A method for regulating the water level on a water contact surface having at least one drain and configured as a roof surface having a water drain system, comprising: - a closing body arranged inside or above the at least one drain and configured to prevent water from entering the at least one drain, - an actuator by which the closing body can be moved to allow water to enter the at least one drain, - a control apparatus for controlling the actuator, which is configured to control the actuator in dependence on evaluated weather data, comprising externally received and evaluated weather forecast data, comprising forecast precipitation amounts, and - a receiving device for receiving the weather forecast data, which is connected to the control apparatus for the purpose of transmitting the received weather forecast data, comprising the steps of: - receiving weather data by means of the receiving device, - transmitting the weather data from the receiving device to the control apparatus, - comparing the weather forecast data with reference values stored in a memory of the control apparatus, - selecting control parameters according to the determined reference values and transmitting the control parameters to the actuator, - if necessary, actuating the actuator and moving the closing body to adjust the water level on the water contact surface, characterized in that the water contact area is the roof surface of a green roof, and the inflow volumes are calculated for the precipitation amounts forecast by a weather app or other weather forecast taking into account the weather forecast data, the storage volumes that must be provided to absorb the forecast precipitation amounts are calculated based on these inflow volumes, the storage volumes are used to calculate the amounts of water per time that must be drained from the water contact surface to provide space for the expected precipitation, it is calculated for how long and, optionally, how far the at least one drain needs to be opened in order to drain the calculated discharge volume before the forecast precipitation event occurs, and the control apparatus actuates the actuator, which in turn, according to the calculated parameters, opens the drain completely or partially for the calculated period of time by moving the closing body in order to drain the calculated discharge volume. - a closing body arranged inside or above the at least one drain and configured to prevent water from entering the at least one drain, - an actuator by which the closing body can be moved to allow water to enter the at least one drain, - a control apparatus for controlling the actuator, which is configured to control the actuator in dependence on evaluated weather data, comprising externally received and evaluated weather forecast data, comprising forecast precipitation amounts, and - a receiving device for receiving the weather forecast data, which is connected to the control apparatus for the purpose of transmitting the received weather forecast data, comprising the steps of: - receiving weather data by means of the receiving device, - transmitting the weather data from the receiving device to the control apparatus, - comparing the weather forecast data with reference values stored in a memory of the control apparatus, - selecting control parameters according to the determined reference values and transmitting the control parameters to the actuator, - if necessary, actuating the actuator and moving the closing body to adjust the water level on the water contact surface, - receiving weather data by means of the receiving device, - transmitting the weather data from the receiving device to the control apparatus, - comparing the weather forecast data with reference values stored in a memory of the control apparatus, - selecting control parameters according to the determined reference values and transmitting the control parameters to the actuator, - if necessary, actuating the actuator and moving the closing body to adjust the water level on the water contact surface, the water contact area is the roof surface of a green roof, and the inflow volumes are calculated for the precipitation amounts forecast by a weather app or other weather forecast taking into account the weather forecast data, the storage volumes that must be provided to absorb the forecast precipitation amounts are calculated based on these inflow volumes, the storage volumes are used to calculate the amounts of water per time that must be drained from the water contact surface to provide space for the expected precipitation, it is calculated for how long and, optionally, how far the at least one drain needs to be opened in order to drain the calculated discharge volume before the forecast precipitation event occurs, and the control apparatus actuates the actuator, which in turn, according to the calculated parameters, opens the drain completely or partially for the calculated period of time by moving the closing body in order to drain the calculated discharge volume.