When raindrops fall on uninhabited terrain, many things can happen. Precipitation that lands on craggy mountainsides flows downhill to streams. Drops that hit soil often soak in; some of that water later evaporates, while much of the rest seeps through the earth to replenish aquifers and nourish springs. Raindrops that splash into rivers immediately join a headlong tumble toward the sea. Consider, however, raindrops that fall on populated areas. Development brings homes, shopping centers, streets, sidewalks, driveways, parking lots, tennis courts—all of which typically shed water more effectively than the proverbial duck's back. On the green space that's left, even small amounts of traffic—tractors, golf carts, lawn mowers, mere human footfalls—can compress the soil and reduce the rate at which it absorbs precipitation.
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