Sum the flow from each section to obtain the total flow rate of the stream. A weir is a notch of a specific shape through which water may flow. Weirs are probably the most common device for measuring irrigation water in open ditches. Weirs are simple to construct and fairly accurate if properly maintained. They require enough slope in the ditch to allow the water to be partially held back and spill over the weir.
Air space is necessary under the falling sheet of water for accurate flow measurement.
The bottom of the notch over which water flows is the crest of the weir. Of the three common shapes of weirs, the rectangular and trapezoidal weirs are useful for measuring large streams. Note the probable head. Sometimes metal strips are fastened to plywood to provide a sturdy crest. The crest should be fairly sharp on the upstream side.