More than half the world’s population lives in metropolitan centers. The built environment of a city is very different from that of rural and natural areas. When it rains over a rural landscape, much of the rainwater sinks into the ground or is evaporated or transpirated by trees, crops and other plants.
Transpiration is the biological process in which plants pull moisture out of the soil by their roots and release water vapor to the atmosphere through small openings in their leaves. Every day, plants release large quantities of water vapor. Growing plants can transpire up to 10 times as much water as they hold in their stems and leaves. These slow, natural processes allow precipitation to replenish groundwater and sustain vegetation, leaving only a small amount of water as overland flow or runoff.
