Stormwater harvesting is the collection, accumulation, treatment or purification, and storing of stormwater

Stormwater

Stormwater is water that originates during precipitation events. It may also be used to apply to water that originates with snowmelt that enters the stormwater system...

 for its eventual reuse. It differs from rainwater harvesting as the runoff is collected from drains or creeks, rather than roofs. It can also include other catchment areas from man made surfaces, such as roads, or other urban environments such as parks, gardens and playing fields.
Water that comes in contact with impervious surfaces becomes polluted and is denominated surface runoff

Surface runoff

Surface runoff is the water flow that occurs when soil is infiltrated to full capacity and excess water from rain, meltwater, or other sources flows over the land. This is a major component of the water cycle. Runoff that occurs on surfaces before reaching a channel is also called a nonpoint source...

. As the water travels more distance over impervious surfaces it collects an increasing amount of pollutants.

The main challenge stormwater harvesting poses is the removal of pollutants in order to make this water available for reuse.

Ground catchment systems

Ground catchments systems channel water from a prepared catchment area into storage. Generally they are only considered in areas where rainwater is very scarce and other sources of water are not available. They are more suited to small communities than individual families. If properly designed, ground catchment systems can collect large quantities of rainwater.