Rainwater harvesting can ensure an independent water supply during mains water restrictions and are effective in 'green droughts', capturing low rainfall events where runoff coefficients are not sufficient to cause flow into dam storages. Though somewhat dependent on end-use and maintenance, yields are usually of acceptable quality for most household needs and renewable at acceptable volumes, despite climate change forecasts. It produces beneficial effects by reducing peak storm water runoff and processing costs. Rainwater harvesting systems are simple to install and operate. Running costs are negligible, and they provide water at the point of consumption.
In municipalities with systems, reducing storm runoff is especially important, because excess runoff during heavy storms leads to the discharge of raw sewage when treatment plant capacity cannot handle the combined flow. Rainwater harvesting in urban communities has been made possible by various companies. Sometimes a practice is adopted of directing collected rainwater deep inside the ground through pipes or shafts. Such a practice will carry surface pollutants to the ground water and thus pollute it. The best practice is to store water in ponds, lakes and other unlined surface reservoirs so that the water gets filtered through before entering the ground.