Forests purify water and help to regulate water flows to downstream areas. Forests, especially forest soils, act like massive filters, purifying water as it drips through the forest ecosystem. This filtration service provides drinking water to over 60 million of the world’s population who dwell in tropical rainforests and to some of the world’s largest cities, at least one-third of which depend on forest protected areas for their water supply. The trunks and roots of forest ecosystems also act like a sponge, controlling the flow of surface and ground water into river systems, which helps to regulate cycles of flood and drought. Furthermore, the recycling of water vapour by forests back into air currents helps to maintain rainfall regimes over vast areas. For example, much of the rainfall in the Andes that feeds glaciers and high-altitude populations has been recycled over lowland Amazonian forests.