Evaporation, transpiration and precipitation are flows that transfer water between storages in the hydrological cycle. Evaporation and transpiration move water into the atmosphere, while precipitation returns atmospheric water to Earth's surface.
This is core IB ESS SL and HL content in Subtopic 4.1, Water systems. A flow is a process that moves water between storages, such as oceans, the atmosphere, soil, groundwater, glaciers and organisms. Solar radiation and gravity drive these movements.
| Flow | Mechanism and direction |
|---|---|
| Evaporation | Solar energy converts liquid water into water vapour. Water moves mainly from oceans, lakes, rivers and soil into the atmosphere. |
| Transpiration | Water absorbed by plant roots moves through the plant and is released as water vapour through stomata in leaves. It transfers water from soil, through organisms, into the atmosphere. |
| Precipitation | Atmospheric water returns to Earth's surface as rain, snow, sleet or hail. Gravity moves it into terrestrial or oceanic storages. |
Evaporation and transpiration are sometimes combined as evapotranspiration. Before precipitation occurs, water vapour cools and undergoes condensation, forming liquid droplets or ice crystals in clouds.
These three processes are part of a larger system. Other hydrological flows include sublimation, condensation, advection, melting, freezing, surface run-off, infiltration, percolation, streamflow and groundwater flow. Human activities can alter their rates: for example, deforestation reduces transpiration, while urbanization increases surface run-off and reduces infiltration.
A common misconception is that evaporation, transpiration and precipitation are water storages. They are processes, so they are flows; the ocean, atmosphere and soil are storages.
Exam technique: For a command term such as outline or explain, state the starting and ending storage and identify the driving mechanism. In a systems diagram, draw storages as rectangular boxes and flows as labelled arrows pointing in the direction of water movement.
