Syllabus Edition
First teaching 2015
Last exams 2025
The Hydrological Cycle (DP IB Environmental Systems & Societies (ESS))
Revision Note
The Hydrological Cycle
Water on Earth
Fresh water only makes up a small fraction (approximately 2.5% by volume) of the Earth’s water storages
Of this fresh water, approximately 68.7% is stored in glaciers and ice sheets and 30% is stored as groundwater
The remaining 1.3% of freshwater is in rivers, lakes and the atmosphere
All water is part of the hydrological cycle
Sources of water on Earth
Comparison of the world's freshwater stores
Hydrological Cycle
The hydrological cycle is a closed system
Within the hydrological cycle, there are stores and flows
The hydrological cycle is a series of processes in which water is constantly recycled through the system
The cycle also shapes landscapes, transports minerals and is essential to life on Earth
The main flows occurring within the hydrological cycle are:
Transformations:
Evaporation - the sun evaporates surface water into vapour
Condensation - water vapour condenses and precipitates
Transfers:
Water runs off the surface into streams and reservoirs or beneath the surface as ground flow
These processes move the water on Earth from one store to another (river to ocean or ocean to atmosphere)
The hydrological cycle involves energy exchange, leading to local temperature fluctuations
As water evaporates, it absorbs energy from its surroundings
This effectively cools the environment
The reverse happens when water condenses (heat is released)
This heat exchange influences the local climate
The hydrological cycle
Storages
Storages in the hydrological cycle include:
Rivers, lakes and oceans
Groundwater (aquifers)
Soils
The atmosphere
Glaciers and ice caps
Organisms (e.g. trees)
Flows
Flows in the hydrological cycle include:
Evapotranspiration (transformation)
Sublimation (transformation)
Evaporation (transformation)
Condensation (transformation)
Melting (transformation)
Freezing (transformation)
Advection (transfer)
Precipitation (transfer)
Flooding (transfer)
Surface run-off (transfer)
Infiltration (transfer)
Percolation (transfer)
Stream-flow or currents (transfer)
Flows in the Hydrological Cycle
Flow | Description |
Evaporation | The process by which liquid water changes into a gaseous state (water vapour) and enters the atmosphere from water bodies such as oceans, lakes, and rivers |
Transpiration | The process by which plants absorb water from the soil through their roots and release it as water vapour through tiny openings called stomata in their leaves |
Evapotranspiration | The combined process of water vaporisation from the Earth's surface (evaporation) and the release of water vapour by plants through transpiration |
Sublimation | The direct transition of water from a solid (ice or snow) to a vapour state without melting first |
Condensation | The process in which water vapour in the atmosphere transforms into liquid water, forming clouds or dew, as a result of cooling |
Advection | The horizontal movement of water vapour, clouds, or precipitation caused by the prevailing wind patterns |
Precipitation | The process of water falling from the atmosphere to the Earth's surface in the form of rain, snow, sleet, or hail |
Melting | The process by which solid ice or snow changes into liquid water due to an increase in temperature |
Freezing | The process by which liquid water changes into a solid state (ice or snow) due to a decrease in temperature |
Flooding | The overflow of water onto normally dry land, often caused by heavy rainfall, melting snow, or dam failure |
Surface run-off | The movement of water over the Earth's surface, typically occurring when the ground is saturated or impermeable, leading to excess water |
Infiltration | The process of water seeping into the soil from the surface, entering the soil layers and becoming groundwater |
Percolation | The downward movement of water through the soil and underlying rock layers, eventually reaches aquifers or groundwater reservoirs |
Stream-flow or currents | The movement of water in streams, rivers, or other water bodies, driven by gravity and the slope of the land, ultimately leads to oceans or lakes |
Examiner Tip
Remember that percolation and infiltration are not the same. Percolation happens after the water has infiltrated the soil.
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