The height of liquid upstream of the weir depends on the flow rate (volumetric Q or mass W)
as well as the effective area of the notch through which the fluid must pass. Unlike an orifice plate,
this area changes with flow rate in both weirs and flumes. One way to envision this by comparison
is to imagine a weir as acting like an elastic orifice plate, whose bore area increases with flow rate.
This flow-dependent notch area exhibited by both weirs and flumes means that these devices become
more sensitive to changes in flow as the flow rate becomes smaller.
A comparison of transfer function graphs for closed-pipe head elements such as orifice plates and
venturi tubes versus weirs and flumes shows this striking difference in characteristics: