As the demand for water grows ever greater and new sources of water become harder to find, efforts must be made to manage existing water supplies more efficiently. One way to do this is to ensure the accurate measurement of flows in water channels. This enables regulation of quantity used. For the effective management of surface water for whatever purpose (e.g. irrigation, municipal supplies and treatment, watershed hydrology, flood-flow monitoring), the flow must accurately measured.
It is recommended that a water-measuring capability be included in all new water projects, and that existing water projects be retro-fitted for water measurement. Water measurements should be planned at all points where it can be reasonably established that informationon the flow rate will affect management decisions. Water should be measured at all bifurcations or divisions in flow, at all delivery outlets, and in the stream or river from which the water is diverted. Critical-flow devices are often used to measure flow in open channels. Most of these devices require laboratory calibrations because the discharge is not theoretically predictable, except through empirically derived coefficients. Two flow devices whose discharges can be theoretically predicted without the need for such coefficients are the "long-throated critical-depth flow-measuring flume," often shortened to "long-throated flume," and the "broad-crested weir." Both have similar hydraulic properties. A flume is usually designed and installed to measure and regulate the flow of water in irrigation canals, and to measure the discharge of non-navigable streams, drains, free flowing sewers and similar flows.