In open channel flow measurement, which parameter is used to infer the flow rate as part of a weir measurement?

• A. Velocity
• B. Water temperature
• C. Height (depth) of the water
• D. Water color

Answer: (C)

In open-channel flow with a weir, the key variable is the height of the water above the weir crest. This head controls how much water can pass the crest, and there’s a calibrated relationship between that head and the discharge for a given weir shape. So you measure the water depth upstream of the crest with a staff gauge or depth sensor, then apply the weir equation to infer the flow rate. That head-to-discharge relationship is why depth is used to determine flow.

Water temperature and water color don’t tell you how much water is moving; they don’t relate to discharge in a practical calculation. Velocity is a different approach to measuring flow (A times V) but isn’t the method used for a weir—where the known head over the crest provides the discharge directly through the weir’s calibration.