The role of water storages at greenhouses will change in the future due to three trends in climate change, upscaling of greenhouses and changes in regulations. These three trends force the local water authority and the greenhouse sector to anticipate on future problems. First step in this process is to understand the hydrological response of greenhouse reservoirs. In this research delays and losses of the precipitation collection system are measured in situ. With the knowledge gained a model is created to do a long simulation of the storage in the reservoir to determine the volume that is usable for irrigation. For a generalised greenhouse the amount of precipitation that is usable for irrigation slightly varies for different reservoir-roof ratios. However the use of additional water sources and overflows into the surface waters are strongly influenced by this ratio.