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 transfers (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 processes occurring within the hydrological cycle are:
- Evaporation - the sun evaporates surface water into vapour
- Condensation - water vapour condenses and precipitates
- Flows - water runs off the surface into streams and reservoirs or beneath the surface as ground flow
- These processes transfer 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
- Sublimation
- Evaporation
- Condensation
- Advection
- Precipitation
- Melting
- Freezing
- Flooding
- Surface run-off
- Infiltration
- Percolation
- Stream-flow or currents
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 |
Exam Tip
Remember that percolation and infiltration are not the same. Percolation happens after the water has infiltrated the soil.