept rounded values and you avoid arguing over decimals.
Residence time is a useful extra: fast stores like the atmosphere recover quickly, slow stores like deep aquifers do not.
⚠ Common mix-up
Infiltration vs percolation. In first, down second. Do not use them as if they mean the same thing.
Transpiration vs evapotranspiration. Transpiration is plants only; evapotranspiration is plants plus open water and soil together.
Calling the cycle a closed system full stop. It is closed for matter but open for energy — say both.
Confusing precipitation with condensation. Condensation makes the cloud; precipitation makes it fall.
Thinking fresh water means available water. Nearly all of it is frozen or deep underground.
Treating run-off as the same as streamflow. Run-off is unchannelled flow across the surface; streamflow is water already in a channel.
Up next: How Humans Change the Water Cycle — what happens to these flows when we irrigate fields, cut down forests and cover the ground in concrete.
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