A notch may be defined as an opening provided in the side of a tank or vessel such that the liquid surface in the tank is below the top edge of the opening. It is generally made of a metallic plate. It is used for measuring the rate of flow of a liquid through a small channel or a tank.
A weir may be defined as any regular obstruction in an open channel over which the flow takes place. It is made of masonry or concrete. It is used for measuring the rate of flow of water in rivers or streams.