Because pavements and roofs have much less surface area to wet in a rainstorm compared to plant-covered lands, more water is also free to run off these simpler, impervious smooth surfaces. Because they are smoother, water also runs off them faster. Instead of flowing off slowly over a long period of time, a larger volume of water arrives downstream at the same time much like rush hour on highway networks. This addition of impervious surfaces associated with urbanization can significantly contribute to lowering the water table, both locally and regionally. Development under conventional zoning does little to minimize, much less prevent, these ill effects.
Curbs hold water in the roadway, requiring storm sewers to let it out.
The round pipes used for storm sewers move masses of water very efficiently. But instead of moving runoff slowly over natural surfaces so it soaks in, runoff moves rapidly once it's inside storm sewers, with no opportunity to infiltrate the soil. The high speed of flow keeps pollutants suspended in the runoff. Constructed ponds are then required to remove pollutants and reduce peak flows. Since storm sewers are designed to flow without pumps, they tend to be put in the lowest portions of the landscape which are natural drainage-ways.