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.
Source: In Stream Corridor Restoration: Principles and Practices 10/98
Hard surfaces such as streets, parking lots and roofs are called impervious surfaces — surfaces that water cannot pass through. Think of plastic wrap. Covering a landscape with impervious surfaces is like wrapping it in plastic — water cannot soak in, so must run off. This shift from soaking in to running off is at the center of water-quality problems cities experience.
Rivers and lakes are supplied with water in one of two ways: by water running off the surface of the ground, or by water seeping through it. Of the two, they are healthier when supplied by water seeping through the ground. Water seeping through the ground, or groundwater, runs cool, clean, steady and slow. In contrast, water running off the surface of the ground behaves entirely differently. It flows fast, warm in the summer, and dirty — carrying stormwater pollutants it picks up along the way.
Not surprisingly, rivers supplied mainly by surface runoff have health issues! Instead of having a steady flow throughout the seasons, they tend to behave erratically, running fast and full — sometimes flooding — when it is raining, and then going down to trickle when it is not. They also are dirtier, having to deal with stormwater pollutants that runoff water picks up along its way.
Because of the increased amounts of runoff coming from hard surfaces, flooding of city streets and neighborhoods would be common if it were not for storm sewers. Like a drain in a bathtub, storm drains in street curbs catch runoff water and take it by underground pipe to nearby rivers, lakes and wetlands. This solves the problem of flooded streets, but causes problems in the water bodies runoff goes to, by delivering too much water that is moving too fast and is too full of pollutants.
As
landscapes develop into cities, they become "harder" with more of the ground
covered by roads, parking lots, and buildings. This is an aerial photo
of Apple Valley at Highways 77 and 42.Source: Dakota Soil and Water Conservation District
Whether called hard surfaces or impervious surfaces, streets, parking lots and buildings are at the center of water-quality problems in cities. As the amount of hard surfaces increases in a city, the amount of water-quality problems increases too. It is not surprising that efforts to improve water quality in cities often involve reducing hard surfaces, and creating "soft" areas where rain and snow runoff can be caught and soaked into the ground.