Why is water stored in reservoirs for several years at a time?

Water reservoirs are used to manage the fluctuating supply of water over long periods of time. By storing excess water when it is available, and holding it for use in times when no excess occurs, a more or less constant supply can be made available for use.

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The time frame depends on several factors, many overlapping. These factors include reservoir capacities, rainfall, drought cycles, human migrations, pollution, usage rates, and anticipated future supplies and demands.

Storage capacity (reservoirs) and water management policies can buffer the supply vs. demand tension by holding and protecting water gathered in times of high rainfall to ensure that adequate supplies are available during times of drought.

Even in areas that have relatively long drought cycles, adequate supplies can usually be maintained by exercising proper management of water resources. However, water shortages occur somewhere in the world every year. Thailand is a current example.

As the world's population grows, these shortages will become more frequent and more severe. But there will be enough water for everyone for a long time if we can develop new technology and all learn to manage our existing water resources more efficiently.