The hydrological cycle: characteristics, stores and transfers
This unit is examined in Section A of Paper 1, Physical geography.
Section A prints three 25-mark questions, one each on rivers, coasts and hazards, and you answer two of them.
Question 1 is always River environments.
It has nine parts that run from two multiple-choice items to an 8-mark Analyse on a figure, and across the three most recent series its marks fell most heavily on water use, water quality and flooding, then on landforms, then on the cycle and regimes.
Every topic in this unit is examined from a figure or from what you know; no part has asked you to name a case study.
Definitions are worth single marks in almost every question, and the examiners have reported weak definitions of discharge in particular.
So this first section opens with every term the unit uses.
Learn each line as it is written.
Each later section repeats its own terms.
- Hydrological cycle: the never-ending circulation of water between the sea, the air and the land, powered by the Sun's heat and by gravity.
- Closed system: a system in which the total amount of material (here, water) stays the same, because nothing enters it from outside or leaves it.
- Store: a place where water is held for a time, such as the oceans, an ice sheet, a lake, the soil or an aquifer.
- Transfer (flow): a process by which water passes between stores, such as precipitation or throughflow.
- Precipitation: water falling from clouds to the ground as rain, snow, sleet or hail.
- Interception: precipitation caught and held on the leaves and stems of plants before it reaches the ground.
- Infiltration: water soaking downwards from the ground surface into the soil.
- Percolation: water seeping downwards out of the soil into the rock beneath it.
- Throughflow: the sideways movement of water through the soil, downslope towards a river.
- Overland flow (surface runoff): water flowing across the surface of the ground.
- Groundwater flow: the slow movement of water through permeable rock towards a river, lake or the sea.
- Groundwater: water stored in the pores and cracks of rock below the water table.
- Aquifer: a permeable rock that stores groundwater and lets it move through it.
- Water table: the upper surface of the zone in which the rock is saturated with water.
- Evaporation: the change of liquid water into water vapour, using heat energy.
- Transpiration: the loss of water vapour from plants, mainly through the pores (stomata) in their leaves.
- Evapotranspiration: evaporation and transpiration added together.
- Condensation: the change of water vapour into liquid water droplets as air cools.
- Drainage basin: the whole area of land from which a river and its tributaries collect their water.
- Source: the place where a river begins.
- Watershed: the boundary of a drainage basin, along high ground, which separates it from the next basin.
- Tributary: a smaller river or stream that joins a larger one.
- Confluence: the point where two rivers meet.
- Channel network: the pattern made by all the river channels in a drainage basin.
- Mouth: the place where a river ends, as it flows into the sea, a lake or a larger river.
- Discharge: the volume of water passing a point in a river channel per unit time, measured in cubic metres per second (m³/s).
- River regime: the yearly pattern of discharge in a river.
- Storm hydrograph: a graph of the discharge of a river at one point over a short period (hours or days) after a single storm, usually shown with the rainfall that caused it.
- Lag time: the gap in time from peak rainfall to peak discharge.
- Peak discharge: the greatest discharge a river reaches after a storm.
- Base flow: the usual discharge of a river between storms, supplied mostly by groundwater.
- Water abstraction: taking water out of a river, lake or aquifer for people to use.
- Velocity: the speed at which water flows in the channel, in metres per second (m/s).
- Gradient: the steepness of the river bed, the fall in height over a horizontal distance.
- Long profile: a section showing how the height of the river bed changes from source to mouth.
- Cross profile: a section across the river valley from one side to the other.
- Relief: the height and shape of the land, including how steep its slopes are.
- Altitude: the height of the land above sea level.
- Aspect: the direction in which a slope faces.
- Erosion: the removal of rock and soil by moving water (or ice, wind or waves), which wears the land away.
- Weathering: the breakdown of rock in place, by the weather, plants and animals, with no movement of the material.
- Mass movement: the downslope movement of rock and soil under gravity.
- Load: the material (sediment) a river carries.
- Deposition: the dropping of load when a river loses the energy to carry it.
- Flood: water spilling over the banks of a river onto land that is normally dry.
- Flash flood: a flood that arrives within a few hours of intense rain, with very little warning.
- Water surplus: an area where water supply is greater than demand.
- Water shortage: an area where demand for water is greater than supply.
Notice what the definitions do not do.
None of them gives an example in place of a meaning, and the river ones name the variable (volume, time, height).
"Discharge is how much water is in the river" fails, because it names neither the time nor the point.
The idea: one fixed amount of water, always moving
On a winter night a storm drifts east from the Mediterranean over the Taurus Mountains of eastern Türkiye.
Snow builds up on the high slopes above the headwaters of the Euphrates.
In spring the snow melts; some soaks into the ground, some runs into streams, and months later the same water may be pumped onto a wheat field in Iraq, evaporate from the leaves and drift away as vapour.
None of that water is new.
It has moved round the same loop many times.
That loop is the hydrological cycle.
Three characteristics matter in an exam:
1. It is a closed system at the global scale. The total amount of water on Earth does not change. Water is not created or destroyed; it only moves between stores and changes state (solid ice, liquid water, gas vapour).
2. It is driven by two energy sources. Heat from the Sun evaporates water and lifts it into the atmosphere. Gravity brings it back down as precipitation and pulls it downhill over and through the land to the sea.
3. Water spends very different times in different stores. A water molecule may stay in the atmosphere for about nine days, in a river for weeks, and in deep groundwater or an ice sheet for thousands of years. This is why rivers respond to a storm within hours but an emptied aquifer takes generations to refill.
A drainage basin, which you meet in the next section, is different: it is an open system, because precipitation enters it from outside and water leaves it as river discharge to the sea and as evaporation.
Mixing the two up is a common slip: the global cycle is closed, a basin is open.
Stores and transfers
The figure below shows the main stores and the transfers that link them.
Read it in two passes: first find every store (a place where water sits), then every arrow (a process that moves water).
Stores (where water is held)
Transfers (how water moves)
Oceans and seas
Evaporation (sea, lakes, wet ground to air)
Ice sheets, glaciers and snow
Transpiration (plants to air)
Atmosphere (vapour and cloud)
Condensation (vapour to droplets)
Lakes and reservoirs
Precipitation (cloud to ground)
Rivers (channel store)
Interception (onto plants)
Soil (soil moisture)
Infiltration (surface into soil)
Groundwater in aquifers
Percolation (soil into rock)
Vegetation (water on and in plants)
Throughflow (sideways through soil)
Overland flow (over the surface)
Groundwater flow (through rock)
Channel flow (river to sea)
A simple test sorts any word: can you point to it as a place? Then it is a store.
Is it something water does, a movement or a change of state? Then it is a transfer.
"Groundwater" is a store; "groundwater flow" is a transfer.
"Lake" is a store; "evaporation" from it is a transfer.
How much water sits in each store
Almost all of Earth's water, about 96.5%, is salt water in the oceans.
Only about 2.5% is fresh, and most of that is locked away where people cannot easily reach it, as the chart shows.
The chart explains a great deal about water problems later in this unit.
Rivers and lakes, the stores people use most, are a tiny share of the fresh water on Earth.
Groundwater is a far bigger store, which is why the Gulf states and Jordan pump so much from aquifers.
But groundwater moves slowly and refills slowly, so pumping it faster than rain can recharge it empties the store.
The chart is a picture of a closed system: the total is fixed, so more water in one store means less in another.
Why it matters: linking the cycle to people
Every later section in this unit is the cycle at work.
A storm hydrograph is a record of how fast precipitation becomes channel flow.
A flood is too much water reaching the channel store too quickly.
Water shortage is a place where precipitation is low and evaporation high, so stores stay small.
Pollution travels with throughflow and overland flow into rivers.
When you explain any of these, name the store and the transfer involved; it turns a vague answer into a precise one.
Identify the best definition of percolation.
(1)
- A Water moving sideways through the soil towards a river
- B Water seeping down out of the soil into the rock beneath
- C Water soaking into the ground surface
- D Water flowing across the surface of the ground
1. Picture where percolation happens: below the soil, in the rock.
2. Check each distractor: A is throughflow, C is infiltration, D is overland flow. All are real transfers, which is why they tempt you.
Answer: B (1).
The distractors in these items are always other real terms from the same topic, so learn each transfer's direction (down, sideways, over) and its layer (surface, soil, rock).
Study the hydrological cycle figure above.
Identify one store shown in the figure where water can be held for thousands of years.
(1)
1. List the stores you can see: atmosphere, snow and ice, soil, groundwater in the aquifer, lake, river, sea.
2. Choose one with a long residence time that is actually drawn: the groundwater in the aquifer (the saturated sandstone) is drawn, and so is the snow and ice.
Answer: Groundwater (aquifer) (1).
The examiners refused "aquifer" on a figure that showed no rock layer, because a store must be visible in the figure to score.
Here the aquifer is drawn, so it is safe.
Writing "percolation" would score nothing: it is a transfer, not a store.
Explain how water is transferred from the sea to the atmosphere.
(2)
1. Name the process and what it does: heat from the Sun warms the sea surface, so liquid water turns into water vapour (evaporation) (1).
2. Develop it with why or how: the warm, moist air is lighter, so it rises and carries the vapour up into the atmosphere, where it is stored until it cools (1).
Answer: 2 marks, one point (evaporation) and one development (how the vapour reaches the atmosphere).
Writing "the water evaporates" alone repeats the idea and earns 1.
Explain one way the hydrological cycle is a closed system.
(3)
1. Point: the total amount of water on Earth stays the same (1).
2. First link: water is not gained from space or lost to it, it only moves between stores such as the oceans, the atmosphere and ice (1).
3. Second link: so when one store grows, another must shrink, for example sea level falls when more water is locked up as ice on land (1).
Answer: 3 marks for one idea developed twice.
A second, separate idea ("it is driven by the Sun") would not earn the third mark.
When a figure asks you to identify a store or a transfer, check two things before you write: that your word is the right kind (a place for a store, a process for a transfer) and that it is actually drawn or labelled on that figure.
A correct term that the figure does not show scores nothing.
- Giving a transfer when a store is asked, or the reverse. It is tempting because the words come in pairs (groundwater and groundwater flow). Fix: use the place-or-process test on every answer.
- Confusing infiltration and percolation. Both move water down. Infiltration is from the surface into the soil; percolation is from the soil into the rock.
- Calling a drainage basin a closed system. Water enters a basin as precipitation and leaves it as river discharge and evapotranspiration, so a basin is open. Only the global cycle is closed.
- Writing "water goes up" for evaporation. Name the change of state (liquid to vapour) and the energy (heat from the Sun).
Check your understanding
1. Identify the best definition of throughflow. (1)
- A Water moving downwards through rock
- B Water moving sideways through the soil towards a river
- C Water held on leaves before it reaches the ground
- D Water flowing across the ground surface Total [1]
2. Define the term interception. (1) Total [1]
3. Study the hydrological cycle figure. Identify one transfer shown in the figure that moves water from the land to the atmosphere. (1) Total [1]
4. Explain one way that water moves from the soil to a river. (2) Total [2]
5. A student writes: "Groundwater is the largest store of water on Earth, so countries can pump as much as they like." Explain one reason why the student is wrong. (3) Total [3]
6. Explain two characteristics of the hydrological cycle. (4) Total [4]
Answers
1. The only correct answer is B (1). A is percolation or groundwater flow; C is interception; D is overland flow.
2. Precipitation caught and held on the leaves and stems of plants before it reaches the ground (1). "Rain hitting trees" is too vague to score: say the water is held before reaching the ground.
3. Transpiration / evaporation (from the lake) (1). Condensation scores nothing here: it is drawn in the atmosphere and turns vapour into droplets, it does not move water from the land.
4. Rain soaks into the soil and moves sideways downslope through it as throughflow (1) because the soil is saturated or sits on less permeable rock, so water cannot sink further and drains towards the river channel (1). Accept overland flow developed: the soil is saturated, so further rain cannot infiltrate (1) and runs over the surface into the river (1).
5. The ocean, not groundwater, is the largest store, and most fresh water is ice; groundwater is about 30% of fresh water (1). Groundwater is refilled only slowly, by percolation of rain through soil and rock (1), so pumping it faster than it is recharged makes the water table fall and the store shrinks or runs out (1). Award the chain on recharge even without the first correction. No credit for "it will run out" with no link to recharge.
6. AO1 (2 marks)/AO2 (2 marks). Award 1 mark for each characteristic and a further mark for its development, twice.
- It is a closed system (1): the total amount of water stays the same and only moves between stores (1).
- It is driven by the Sun's energy (1): heat evaporates water from the sea and land, lifting vapour into the atmosphere (1).
- Gravity drives the return flows (1): precipitation falls and water runs downhill over and through the land to the sea (1).
- Water changes state (1): it moves between ice, liquid and vapour, for example condensing into clouds as air cools (1). Do not credit two developments of the same characteristic.