Key Terms: The Water Cycle (AQA A Level Geography): Revision Note

Systems approach - key terms glossary

Cascading System – A series of subsystems where the output from one becomes the input for another, such as sediment from a river entering a coastal system.

Closed System – A system where energy can enter or leave, but matter cannot. The global hydrological cycle is an example.

Dynamic Equilibrium – A state of balance in a system where small, constant changes maintain the overall system condition.

Energy – The power or driving force in a system, such as solar radiation.

Feedback Loop – A mechanism that maintains or alters system equilibrium. Positive feedback amplifies changes; negative feedback resists change.

Input – Matter or energy entering a system, like precipitation entering a drainage basin.

Isolated System – A theoretical system with no exchange of matter or energy, e.g. a coconut.

Open System – A system that exchanges both energy and matter with its surroundings.

Output – Matter or energy leaving a system, such as runoff from a basin.

Store/Component – Elements where matter or energy is held, e.g. soil or vegetation.

System – A collection of interacting components with inputs, outputs, and flows.

Throughput – Movement of matter or energy through a system.

The global water cycle - key terms

Aquifer – A water-bearing rock that can store and transmit groundwater. May be confined or unconfined.

Atmosphere – The layer of gases surrounding the Earth, which stores water vapour and facilitates condensation and precipitation.

Condensation – The process by which water vapour changes to liquid, forming clouds.

Cryosphere – The frozen parts of the Earth system, including glaciers and ice caps.

Evaporation – The process where liquid water turns into water vapour due to heat.

Freshwater – Water with low salt content, only 2.5% of all Earth's water.

Groundwater – Water stored in the pore spaces of soil and rock beneath the surface.

Hydrological Cycle – The continuous movement of water on, above and below the Earth’s surface.

Hydrosphere – All water on Earth including oceans, lakes, rivers, groundwater, and glaciers.

Permeability – The ability of rock or soil to allow water to flow through it.

Porosity – The volume of pore spaces in soil or rock that can hold water.

Precipitation – Any form of water, such as rain, snow, or hail, that falls from the atmosphere to the Earth.

Surface Store – Places where water is held on the surface, such as lakes, puddles or reservoirs.

Changes in magnitude of water stores - key terms

Climate change – Long-term alteration in climate patterns affecting global and local water stores.

Condensation - Transfer of water from a gaseous (vapour) state to a liquid state as in cloud formation

Cryospheric processes – Natural processes affecting ice masses, such as melting or calving.

Evapotranspiration – The combined process of evaporation from land and transpiration from plants.

Groundwater flow – Slow horizontal movement of water through rock below the water table.

Infiltration – The movement of water into the soil from the surface.

Overland flow – Water that flows across the ground surface when rainfall exceeds infiltration capacity.

Percolation – The downward flow of water through soil into underlying rocks.

Precipitation - Transfer of water from the atmosphere to the Earth's surface in the form of rain, hail, snow and dew

Residence time – The average time water spends in a particular store, e.g. groundwater or atmospheric moisture.

Sublimation – The change of water directly from solid (ice) to gas (vapour) or vice versa, without becoming liquid.

Throughflow – Lateral movement of water through the soil towards a river channel.

Trunk or stem flow - This is the flow of water down the stems of plants or trunks of trees.

Zone of saturation - The height of the water table will vary according to the season. Where there is permanent saturation, this is called the phreatic zone

Drainage basin system - key terms

Baseflow – The main supply of water to the channel under normal conditions.

Confluence – The point where two rivers meet.

Drainage basin – The area of land drained by a river and its tributaries. Also called a catchment area.

Drainage density – A measure of how many streams or rivers exist in a drainage basin.

Interception – The capture of precipitation by vegetation or buildings before it reaches the ground.

Mouth – The point where a river flows into another water body like a sea or lake.

Source – The beginning or start point of a river.

Surface storage - when water is stored on the surface in natural features such as lakes, puddles, reservoirs and wetlands but also in urban areas because of impermeable surfaces 

Tributary – A smaller river or stream that flows into a larger one.

Watershed – The boundary dividing one drainage basin from another.

Hydrographs - key terms

Annual Hydrograph – Also called a river regime; shows the pattern of a river's discharge over a year.

Antecedent conditions - The environmental or climatic features which existed before a precipitation event.

Discharge – The volume of water flowing in a river, measured in cumecs (m³/s).

Flashy Hydrograph – A storm hydrograph with a short lag time and high peak discharge, indicating rapid runoff.

Flat Hydrograph – A storm hydrograph with a long lag time and low peak discharge, indicating slower response.

Lag Time – The delay between peak rainfall and peak discharge in a river.

Peak Discharge – The highest flow in a river following a rainfall event.

Rising Limb – The part of a hydrograph showing increasing discharge.

Storm Hydrograph – A graph showing river discharge before, during and after a storm.

Impacts on the water cycle - key terms

Deforestation – The removal of trees, reducing interception and transpiration and increasing runoff.

Desertification – Degradation of land in arid areas, reducing its ability to store and absorb water.

Ecosystem change – Natural changes in vegetation and wildlife which can influence water movement and storage.

Industrialisation – Human activity that increases emissions, alters precipitation, and affects water quality.

Irrigation – Artificial application of water to land, often drawing from aquifers and altering local water balances.

Over-abstraction – Excessive removal of water from natural sources, especially groundwater.

Urbanisation – Development that increases impermeable surfaces, reducing infiltration and increasing surface runoff.