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The water balance of urban areas differs considerably from the landscape water balance. Increased surface runoff, reduced groundwater recharge and evaporation change the hydrological regime, the morphology and ecology of water bodies close to the cities, the groundwater in the urban area and the urban climate. Today’s urban drainage systems are designed to prevent, reduce, drain, seep away, evaporate or discharge precipitation into nearby surface waters with considerable delays. In doing so, it follows the principles of the German Water Resources Act (WHG) and the objectives of the relevant technical regulations DWA-A 102 to keep changes in the natural water balance by settlement activities as low as ecologically, technically and economically acceptable. A reference for the “natural” water balance has to be defined as a planning objective in order to quantify the hydrological changes in …