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The objective of field water measurement is to conserve water by improving management of its distribution and field application. A simple mobile flume to measure a discharge through small rectangular open channels in agricultural fields has been experimentally investigated. The flume consisting of a vertical cylinder inserted axially into the horizontal prismatic rectangular channel, referred as a simple cylindrical flume, has been calibrated. The flow rate in rectangular channel can be measured by constricting the flow due to presence of cylinder, resulting in critical flow conditions. Experiments have been performed on two simple cylindrical flumes of different diameters, to evaluate the hydraulic characteristics of subcritical incoming flow under free flow conditions. The results of laboratory experiments on the flume have been analysed and two different discharge prediction models have been developed. The two models developed for the prediction of discharge for simple cylindrical flumes developed for use in rectangular channel sections, are based on the energy concept and the direct regression approach, respectively. Both the proposed models have been validated using the limited experimental data available in the literature. Formation of critical depth at the throat section has also been verified. Plots have also been developed for the dimensionless column head and the corresponding Froude number of the incoming flow. The discharge prediction model giving the least error has been proposed for use in practice.