The Water Cycle: Pathways, Stores & Drivers — Print · Grade 6 Science

The Water Cycle: Pathways, Stores & Drivers

Learning goals

I can name the processes that move water between stores and what drives each.
I can explain what infiltration is and why paving a surface changes a river.
I can compare how long water stays in different stores.

Readings for this unit

A Rainy Day and a Longer Record

Before this unit

Read the chapter opening and the named observation or diagram. Clarify one word or unit that blocks understanding. Recall the relevant earlier measurement, matter, or energy idea.

Study the unit across three 35–45-minute class sessions. Use the worked examples after an attempt and the supplied records when actual equipment is not available.

Energy from the Sun evaporates water from oceans, lakes and soil, and plants release more through their leaves by transpiration. The vapour rises, cools, and condenses onto tiny particles to form clouds. When droplets grow heavy enough they fall as precipitation, and gravity takes over from there.

On the ground, water either runs off the surface into streams and rivers, or soaks into the soil and rock by infiltration, where it becomes groundwater and moves slowly downhill underground. Both routes end in the ocean, one in hours and the other sometimes in centuries.

The quantities involved are enormous and the stores are very unequal. About 97 per cent of Earth's water is salt water in the oceans, roughly 2 per cent is frozen in ice sheets and glaciers, and all the fresh liquid water in rivers, lakes and the ground amounts to well under 1 per cent.

Build your understanding

Where this fits

This chapter follows water and air around the planet, using the systems from the last chapter. The water cycle here supplies the moisture that makes weather in the next two lessons, and the infiltration in the third section is the process the wetland question in chapter 5 turned on.

Use three flexible 40-45-minute sessions: the processes and what drives each; the stores and how long water stays in them; infiltration, runoff and what changes them, then the practice set, revision and the exit check. Workshop, review, quiz and exams have separate time.

Part 1 · Two engines and nothing else

Every movement in the water cycle is driven either by the Sun or by gravity, and sorting them is a useful exercise. Evaporation, transpiration and the rising of warm moist air all need energy, and that energy comes from sunlight; it takes about 2,260,000 J to evaporate 1 kg of water, which is why evaporation is such an effective way of cooling a surface. Precipitation, runoff, infiltration and the slow movement of groundwater are all downhill, driven by gravity, and need no energy input at all.

Condensation is the step students most often get wrong, because it looks like something is being created. Warm air holds more water vapour than cool air, so when air rises and cools it reaches a point where it can no longer hold all the vapour it carries, and the excess condenses onto microscopic particles of dust, salt or smoke. Those particles matter: completely clean air can be cooled well past the point of saturation without any cloud forming at all, because the droplets have nothing to form on. Cloud is not water appearing; it is water becoming visible. There is a fourth process worth naming because it skips the liquid stage entirely. Sublimation turns ice directly into vapour without melting first, which is how snow slowly disappears on a dry sunny day well below 0 degrees Celsius, and the reverse, deposition, forms frost directly from vapour on a cold window. Neither is a major part of the global cycle, and both are useful evidence that the cycle is about changes of state rather than about liquid water moving around.

This diagram selects several possible pathways; water need not follow every arrow in order. Solar energy helps support evaporation and transpiration, while gravity contributes to rainfall and runoff. Stores and flow times vary; sizes, positions, and arrow lengths are not quantitative.

Pause and explain

Which processes in the water cycle need energy from the Sun, and which do not?

Check your reasoning after answering

Evaporation and transpiration need energy, since separating water molecules into vapour takes about 2,260,000 J per kilogram, and the rising of warm air is driven by solar heating too. Precipitation, runoff, infiltration and groundwater flow all move water downhill and are driven by gravity, requiring no energy input at all.

Part 2 · How long water stays put

The time water spends in a store before moving on is called its residence time, and the range is extraordinary. A water molecule stays in the atmosphere about nine days on average, which is why weather changes within a week and why a drought cannot be fixed by waiting for the air to refill. A river molecule reaches the sea in weeks. Water in a large lake may stay for years to decades, in the deep ocean for several thousand years, and in a deep aquifer or an ice sheet for tens of thousands of years.

These figures turn an abstract diagram into a set of practical consequences. Groundwater pumped from a deep aquifer may have fallen as rain before the last ice age, which means that using it faster than rain replaces it is mining rather than harvesting, and several of the world's major aquifers are being drawn down in exactly that way. The nine-day atmospheric figure also explains something hopeful: pollutants washed out of the air clear quickly, while a contaminant that reaches deep groundwater may stay for centuries. One more figure puts the atmospheric store in proportion. At any moment the whole atmosphere holds only about 12,900 cubic kilometres of water, which if it all fell at once would cover the planet to a depth of about 25 mm. The reason far more rain than that falls each year is the nine-day turnover: the same store is emptied and refilled roughly forty times a year, which is why rainfall totals are so much larger than the amount of water in the sky at any instant.

Pause and explain

Water stays in the atmosphere about nine days and in a deep aquifer for tens of thousands of years. What practical difference does this make?

Check your reasoning after answering

It decides how quickly each store recovers. The atmosphere refreshes within a fortnight, so airborne pollution washes out fast and weather changes week to week. A deep aquifer is replaced so slowly that pumping it faster than rainfall refills it is effectively using a finite resource, and any contamination reaching it may remain for centuries.

Part 3 · Into the ground or across it

When rain reaches the surface it takes one of two routes, and which one dominates decides almost everything about the local river. Infiltration is water soaking downward into soil and rock, where it fills the spaces between grains and moves slowly, often only a metre or so a day, recharging the groundwater that feeds wells and keeps streams flowing in dry weather. Runoff is water flowing across the surface, reaching a stream within minutes or hours and carrying soil with it.

The balance between them depends on the surface. Woodland soil, loosened by roots and covered by leaf litter, absorbs most of what falls on it. Bare compacted earth absorbs far less. Concrete and tarmac absorb nothing at all, so a town converts almost all its rainfall into runoff, which is why urban rivers rise sharply within an hour of a storm and fall just as fast, while a wooded catchment produces a slower, lower and longer flood. Rainfall intensity matters too: soil that absorbs 10 mm of rain in an hour comfortably will shed most of 50 mm falling in the same hour simply because the water arrives faster than it can soak in. Slope and existing wetness matter as much as the surface material. Rain falling on a steep hillside runs off before it has time to sink in, and soil that is already saturated from previous days cannot accept any more however loose it is, which is why the most damaging floods usually follow a wet fortnight rather than a single dramatic storm. Forecasters therefore track soil moisture as closely as they track the rain itself.

Pause and explain

Why does a town's river rise faster after a storm than a river draining woodland?

Check your reasoning after answering

Because paved surfaces prevent infiltration. In woodland most rain soaks into loose, root-filled soil and reaches the stream slowly through the ground, spreading the flow over days. Concrete and tarmac absorb nothing, so almost all of the rainfall becomes runoff and arrives within an hour, producing a sharp rise and an equally sharp fall.

Part 4 · The cycle is closed, the supply is not

No water is created or destroyed in any of this, and the total on Earth has been roughly constant for billions of years. That fact is sometimes used to argue that water shortages are impossible, which does not follow at all. The problem is never the total; it is whether fresh liquid water is in the right place, at the right time, and clean enough to use. Of all Earth's water, the rivers and lakes that supply most cities hold around one hundredth of one per cent.

Human activity changes distribution rather than quantity. Paving a catchment moves water from infiltration to runoff. Damming a river moves water from the sea to a reservoir and increases evaporation from its surface. Irrigating a field moves groundwater to the atmosphere through crops. Each of these conserves the total exactly while changing where the water is and how fast it moves, and that is what a water shortage actually consists of. Climate change acts on the same distribution rather than on the total. Warmer air holds more water vapour, roughly 7 per cent more for each degree Celsius of warming, so a warmer atmosphere carries more moisture and tends to release it in heavier bursts. The result is not less water overall but a shift towards longer dry spells punctuated by more intense rainfall, which is harder to store and more likely to run off than steady rain. The last lesson of this chapter returns to the evidence for that warming.

Pause and explain

If the total amount of water on Earth never changes, how can there be a water shortage?

Check your reasoning after answering

Because availability depends on location, timing and quality rather than on the total. Nearly all of Earth's water is salt or frozen, and the fresh liquid water in rivers and lakes is a hundredth of one per cent of it. Human activity shifts water between stores without destroying any, and a shortage is water being in the wrong store for the use required.

Apply the whole unit · Predict the flood

Two neighbouring catchments each receive 40 mm of rain in two hours. Catchment X is woodland with deep soil. Catchment Y was woodland ten years ago and is now a housing estate with roads, roofs and drains.

Your task

Predict how each river responds and explain the difference using infiltration and runoff.

What your response should show

Describes both the height and the timing of each flood, not just which is worse
Attributes the difference to infiltration against runoff
States that the total volume is similar and the rate is what differs
Proposes a measure and names the process it restores

Response guidance — after your attempt

Describe the shape and timing of the flood in each catchment.
Explain the difference in terms of the two routes water can take.
Suggest one change to catchment Y that would reduce the difference, and say which process it restores.

In catchment X most of the 40 mm infiltrates into deep woodland soil, where roots and leaf litter have kept it open, so it travels to the stream slowly underground. The river rises gradually over a day or more, peaks modestly and falls slowly over several days. In catchment Y the roofs and roads absorb nothing and the drains are built to remove water quickly, so nearly all 40 mm becomes runoff. The river rises sharply within an hour, reaches a much higher peak, and drops back almost as fast once the rain stops. The total volume reaching the river is similar in both cases; what differs is how long it takes to arrive, and a flood is caused by rate rather than by total. One useful change in catchment Y would be a retention pond, or permeable paving and soakaways, each of which restores infiltration or at least delays runoff, flattening the peak by spreading the same water over a longer period.

Scientific references for this unit

Parent / teacher teaching notes

Begin with the literal observation, quantity, or diagram relationship. Ask the learner to explain why the evidence supports the claim, check a relevant unit or boundary, then discuss an alternative. The supplied numbers are illustrative, not real observations. A parent can read aloud and allow a spoken rehearsal before writing. Judge science and evidence rather than political, faith, or family agreement. Use the complete data option whenever an actual procedure is unavailable or unsuitable.

The key idea

The water cycle is driven by the Sun and by gravity, and nothing else. Water does not disappear when a puddle dries; it moves between a small number of stores, and the time it spends in each one ranges from about nine days in the atmosphere to thousands of years underground.

Worked examples

Example 1 · A water-budget model

Check the account and explain what a missing transfer would change.

Add the stated output and storage terms.
Retain the area/time interpretation of millimeters.
Identify the idealized boundary and omitted possible flow.

Answer: 35 + 25 + 20 = 80 mm, so the listed terms match the input under the model. Increased storage is not an outgoing flow. If unrecorded groundwater entered or left, the account would need that transfer; balance here does not prove every real pathway was measured.

Example 2 · Transfer to a different case

A diagram requires every rain drop to enter a plant before returning to the ocean. Is that a complete pathway model?

Identify the distinction and conditions in this new case.
Use the relevant mechanism or quantity; preserve units and the chosen boundary.
Explain why the answer fits and what further evidence would be needed.

Answer: No. Runoff, groundwater flow, evaporation, and other routes can bypass plant uptake. Show alternatives rather than one compulsory route.

Example 3 · Trace infiltration and groundwater · a focused example

Why can water remain in the ground for a long interval?

State the literal condition or observation in the example.
Apply the distinction explained in this teaching part.
Check whether the claim stays within the given evidence.

Answer: Storage and flow depend on pore connections, geology, gradients, and conditions; a cycle does not require rapid circulation.

Avoid these mistakes

Misleading claim: Every water particle follows one identical circular route.
Misleading claim: A cloud is simply a mass of visible water vapor.
Omitting units, measurement timing, or a model boundary makes the evidence hard to interpret.
Treating illustrative classroom data as real field measurements or copying a model is not an independent explanation.

Check your understanding

Which stage changes gas to liquid?

Discuss the answer

Condensation. Rainfall is a later transport pathway; water vapor itself is ordinarily invisible.

Why can water remain in the ground for a long interval?

Discuss the answer

Storage and flow depend on pore connections, geology, gradients, and conditions; a cycle does not require rapid circulation.

Does runoff require water to boil?

Discuss the answer

No. It is movement of liquid over a surface, often influenced by gravity, not a boiling process.

Why must the interval be stated?

Discuss the answer

Pathways and storage continue to change. An immediate rain account and a week-long account can include different transfers.

Sustained scientific explanation and revision

These activities are scheduled in the parent plan. Save a first response, a correction, and the reason for the change in the science notebook.

Activity 1 · The Water Cycle: Pathways, Stores & Drivers · Model and explain

Assigned reading and preparation

Read A Rainy Day and a Longer Record and the linked unit teaching; use its complete supplied case and diagrams.

Your task

For The Water Cycle: Pathways, Stores & Drivers, study Parts 3–4 and attempt the worked case: Check the account and explain what a missing transfer would change. Explain the steps before opening the model.

Success criteria

An accurate scientific claim answering the actual question.
Relevant evidence or a labeled model with quantities and units when applicable.
Reasoning that connects evidence with the mechanism rather than only repeats it.
A specific method, boundary, scale, or alternative-explanation limit.

Hint

Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.

Response guidance — discuss after your attempt

35 + 25 + 20 = 80 mm, so the listed terms match the input under the model. Increased storage is not an outgoing flow. If unrecorded groundwater entered or left, the account would need that transfer; balance here does not prove every real pathway was measured.

Activity 2 · The Water Cycle: Pathways, Stores & Drivers · Practice, revise and exit

Assigned reading and preparation

Read A Rainy Day and a Longer Record and the linked unit teaching; use its complete supplied case and diagrams.

Your task

For The Water Cycle: Pathways, Stores & Drivers, complete practice questions 1–6 and both choice checks, then answer independently: A diagram requires every rain drop to enter a plant before returning to the ocean. Is that a complete pathway model? Save a correction explaining one changed claim or method.

Success criteria

An accurate scientific claim answering the actual question.
Relevant evidence or a labeled model with quantities and units when applicable.
Reasoning that connects evidence with the mechanism rather than only repeats it.
A specific method, boundary, scale, or alternative-explanation limit.

Hint

Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.

Response guidance — discuss after your attempt

No. Runoff, groundwater flow, evaporation, and other routes can bypass plant uptake. Show alternatives rather than one compulsory route.

Words to know

infiltration: Water entering spaces in soil or other ground material.
transpiration: Water passing through plants and entering the atmosphere as vapor.
evidence: A relevant observation, measurement, or record used to evaluate a claim.
system boundary: The chosen limit specifying what is included and which exchanges cross it.
uncertainty: A stated limit in measurement or knowledge, distinct from a careless mistake.

In the real world

City water departments depend on the water cycle: they protect the forests and reservoirs where rain and snow collect, because that precipitation is exactly where your tap water begins its journey.

Exit check

A diagram requires every rain drop to enter a plant before returning to the ocean. Is that a complete pathway model?

Check your explanation after answering

No. Runoff, groundwater flow, evaporation, and other routes can bypass plant uptake. Show alternatives rather than one compulsory route.

Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.