Hydrological Cycle (DP IB Environmental Systems & Societies (ESS))
Revision Note
Hydrological Cycle
Water on Earth
The hydrosphere includes all Earth's water, such as oceans, rivers, lakes and atmospheric moisture
Fresh water only makes up a small fraction (approximately 2.5% by volume) of the Earth’s water storages
Of this fresh water, approximately 69% is stored in glaciers and ice sheets and 30% is stored as groundwater
The remaining 1% of freshwater is in rivers, lakes and the atmosphere
All water is part of the hydrological cycle
Driving forces of the hydrological cycle
Gravity and solar radiation both influence the movement of water in the hydrosphere
The Sun's heat causes water to evaporate from oceans, lakes, and rivers
Water vapour cools and condenses into clouds, releasing heat
Gravity pulls condensed water back to Earth via the process of precipitation (rain, snow, sleet, or hail).
Gravity causes water to flow over land into rivers and streams (runoff) and drain through soil
Rivers flow downhill due to gravity, moving water from inland back to the oceans
Components of the hydrological cycle
The global 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 stores occurring within the hydrological cycle are:
Oceans
Glaciers and ice caps
Groundwater and aquifers
Surface freshwater (rivers and lakes)
Atmosphere
The main flows occurring within the hydrological cycle are:
Transformations: processes where the state or form of water changes, e.g.
Evaporation (the sun evaporates surface water into vapour)
Condensation (water vapour condenses and precipitates)
Transfers: movements of water from one location to another without changing state, e.g.
Water runs off the surface into streams and reservoirs or beneath the surface as ground flow
These flows move the water on Earth from one store to another (river to ocean or ocean to atmosphere)
Flows in the hydrological cycle
Flows in the hydrological cycle include the following:
Flows in the Hydrological Cycle
Flow | Type | Description |
|---|---|---|
Evaporation | Transformation | 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 | Transformation | 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 | Transformation | The combined process of water vaporisation from the Earth's surface (evaporation) and the release of water vapour by plants ( transpiration) |
Sublimation | Transformation | The direct transition of water from a solid state (ice or snow) to a vapour state without melting first |
Condensation | Transformation | The process by which water vapour in the atmosphere transforms into liquid water, forming clouds or dew, as a result of cooling |
Melting | Transformation | The process by which solid ice or snow changes into liquid water due to an increase in temperature |
Freezing | Transformation | The process by which liquid water changes into a solid state (ice or snow) due to a decrease in temperature |
Advection | Transfer | The wind-blown movement of water vapour or condensed/frozen water droplets (clouds) |
Precipitation | Transfer | The process of water falling from the atmosphere to the Earth's surface in the form of rain, snow, sleet, or hail |
Surface run-off | Transfer | The movement of water over the Earth's surface typically occurs when the ground is saturated or impermeable, leading to excess water |
Infiltration | Transfer | The process of water seeping into the soil from the surface, entering the soil layers and becoming groundwater |
Percolation | Transfer | The downward movement of water through the soil and underlying rock layers, eventually reaches aquifers or groundwater reservoirs |
Streamflow | Transfer | 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 |
Groundwater flow | Transfer | The movement of water through the pores and spaces in underground soil and rock layers, often moving towards rivers, lakes or oceans |
Exam Tip
Remember that percolation and infiltration are not the same. Percolation happens after the water has infiltrated the soil.
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