This is done by observing the direction of general features of the flow. From volume flow rate and the measurements the height of the water above the notch base, we can find the corresponding value of discharge coefficient. The Bernoulli equation is then applied and for the conservation of to a flow model of rectangular notch weir, the theoretical discharge for the total ideal flow can be found. But due to the smaller effective flow area, the actual flow will be less than the ideal flow.
