This figure illustrates how the hydrologic cycle and water balance change as a result of urbanization. In a forested setting with no urbanization, about one-third of annual precipitation is returned to the atmosphere through evapotranspiration, while approximately another third infiltrates deeply into soil and recharges groundwater. Very little overland runoff occurs. Impervious surfaces created through urbanization prevent infiltration and rapidly convey water from the lanscape. In this illustration, overland runoff is 30 percent of annual precipitation. Recharge and evapotranspiration are greatly reduced. Note that the amount of runoff varies with the extent and connectedness of impervious surfaces. Ultra-urban settings may result in runoff exceeding 70 percent of annual precipitation.
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