In the Hydrologic Cycle, the Maximum Sustainable Yield is the amount of Fresh Water that can be processed by aquifers or wetlands and returned to the environment in lakes or rivers in a given period of time. Since every environment processes water differently, various locations will have different Maximum Sustainable Yields, and an increase or decrease in the Yield can have major effects on both the local ecology and the ability of the location to continue its Yield consistently.
A wetland, for example, processes water mainly through filtration through the saturated ground. Heavier elements fall while the water remains high, and the fresh water is taken up by plants and animals.
In the ocean, the water is highly saturated with salt, and while there are organisms that live in salt water, the majority of land animals need fresh water to survive. Water evaporates from the ocean, leaving the salt behind, and is precipitated onto land and lakes by rain and snow. Another method of filtering water is through ice; since the salt cannot freeze with the water, all icepacks, icebergs, and glaciers are composed of fresh water, which can float to warmer areas and melt into fresh water, some of which evaporates.
In all cases, Sustainable Yield is damaged when the ecosystem changes. Development of wetlands erases the fresh water that can be taken from them, while damming to create lakes can collect too much water into one area to be processed by natural means. Dry areas, like Australia's Sydney Catchment Authority, use technology to collect and purify water in large amounts. The collection of water in reservoirs often creates new Sustainable Yields as the land around the reservoir adjusts to the new ecology.
To promote the best Sustainable Yield, there has been a recent push to reclaim many wetlands where mass processing of water takes place. By allowing the natural processes of water purification and freshining to continue, we can keep the environment alive and continue to enjoy natural, fresh water.