Chapters
The water cycle is a crucial part of the A Level curriculum as well as being important for us to all understand as part of our daily lives. We all rely on a working water cycle and whether we appreciat eit or not, it is a huge part of everyone’s lives.
Our guide is aimed at AQA students looking to understand the water cycle.
Global Distribution and Size of Major Stores of Water
Storage of water is one of the most important aspects. Water on Earth is stored in four key areas: the lithosphere, hydrosphere, cryosphere, and atmosphere. Each of these stores plays a role in maintaining the water cycle.
- Lithosphere: This is the Earth's crust and upper mantle. Water is stored here in groundwater, aquifers, and soil moisture, though it accounts for only a small percentage of global water.
- Hydrosphere: The hydrosphere refers to all liquid water on Earth, primarily found in oceans. Oceans hold around 97% of Earth’s water, making them the largest store.
- Cryosphere: The cryosphere includes all frozen water, such as glaciers, ice caps, and permafrost. Around 1.7% of the planet’s water is stored here, playing a crucial role in regulating global climate patterns.
- Atmosphere: The atmosphere contains water vapour, which makes up a small percentage of the Earth’s water, but it is essential in the water cycle due to its role in cloud formation and precipitation.
Processes Driving Changes in Water Stores
The magnitude of these water stores fluctuates due to processes that drive water between them. We all see this on a very basic level, with things like rain and condensation.
| Process | Explanation |
|---|---|
| Evaporation | The process where liquid water is heated and transformed into water vapour. It occurs primarily from oceans and other water bodies, transferring water into the atmosphere. |
| Condensation | Water vapour cools and turns back into liquid, forming clouds. This process is crucial for precipitation. |
| Cloud Formation | As water vapour rises and cools, it condenses to form tiny water droplets or ice crystals, creating clouds. |
| Precipitation | Water falls from the atmosphere as rain, snow, sleet, or hail. The cause of precipitation often stems from moisture-laden air masses cooling as they rise over mountains or encounter colder air. |
| Cryospheric Processes | Cryospheric processes refer to the movement of water through frozen regions. Ice caps and glaciers advance or retreat and in the process release or store water, and melting ice contributes to sea-level rise. These processes operate on various timescales, from short-term (seasonal) to long-term (centuries). |
Drainage Basins as Open Systems
A drainage basin is an open system where water flows in and out, creating a cycle of inputs, outputs, and storage.
Precipitation is the main input, entering the system as rain or snowfall.
Evapotranspiration (water evaporating and plants releasing water vapour) and runoff are key outputs.
In a drainage basin, water is stored in various forms:
Interception – Water held temporarily by vegetation.
Surface storage – Puddles, lakes, and reservoirs.
Soil and groundwater – Water stored in soil and underground aquifers.
Channel storage – Water held in rivers and streams.
Flows – Water moves through the system in different ways:
Infiltration – Water soaks into the soil.
Stemflow – Water flowing down plant stems and trunks.
Overland flow – Water flowing across the surface to rivers and streams.
Channel flow – Water moving through rivers towards the sea.
The concept of the water balance involves a delicate relationship between inputs, stores, and outputs within a drainage basin.
Runoff Variation and the Flood Hydrograph
The flood hydrograph is a graphical representation showing how a river's discharge changes in response to precipitation.
Rising Limb – This shows the increasing discharge as rainfall flows into the river.
Peak Discharge – The highest flow in the river after rainfall.
Lag Time – The delay between peak rainfall and peak discharge.
Receding Limb – The falling discharge as water moves downstream and less water enters the river system.
Runoff variations depend on factors such as rainfall intensity and land use. A short lag time and steep rising limb can indicate a greater risk of flooding, for example.
Flood and storm hydrographs are absolutely crucial when it comes to scientific modelling and understanding flood risks.
Changes in the Water Cycle Over Time
There are natural changes that occur in the water cycle, and seasonal changes influence evaporation and precipitation. Natural weather and storm events lead to short-term spikes in water movement also.
Human Impact
The human impact on the water cycle is significant and can include:
Farming Practices: Intensive agriculture, deforestation, and soil compaction reduce infiltration and increase surface runoff, affecting drainage basins.
Land Use Change: Urbanisation increases impermeable surfaces, which leads to more runoff and reduces groundwater recharge.
Water Abstraction: Excessive extraction from rivers and aquifers for agricultural, industrial, or domestic use can alter natural flows and deplete stores.
The Global Water Cycle
The global water cycle is a closed system where water continuously circulates between the atmosphere and land. Water changes state constantly, between solid, liquid, and gas, and this process is driven by solar energy and gravity (rain, for instance).
Different types of water stored in the global water cycle include:
Oceanic Water. Oceans are the main driver of the water cycle due to their vast surface area and high evaporation rates.
Cryospheric Water. Frozen water in glaciers and ice caps stores vast amounts of freshwater, playing a crucial role in regulating sea levels.
Terrestrial Water. Water stored in lakes, rivers, and groundwater provides freshwater for ecosystems and human activities.
Atmospheric Water. The small percentage of atmospheric water (as vapour) plays an outsized role in transferring water between the other stores through precipitation and condensation.
Storm and Flood Hydrographs
A storm hydrograph illustrates the impact of a rainfall event on a river’s flow. This can be used to better understand the impacts of storms and the condition of bodies of water, including flood risks.
Key terms to understand include:
- Rising Limb: Indicates the speed at which water enters the river.
- Peak Discharge: The maximum flow in the river.
- Lag Time: The delay between rainfall and peak discharge.
- Receding Limb: This represents the decline in flow as water leaves the system.
Factors such as land use, vegetation cover, and soil type influence the shape of a hydrograph. Urban areas with impermeable surfaces typically experience shorter lag times and higher peak discharges, increasing flood risk.
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