Rainwater harvesting can ensure an independent water supply during water restrictions, though somewhat dependent on end-use and maintenance, usually of acceptable quality for household needs and renewable at acceptable volumes, despite forecasted climate change (CSIRO, 2003). It produces beneficial externalities by reducing peak storm water runoff and processing costs. In municipalities with combined sewer systems, reducing storm runoff is especially important, because excess runoff during heavy storms leads to the discharge of raw sewage from outfalls when treatment plant capacity cannot handle the combined flow. Rainwater harvesting systems are simple to install and operate. Running costs are negligible, and they provide water at the point of consumption. Rainwater harvesting in urban communities has been made possible by various companies. Their tanks provide an attractive yet effective solution to rainwater catchment.

Rain water harvesting law Some U.S. states have water law in favor of land owners, and some states own all water rights, leaving the owner only water rights the states are willing to grant through permits. For example, in Colorado you may not catch, collect or harvest rainwater from your roof unless you first buy a permit.

In Australia In Australia it is common to have a rain water tank with rainwater running off the house roof to fill tank, it is common to flush toilets with this water and have a back up valve like an Acquasaver that gives the home owner mains(town water) as a back up in case rainwater supply is exhausted or power failure. These types of valves automatically switch from rain water to mains water supply when available, so the homeowner never needs manually operate this valve.

In Sri Lanka Urban Development Authority of Sri Lanka has been directed to include rain water harvesting by changes to its act of establishment.