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Urban runoff is surface runoff of rainwater, landscape irrigation, and car washing[1] created by urbanization. Impervious surfaces (roads, parking lots and sidewalks) are constructed during land development. During rain, storms, and other precipitation events, these surfaces (built from materials such as asphalt and concrete), along with rooftops, carry polluted stormwater to storm drains, instead of allowing the water to percolate through soil.[2] This causes lowering of the water table (because groundwater recharge is lessened) and flooding since the amount of water that remains on the surface is greater.[3][4] Most municipal storm sewer systems discharge untreated stormwater to streams, rivers, and bays. This excess water can also make its way into people's properties through basement backups and seepage through building wall and floors.
Urban runoff can be a major source of urban flooding and water pollution in urban communities worldwide.