Have you ever wondered what happens to rainwater? Some of it, particularly when it rains hard, runs off to rivers and streams, potentially causing floods. But, the fate of the rest is subtle—some evaporates from the surface and some soaks into the ground, feeding plants and replenishing groundwater.
Recently, I’ve been examining studies that have assessed plant uptake of water. On the average, from deserts to tropical rainforests, about 60% of the rain that falls on Earth surface is taken up by plants. Plants transpire water into the atmosphere—a special form of evaporation that accompanies photosynthesis. When plants are removed, as in forest cutting, runoff increases, sometimes by 25 to 40%. Additional waters may also percolate to groundwater.
So, what would a world with fewer plants be like? We can get a feel for this, when we visit cities. Without the uptake and transpiration of water by trees, which carries away a lot of heat, the air in cities is typically warmer than in forested areas in the same region. We call this the heat island effect of cities.
Plants slow the runoff of water, allowing greater infiltration into the soil. Areas devoid of vegetation are often prone to flash floods.
The transpiration from plants also adds water vapor to the atmosphere, which is the source of rainfall in downwind areas. We could expect that the removal of large areas of vegetation would reduce regional rainfall totals.
Indeed, land plants moderate the environment for all of us, both local and distant from where they grow.
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Photo, taken on January 27, 2008, courtesy of Samantha Celera via Flickr.
Earth Wise is a production of WAMC Northeast Public Radio. Support for Earth Wise comes from the Cary Institute of Ecosystem Studies in Millbrook, NY.