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The notch used for irrigation streams is known as a Parshall flume. A Parshall flume is a specially designed channel that allows for the measurement of flow rates in an open channel system. It consists of a converging section, a throat (the narrowest part), and a diverging section. The unique shape of the flume creates a specific relationship between the flow rate and the water level within the throat, allowing for accurate flow measurements.

When water flows through the flume, the reduction in width at the throat causes an increase in velocity and a corresponding drop in pressure, which facilitates the precise measurement of flow rates. This is particularly critical in agricultural engineering where managing water resources efficiently can significantly affect irrigation practices.

In comparison, while a weir also measures the flow of water, it does so differently and is typically used in more varied water bodies. A flume gate regulates flow but is not primarily designed for measurement like the Parshall flume. An overflow channel serves to control or redirect excess water flow, rather than measuring it. Hence, the Parshall flume is specifically recognized for its measurement capabilities in irrigation systems.