With rainwater harvesting, precipitation discharge from roofs of public and private buildings can be collected in cisterns and withdrawn by pumps. The collected water replaces drinking water in areas where drinking water quality is not necessary to be obtained, such as in the household for flush, running the washing machine, to irrigate the garden but also in industrial areas. The main purpose of Rainwater Harvesting is saving drinking water, which helps to keep the groundwater resources. Furthermore combination with storage systems or infiltration system are possible, to keep all rainwater on site Regarding the hydrological balance, rainwater harvesting has similar effects as rainwater infiltration. The second impact of rainwater harvesting is the relief of subsequent drainage and treatment systems, since the used rainwater is detracted from run- off. The degree of reduction depends on the configuration of rainwater harvesting systems. The volume of a cistern has to be larger than 6 m³ to achieve significant reduction (Hermann (1996)).