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Water that flows over land until it reaches lakes, rivers, or other areas such as oceans.
The amount of surface runoff that we can generally count on as a source of freshwater from year to year.
Zone of aeration
The upper zone which usually isn't completely filled with water, but with rocks and soil too.
Underground surface below which the ground is wholly saturated with water or the upper layer of the saturated zone.
Subterranean, porous, water-holding rocks that provide millions of wells with steady flows of water.
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