NICOLLE R. FULLER / SCIENCE PHOTO LIBRARY NICOLLE R. FULLER / SCIENCE PHOTO LIBRARY
Fresh water sources. Cutaway illustration of sources of fresh water use by humans, mostly living in cities (far left). Fresh water falls as rain and snow over mountains (upper right). Around 1 percent enters lakes and rivers, and is harnessed by dams, reservoirs and irrigation canals (centre and lower left). Groundwater is used during droughts, but overuse means that reserves are shrinking. Dashed cloud outlines show predicted precipitation changes (more rain and less snow) as the Earth's climate warms.
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