HARD SURFACES

Remember learning the water cycle in school? If you do, your memory probably goes something like this:

The sun shines on lakes and rivers, causing water to evaporate and form clouds. Clouds grow heavy and begin to rain down. Rain runs off the ground, back into lakes and rivers, where the cycle repeats.

Now brace yourself— what you learned is not entirely true! It includes the misleading concept that rain runs off the ground. Oh, in cities it runs off well enough (more on that soon), but in natural areas, only 10% of rain and snow actually runs off the land. Of the other 90%, 50% soaks into the ground and 40% evaporates. Instead of being taught that rain runs off the ground, we should have been taught rain soaks into the ground, since five times more soaks in than runs off.

Natural areas — landscapes covered by grasses, trees and other plants — are "soft" and act like a sponge soaking up water. As landscapes develop into cities, they become "harder," with the ground being covered by roads, parking lots and buildings. In downtown areas where almost all the ground is covered by hard surfaces, 55% of rain and snow runs off the ground, and only 15% soaks in. Notice the difference? Five times more water runs off of downtown areas than what runs off of natural areas.

As landscapes are covered with hard "impervious" surfaces, the amount of water that soaks in (infiltrates) decreases and the amount that runs off increases. Source: In Stream Corridor Restoration: Principles and Practices 10/98

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