4.1 Water systemsIB Environmental Systems and Societies SL: Revision notes

The hydrosphere is all the water on Earth: liquid, ice and vapour. Movements of water through it are driven by two things:

• Solar radiation: provides the energy for evaporation, melting, sublimation and transpiration, and sets up the winds that carry water vapour.
• Gravity: pulls water downhill as run-off and streamflow and downwards through soil and rock as infiltration and percolation, and brings precipitation to the surface.

The global hydrological cycle is a system made of stores (where water is held) and flows (processes that move water between stores). The total amount of water on Earth is almost constant; it is only moved around.

Main stores and the share of the Earth's water they hold:

• Oceans: 96.5%
• Glaciers and ice caps: 1.7%
• Groundwater: 1.7%
• Surface freshwater (lakes, rivers, wetlands): 0.02%
• Atmosphere: 0.001%
• Organisms: 0.0001%

Fresh water that humans can easily use is a tiny fraction of the total. A small store such as the atmosphere can still matter because water moves through it fast.

Flows you need to know:

• Evaporation: liquid water to vapour. Transpiration: water vapour lost from plants. Sublimation: ice or snow directly to vapour.
• Condensation: vapour to liquid droplets. Advection: horizontal movement of moist air.
• Precipitation: water falling to the surface. Melting and freezing: changes between ice and liquid.
• Surface run-off: water flowing over the land surface. Infiltration: water entering the soil. Percolation: water moving down through soil and rock. Streamflow: water flowing in a river channel. Groundwater flow: slow lateral movement of water through rock.

Human activities change flows and stores:

• Agriculture: irrigation removes water from rivers and aquifers; ploughing and compaction cut infiltration.
• Deforestation: less interception and transpiration, more surface run-off, more erosion and flooding, less groundwater recharge.
• Urbanization: paving and roofs stop infiltration; drains send water quickly to rivers, raising flood peaks; abstraction lowers groundwater.

A water body such as a lake or an aquifer is in a steady state when its inputs equal its outputs, so its volume stays constant even though water is flowing through it. This is shown by a flow diagram of inputs and outputs, with arrow sizes showing the size of each flow.

If human activity makes outputs exceed inputs (for example, more abstraction), the store shrinks until a new balance is reached. In exam questions, add up inputs and outputs separately, give units, and state what the comparison shows.