What you'll learn

• Why the global hydrological cycle is described as a closed system.
• The main stores and transfers of water.
• The key features of a drainage basin, including the source, watershed, channel network and mouth.
• How precipitation, temperature, vegetation, land use, water abstraction and dams affect river regimes and storm hydrographs.

1. The hydrological cycle as a closed system

The hydrological cycle is the continuous movement of water between the land, oceans, atmosphere and living things. At a global scale, Earth’s water is recycled again and again: water changes location and state — for example from liquid water to water vapour — but the total amount of water stays almost the same.

At IGCSE level, the key idea is simple: the world’s water supply is finite. The same water is constantly moved, stored and recycled.

2. Stores: where water is held

A store is a place where water is held for a period of time. Some stores hold water for days or weeks; others hold it for thousands of years.

Main global water stores

• Oceans: the largest store, containing about 97% of Earth’s water, mostly salty.
• Ice and snow: water stored in glaciers, ice sheets and snowfields.
• Groundwater: water stored underground in rocks and pores in soil.
• Soil water: water held in the soil, available to plants.
• Rivers and lakes: smaller surface stores, but very important for people and ecosystems.
• Atmosphere: water vapour, droplets and ice crystals in the air.
• Vegetation: water stored in plants.

3. Transfers: how water moves between stores

A transfer is a movement of water from one store to another. Transfers are the “arrows” in the hydrological cycle.

Key transfers you need to know

• Evaporation: liquid water changes into water vapour due to heat, especially from oceans, lakes and rivers.
• Transpiration: water vapour is released from plants through tiny openings in leaves.
• Evapotranspiration: the combined total of evaporation and transpiration.
• Condensation: water vapour cools and changes into tiny water droplets, forming clouds.
• Precipitation: water falls from the atmosphere as rain, snow, sleet or hail.
• Infiltration: water soaks from the ground surface into the soil.
• Percolation: water moves deeper through soil and rock into groundwater stores.
• Surface runoff: water flows over the land surface into streams and rivers.
• Throughflow: water moves sideways through the soil towards a river.
• Groundwater flow: water moves slowly through rocks underground.
• River discharge: water flows through a river channel towards the sea or a lake.

4. Drainage basins: the hydrological cycle at river scale

A drainage basin is the area of land drained by a river and its tributaries. You can think of it as the land area that “feeds” water into one river system.

At the drainage basin scale, the system is usually treated as an open system. It has inputs, stores, transfers and outputs.

• Input: precipitation entering the basin.
• Stores: soil water, groundwater, lakes, vegetation and channel storage.
• Transfers: infiltration, overland flow, throughflow, groundwater flow and river flow.
• Outputs: river discharge leaving the basin at the mouth, plus evapotranspiration.

Key drainage basin features

• Source: the place where a river begins, often in upland areas.
• Watershed: the boundary between one drainage basin and another, usually along higher land.
• Channel network: the pattern of the main river and its tributaries.
• Tributary: a smaller stream or river that flows into a larger river.
• Confluence: the point where two rivers or streams meet.
• Mouth: the place where a river flows into the sea, a lake or a larger river.

A named example you may see in the UK is the River Severn, which rises in the Cambrian Mountains in Wales and flows through western England to the Bristol Channel. Its drainage basin includes tributaries, uplands, towns, farmland and floodplains — all of which affect how water reaches the river.

5. River regimes: how discharge changes over time

A river regime is the seasonal pattern of a river’s discharge over a year. Discharge means the volume of water flowing past a point in a river each second, usually measured in cubic metres per second, written as m³/s.

For example, a river in a snowy mountain region may have high discharge in spring or summer when snow melts. A river in a tropical monsoon climate may have very high discharge during the wet season and much lower discharge during the dry season.

6. Storm hydrographs

A storm hydrograph shows how a river’s discharge changes after a rainfall event. It helps you link rainfall to river response.

Key storm hydrograph terms

• Baseflow: the normal river flow before the storm, mainly supplied by groundwater.
• Rising limb: the part of the graph where discharge increases after rainfall.
• Peak discharge: the highest discharge reached.
• Lag time: the time between peak rainfall and peak discharge.
• Falling limb: the part of the graph where discharge decreases after the peak.

A flashy hydrograph has a steep rising limb, high peak discharge and short lag time. This means water reaches the river quickly, increasing flood risk.

A subdued hydrograph has a gentler rising limb, lower peak discharge and longer lag time. This means water reaches the river more slowly.

7. Factors affecting river regimes and hydrographs

Different drainage basins respond differently to rainfall. The same storm can cause a rapid flood in one basin but a slower, smaller rise in another.

Precipitation

Precipitation is one of the strongest controls on river flow.

• Heavy rainfall increases surface runoff and raises discharge.
• Long-duration rainfall can saturate the soil, reducing infiltration and increasing overland flow.
• Snowfall may be stored as snow or ice, delaying river flow until temperatures rise.

In areas with intense rainfall, such as parts of Bangladesh during the monsoon season, rivers can rise rapidly. In contrast, rivers in dry climates may have very low discharge for long periods.

Temperature

Temperature affects how much water is lost through evapotranspiration and whether precipitation is stored as snow or ice.

• Higher temperatures usually increase evaporation and transpiration, reducing river discharge.
• In cold areas, water may be stored as snow and ice, lowering winter discharge.
• When snow melts in spring or summer, discharge can rise sharply.

Vegetation

Vegetation can slow water down before it reaches the river.

• Leaves and branches cause interception, where rainfall is temporarily stored on vegetation surfaces.
• Roots help create spaces in the soil, increasing infiltration.
• Plants return water to the atmosphere through transpiration.

Forested basins often have longer lag times and lower peak discharge than deforested basins, if other factors are similar.

Land use

Land use means how people use the land, such as farming, forestry, housing, roads or industry.

• Urban areas have impermeable surfaces like concrete and tarmac, so infiltration is reduced.
• Drains and gutters move water quickly into rivers, shortening lag time.
• Compacted farmland can reduce infiltration and increase surface runoff.
• Bare soil after vegetation clearance is more exposed to erosion and rapid runoff.

Water abstraction

Water abstraction means taking water from rivers, lakes or groundwater for human use, such as drinking water, irrigation or industry.

Abstraction can reduce river discharge, especially during dry seasons. For example, heavy water use in the Colorado River basin in the USA and Mexico has reduced flows downstream; in some years, very little water reaches the Gulf of California. Exact figures vary by year and management decisions.

Dams and reservoirs

A dam is a barrier built across a river to store or control water. The reservoir behind it stores water that can be used for water supply, irrigation, flood control or hydroelectric power.

Dams often reduce peak discharge downstream because storm water is stored and released more slowly. They can also increase dry-season flows if water is released from the reservoir. A well-known example is the Aswan High Dam on the River Nile in Egypt, which has helped control seasonal flooding and provide water for irrigation, while also changing sediment movement and downstream ecosystems.