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The wealth of water is a blessing, but population growth has radically altered the natural systems that manage rainfall through transpiration, infiltration, and gradual runoff into surface waters—leading to everything from poor water quality to flooding, severe erosion, and droughts.

When the amount of rain falling exceeds the land's ability to absorb it, the result is stormwater runoff. The volume of runoff and rate at which it flows varies with the intensity and duration of the rainfall—and with the type of land surface upon which it falls. A short light rain falling on permeable soils might produce little to no runoff, while heavy rain landing on an impervious parking lot can produce a substantial amount. Runoff can carry chemicals, metals, bacteria, viruses, organic compounds, and other pollutants directly into creeks, lakes, rivers, and streams. And, stormwater runoff can cause severe erosion and flooding.