How does water move through the hydrosphere, and why is fresh water such a limited resource?
The distribution and stores of water on Earth, the water cycle and the processes that move water between stores, the limited availability of fresh water, and the role of oceans in climate.
A focused answer to AQA A-Level Environmental Science 3.2.2, covering the distribution of water on Earth, the water cycle and its processes, the limited supply of fresh water, and the role of the oceans in regulating climate.
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What this dot point is asking
AQA wants you to describe how water is distributed between stores on Earth, explain the water cycle and the processes that move water between stores, explain why usable fresh water is limited, and describe the role of the oceans in regulating climate. Examiners value precise figures for the distribution of water and the ability to reason with a simple water balance.
Distribution of water on Earth
The stores differ enormously in size and in how long water stays in them (residence time). Ocean water has a residence time of thousands of years; deep groundwater can be even longer; but water in the atmosphere stays only about nine days on average, which is why the atmosphere is a small but fast-moving store. Because most fresh water is frozen or deep underground, the usable supply is both small and unevenly distributed, which is why fresh water is treated as a limited resource despite the planet being mostly water.
The water cycle
Solar energy powers evaporation and lifts water into the atmosphere, while gravity drives precipitation and the flow of water back to the oceans. Within a catchment the inputs and outputs can be written as a water balance: precipitation is partitioned into evapotranspiration, groundwater recharge and runoff (plus any change in stored water). This balance is the basis of catchment management and the worked example below. Water is neither created nor destroyed; the same water is recycled indefinitely between the atmosphere, land and oceans.
Limited fresh water
Although the total amount of water is vast, usable fresh water is scarce because most is salt water or frozen. Three factors compound the problem:
- Uneven distribution: some regions have abundant rainfall while arid regions have very little, and supply often does not match where people live.
- Rising demand: agriculture (the largest user, mainly irrigation), industry and growing urban populations all draw on the same limited supply.
- Degradation of supply: over-abstraction lowers water tables and can cause saltwater intrusion, while pollution contaminates rivers and aquifers, removing water from usable supply.
These pressures mean water stress and conflict over shared rivers and aquifers are growing environmental issues.
The role of the oceans in climate
The oceans have a very high specific heat capacity, so they absorb, store and release enormous quantities of heat with only small temperature changes. This makes them a thermal buffer that moderates climate. Ocean currents form a global conveyor: warm surface currents (such as the Gulf Stream) carry heat from the tropics towards the poles, warming coastal regions such as north-west Europe, while cold, dense, deep currents return towards the equator. The oceans also absorb a large share of atmospheric carbon dioxide, buffering climate change but causing ocean acidification. Through heat storage, currents and carbon uptake, the oceans are central to regulating global climate, not merely a store of water.
Try this
Q1. State approximately what percentage of the Earth's water is salt water in the oceans, and where most fresh water is stored. [2 marks]
- Cue. About 97 percent is salt water; most fresh water is locked in ice caps, glaciers and groundwater.
Q2. Explain how ocean currents help to moderate the climate of coastal regions. [3 marks]
- Cue. Warm currents carry tropical heat polewards (for example the Gulf Stream warming north-west Europe), and the oceans' high heat capacity buffers temperature change, evening out climate.
Q3. A catchment receives 800 mm of precipitation, loses 480 mm to evapotranspiration and recharges 80 mm to groundwater. Calculate the surface runoff and its percentage of precipitation. [3 marks]
- Cue. Runoff ; percentage .
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20186 marksExplain why, although the Earth has a vast amount of water, only a tiny fraction is available as usable fresh water, and describe two ways human activity reduces the usable supply.Show worked answer →
A 6-mark answer needs the distribution argument plus two human pressures.
Distribution. About 97 percent of the Earth's water is salt water in the oceans, which is unusable without expensive desalination. Of the remaining 3 percent that is fresh, roughly two-thirds is locked in ice caps and glaciers and most of the rest is groundwater, some of it deep or saline. That leaves well under 1 percent of total water as accessible surface fresh water in rivers, lakes and shallow aquifers. Fresh water is also unevenly distributed, so even this small fraction is not where many people live.
Human reduction of supply. (1) Over-abstraction: removing groundwater faster than it recharges lowers water tables, dries wells and can cause saltwater intrusion into coastal aquifers, permanently spoiling them. (2) Pollution: discharge of sewage, fertilisers, industrial chemicals and plastics contaminates rivers and aquifers, making water unsafe to use without treatment, so it is effectively lost from supply.
Markers reward (1) the 97 percent salt figure, (2) recognising ice and groundwater lock up most fresh water, (3) the small accessible surface fraction, and (4) two distinct, explained human pressures.
AQA 20224 marksA catchment receives 900 mm of precipitation per year. Of this, 540 mm is lost by evapotranspiration and 90 mm recharges groundwater. Calculate the surface runoff in mm and as a percentage of precipitation.Show worked answer →
A 4-mark calculation needs a water-balance approach with both results.
- Water balance
- Precipitation is split between evapotranspiration, groundwater recharge and surface runoff (assuming no change in storage). So runoff precipitation evapotranspiration recharge.
- Runoff in mm
- Runoff .
- As a percentage
- .
Markers reward setting up the balance correctly, the arithmetic (), and the percentage (). A common error is forgetting groundwater recharge and reporting .
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