There are many stores, or reservoirs of water on Earth, ranging from oceans to ice caps and glaciers, living organisms and the atmosphere. Water is is not just fixed to these reservoirs but can transfer between them in a system known as the hydrological cycle. Processes driving this cycle include evaporation, evapotranspiration, condensation, precipitation, runoff and percolation.
Looking firstly to the oceans, we find that evaporation causes water to transfer from the oceans to the atmosphere as water vapour, with water vapour also entering the atmosphere through evapotranspiration via plants. Water then condenses and leaves the atmosphere as precipitation. This can occur as rain water, which enter rivers as runoff or soaks into the ground through infiltration, or as snowfall which accumulates on top of glaciers and ice sheets. This water eventually returns to the oceans and consequently the cycle repeats.
Water can stay in reservoirs for thousands of years before transferring to another, with the time water spends in a reservoir being known as its residence time.
However human driven changes in the environment and increased consumption have had major impacts on the water cycle, raising concerns over water scarcity and the increased likelihood of future water shortages.
Reservoirs – A store of material that is separate from other stores either in physical location or chemical nature
Evaporation – The processes of a liquid turning into a gaseous state
Evapotranspiration – The process of water moving from the soil, into a plant and then into the atmosphere as water vapour
Condensation – The process of a gas turning into a liquid
Precipitation – Any form of liquid water falling from the atmosphere due to gravity
Runoff – The movement of water from rain, snow, sleet and other sources across the land surface
Percolation – The process of water flowing through the soil due to gravity
Infiltration – The process of water entering into the soil
Residence Time – The average amount of time that a material spends in a store