The hydrological cycle: a closed system and its stores revision notes

Edexcel International GCSE Geography (4GE1), chapter 1: River environments, specification point 1.1a. Free lesson.

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The hydrological cycle is the movement of water between the sea, air and land. It is a closed system, and understanding its stores and transfers helps you describe how water moves around Earth.

Why the hydrological cycle is closed

The hydrological cycle is the movement of water between the sea, the air and the land. It is a closed system because no water enters Earth from outside and none leaves. The total amount of water stays the same, although its location changes. Energy from the Sun lifts water into the air, and gravity brings it back down.

The seven stores of water

A store is a place where water is held for a time. The seven stores differ in where the water is and how it is held. An aquifer is rock that holds water in its small spaces and cracks. Oceans hold salt water and are by far the largest store. Ice caps and glaciers hold fresh water frozen solid, in polar regions and on high mountains. Groundwater is water held beneath the ground in rock. Lakes and rivers are visible surface water. Soil moisture is held between soil grains, where plant roots can reach it. The atmosphere holds water vapour and the tiny droplets…

How Earth's water is distributed

The stores are very unequal. About 97% of all Earth's water is salt water in the oceans, leaving about 3% as fresh water. Of that fresh water, most is frozen or held underground. This means the fresh water easiest to reach in rivers and lakes is only a tiny part of all Earth's water. Ice caps and glaciers hold about 69% of fresh water. Groundwater holds about 30%. Everything else holds about 1%. This includes lakes, rivers, soil, the air and living things.

How to tell a store from a transfer

Every part of the cycle is either a store or a transfer, also called a flow. A store tells you where water is held. A transfer tells you how water moves from one store to another. Examples of stores include oceans, ice and groundwater. Evaporation and precipitation are transfers. Precipitation includes rain, snow and hail. Check what the question asks for: a store is a place, while a transfer is a movement. Giving one when asked for the other earns no marks.

How to explain a store

For a question about a store, name a place where water is held and explain how the water gets there. Evaporation is not a store because it describes water moving. Water can, for example, be held as ice in glaciers when fallen snow does not melt.