How do paved surfaces and roofs increase water runoff?

In the context of garden and landscape design, paved surfaces and roofs increase water runoff because they are impermeable, meaning they prevent the natural absorption of water into the ground.1

While natural earth acts as a drainage vehicle by absorbing a high percentage of rainfall—storing it below ground or allowing it to move through soil patterns—roofed structures and paving eliminate this absorption process entirely. When land is covered by these materials, water that would otherwise be soaked up by the soil is forced to run off the surface instead. As areas transition from natural or agricultural landscapes to residential or commercial developments, this reduction in permeable ground leads to a notable increase in peak water flows, necessitating the installation of extensive drainage systems to manage the excess volume and prevent flooding.12