The Hydrological Cycle (SL IB Environmental Systems & Societies (ESS))

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