IB ESS SLTopic 4 — Water SystemsPaper 1 & 2Core idea~11 min read
How Water Moves Around the Planet
There is exactly as much water on Earth today as there was a billion years ago. None of it is being made and none of it is leaving — it is just moving between stores, changing state as it goes. Almost every water question in ESS comes down to two things: knowing where the water is, and knowing the name of the flow that moves it.
📚 What you need to know
The hydrosphere is all of Earth’s water — oceans, rivers, lakes, ice, groundwater and moisture in the air.
Only about 2.5% of it is fresh water. Of that, roughly 69% is locked in glaciers and ice sheets and about 30% is groundwater, leaving around 1% in rivers, lakes and the atmosphere.
The two driving forces are solar radiation (which lifts water up) and gravity (which brings it back down and moves it downhill).
The global hydrological cycle is a closed system: energy enters and leaves, but matter is recycled.
It is made of stores (where water sits) and flows (how water moves).
Flows split into transformations, where water changes state, and transfers, where it moves without changing state.
Know the flow names precisely — especially infiltration versus percolation.
Where the water actually is
The planet looks blue from space, which makes it easy to assume water is abundant. Break the total down and the picture changes completely.
Every river, lake and swamp on the planet sits inside that last thin band on the bottom bar. Rivers, the source most people picture when they think of fresh water, are 0.44% of 1.2% of 2.5% of the total.
Why this matters later. Because so little water is accessible, small changes in how fast we take it out — or how fast rain soaks in to replace it — have very large effects. That is the whole of the next page.
What drives the cycle
Two forces, and only two, keep everything moving.
Solar radiation supplies the energy. The Sun’s heat evaporates water from oceans, lakes and rivers, lifting it into the atmosphere as vapour. When that vapour cools and condenses into cloud, the heat is released again.
Gravity brings it back. It pulls condensed water down as precipitation, pulls water across the land as run-off, drags it down through soil and rock, and makes rivers flow downhill to the sea.
A neat way to hold it: the Sun does the lifting, gravity does the returning. Every flow in the cycle is one or the other, and if you can say which, you can usually explain the flow.
Stores and flows
The global hydrological cycle is a closed system. Energy passes through it, but the water itself is recycled endlessly rather than gained or lost. Within that system, everything is either a store or a flow.
The two words that organise this topicstore = a place water sits • flow = a process that moves water between stores
The main stores are:
Oceans — by far the largest
Glaciers and ice caps
Groundwater and aquifers — an aquifer is a body of rock that holds and lets through usable groundwater
Surface fresh water — rivers and lakes
The atmosphere — small, but it turns over fastest
The flows split into two kinds:
Transformations — the water changes state or form. Evaporation and condensation are transformations.
Transfers — the water moves from one place to another without changing state. Run-off and groundwater flow are transfers.
Trace one raindrop through it: it condenses, is blown inland by advection, falls as precipitation, either runs off the surface or infiltrates the soil, percolates down to the aquifer, flows slowly to the sea, and evaporates again. Same water, different stores.
The flows, one by one
Learn the type as well as the name. Questions often ask you to classify a flow, and that is a free mark if you know whether the water changes state.
Flow
Type
What happens
Evaporation
Transformation
Liquid water turns to vapour and enters the air from oceans, lakes and rivers
Transpiration
Transformation
Plants draw water up through their roots and lose it as vapour through the stomata in their leaves
Evapotranspiration
Transformation
Evaporation from the surface and transpiration from plants counted together
Sublimation
Transformation
Ice or snow turns straight to vapour without melting first
Condensation
Transformation
Cooling turns vapour back into liquid droplets, forming cloud or dew
Melting
Transformation
Warming turns ice or snow into liquid water
Freezing
Transformation
Cooling turns liquid water into ice or snow
Advection
Transfer
Wind carries vapour or cloud sideways, often from over the sea to over the land
Precipitation
Transfer
Water falls from the air as rain, snow, sleet or hail
Surface run-off
Transfer
Water flows across the ground, usually because the soil is saturated or the surface will not let water through
Infiltration
Transfer
Water soaks from the surface into the soil
Percolation
Transfer
Water already in the soil moves further down through soil and rock towards an aquifer
Throughflow
Transfer
Water moves sideways through the soil, downslope, towards a river
Streamflow
Transfer
Water moves along a stream or river under gravity, heading for a lake or the sea
Groundwater flow
Transfer
Water moves slowly through the pores and cracks in underground rock towards rivers, lakes or the sea
Infiltration is not percolation. Infiltration is water getting into the soil at the surface. Percolation is what happens afterwards, as that water keeps moving down through soil and rock. Same journey, two different stages.
Worked examples
EXAM Q1
Distinguish between a transformation and a transfer in the hydrological cycle, giving one example of each. [3]
Transformation
Water changes state or form, for example evaporation, where liquid becomes vapour.
Transfer
Water moves location without changing state, for example surface run-off across a field.
The test to apply
Ask whether the water is solid, liquid or gas before and after. If that changes, it is a transformation.
Change of state versus change of placegive named examples — the third mark is usually for those
EXAM Q2
Explain why the hydrological cycle is described as a closed system. [2]
Point 1: matter
The same water is recycled between stores; essentially none is gained or lost by the planet.
Point 2: energy
Energy from the Sun does enter and leave, which is what makes it closed rather than isolated.
Matter is recycled; energy passes througha closed system exchanges energy but not matter — an isolated system exchanges neither
EXAM Q3
Only a small fraction of Earth’s fresh water is available to people. Explain, with figures. [3]
Step 1: start from the total
Fresh water is only about 2.5% of all the water on Earth.
Step 2: subtract what is frozen
Roughly 69% of that fresh water is locked in glaciers and ice sheets.
Step 3: subtract what is underground
About 30% is groundwater, much of it deep and expensive to reach.
That leaves around 1% of fresh water in rivers, lakes and the atmospherequote the figures — “hardly any” is not an answer
💡 Exam tip
Learn the store and flow names exactly. “Water goes into the ground” scores nothing; “infiltration” scores.
When you name a flow, be ready to say whether it is a transformation or a transfer.
If asked to draw the cycle, label at least one store, one transformation and one transfer, with arrows showing direction.
Use 2.5%, 69% and 30% in any question about water availability. Named figures earn marks.
Say solar radiation and gravity when asked what drives the cycle, and say what each one actually does.
Evapotranspiration is one word covering two processes. If a question asks for evaporation and transpiration separately, give both.
⚠ Common mix-up
Infiltration confused with percolation. Percolation happens after infiltration, deeper down.
Calling precipitation a transformation. The water has already condensed; falling is a transfer.
Thinking the cycle is an open system. It is closed — energy in and out, water recycled.
Saying groundwater is unlimited. It is about 30% of fresh water, but it recharges slowly, which matters enormously for the next page.
Mixing up transpiration and evaporation. Transpiration is specifically water lost through plant stomata.
Forgetting advection. Without wind moving vapour off the ocean, it would rain almost entirely over the sea.
Up next: How Humans Change the Water Cycle — what happens to all these flows when you add irrigation, chainsaws and concrete.
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