The Hydrological (Water) Cycle
Water continuously cycles between the oceans, atmosphere, land and living organisms.
The hydrological cycle describes the continuous movement of water through the Earth system. It is driven by solar energy (for evaporation and plant transpiration) and gravity (for precipitation and runoff).
Key processes:
| Process | Description |
|---|---|
| Evaporation | Water changes from liquid to vapour, mainly from ocean surfaces |
| Transpiration | Water vapour released by plants through stomata |
| Evapotranspiration | Combined evaporation + transpiration |
| Condensation | Water vapour cools and forms clouds (water droplets on dust particles) |
| Precipitation | Water falls as rain, snow, sleet, hail |
| Surface runoff | Water flows over land into rivers and lakes |
| Infiltration | Water soaks into soil |
| Percolation | Water moves through soil into groundwater (aquifers) |
| Groundwater flow | Slow movement of water through rock and soil toward sea |
| Interception | Vegetation catches precipitation before it reaches the ground |
Water balance: In any given area: Precipitation = Evapotranspiration + Runoff + Change in storage.
- Areas with P > ET = water surplus (humid climates, e.g. UK, tropical rainforests).
- Areas with ET > P = water deficit (arid regions, e.g. Sahara, Middle East).
How human activities affect the water cycle:
- Deforestation: Reduces interception and transpiration β more runoff β flooding; aquifers not recharged as efficiently.
- Urbanisation: Impermeable surfaces β surface runoff instead of infiltration β aquifers not recharged; flooding increases.
- Irrigation: Removes water from rivers and aquifers β reduces flow; returned water may be saline.
- Climate change: Alters precipitation patterns; melts glaciers (short-term increase in meltwater, long-term reduction).
- Solar energy drives evaporation; gravity drives precipitation and runoff.
- Key processes: evaporation β condensation β precipitation β runoff/infiltration.
- Deforestation: less interception and transpiration β more flooding, less groundwater recharge.
- Urbanisation: impermeable surfaces β runoff not infiltrating β flooding.