Measuring flows is not an easy task. We have various way of doing so, e.g. using the ITRC weir stick on which the smart-phone can be clipped. From the picture, the water level upstream that hits the stick on the gauge is read and automatically multiplied with the width and a calibration coefficient. This means no more errors in the computation of the flow.
Another option is taking a picture of a staff gauge upstream of a measurement flume, where again the flow is computed automatically and stored in the database/Water Information System, together with the picture.
A new product is a hystereses-free level that can be clipped on a tilting weir or radial gate that can be photographed to read the gate angle (and from that the gate level).