Urbanization can have a great effect on hydrologic processes,
such as surface-runoff patterns.
Imagine it this way: in a natural environment, think of the land in
the watershed alongside a stream as a sponge (more precisely,
as layers of sponges of different porosities) sloping uphill away
from the stream.
When it rains some water is absorbed into the sponge (infiltration)
and some runs off the surface of the sponge into the stream
(runoff).
Assume a storm lasting one hour occurs and one-half of the
rainfall enters the stream and the rest is absorbed by the sponges