The water at 20°C is pumped from a large reservoir on the surface at a constant speed of 9m³/h to the top of an open experimental absorption tower. The point where the water is carried is 5m above the surface. The friction losses in the 50mm diameter pipeline where the water is transferred from the reservoir to the tower top is 2.5 J/kg. As the carrying power of the pump is 0.1kW, at what height should the water level in the reservoir be kept constant?