How do impervious surfaces change the flow of water?
In natural landscapes such as forests and meadows, spongy soil and plant roots enable water to infiltrate the soil. After water seeps into the ground, it may: • Nourish plants, and cycle back into the air through plant processes • Continue to flow just beneath the surface to nearby streams (as subsurface flow) • Continue into deeper groundwater Physical and chemical processes accomplished by microorganisms and plant roots help to filter and purify this water. Large volumes of water are stored in the soil and in wetlands. Sudden rainstorms in natural areas thus cause only a gradual change in the water level of streams. Evaporation of stored water also helps to cool the air in natural areas. In urban landscapes dominated by impervious surfaces, instead of infiltrating, rainwater flows across the impervious surfaces. A brief rainstorm over a large area urban area can cause a great amount of water to suddenly flow into the storm drains and streams. Since no water is stored in impervious su
