Impacts of green roofs and rain water use on the water balance and groundwater levels in urban areas

the decentralised infiltration of rain water, which accumulates on sealed surfaces in urban areas, causes a change of the water balance parameters (evapotranspiration, surface run-off including sewage water and groundwater recharge) which in consequence leads to a rise of the groundwater level, in particular in areas with an increasing degree of surface sealing and low soil permeability. through adapted infiltration of rain water in combination with green roofs and rain water use systems, the impacts in new housing development areas can be controlled in such a way that the water balance - in particular the groundwater recharge - reaches its initial, pre-development level or that the groundwater returns to its natural level. the quantitative figures necessary for these control measures are provided by an area-differentiated water balance program whose input data can be determined by coupling various analytical and numerical simulation models. Due to proposed control measures in one urban housing development area, for example, rain and groundwater use comparable to the natural, pre-development situation would be possible if 30 % of the roof surfaces are covered with green roofs with subsequent infiltration via swales. in new housing development areas, economically reasonable rain water use in combination with a reduced infiltration of rain water is an effective compensation for the disturbance of the urban water balance.