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What happens to precipitation when it falls
on the ground? It follows various paths. Some of it soaks
into the ground replenishing the groundwater. Some of it goes
back into the atmosphere through transpiration by plants and
evaporation from the ground surface (evapotranspiration). And
finally, some of the precipitation becomes surface runoff and
flows into our streams and lakes.
 One
of the main factors which determines the amounts of evapotranspiration,
infiltration, and surface runoff is land use. Land development
through urbanization plays a significant role in changing the
hydrologic balance in our watersheds. For example, in a woodland
where the natural landscape is not disturbed, precipitation
turns mainly into infiltration and evapotranspiration. As more
and more natural forests and rural farmlands are converted into
residential and commercial communities, more trees are removed
and more permeable (porous) surfaces are turned into less permeable
or impermeable surfaces.
The net outcome of increased imperviousness
is a significant increase in surface runoff in terms of rate,
volume, and frequency. Streams, the main recipients of stormwater
runoff, make room in the channel by eroding the banks and the
bed. The result is a wider and deeper channel. As the stream
erodes vertically and laterally, sediment is washed downstream
where it eventually reaches the Potomac River and then the Chesapeake
Bay. Sediment, and the nutrients attached to the sediment particles,
impair water quality.
An unstable stream channel also decreases
biological productivity and threatens the infrastructure (sidewalks,
streets, and buildings) that borders the stream channel.
Stream degradation will continue as long
as land development within the watershed continues.
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