The establishment of a reliable relationship between the monitored variable stage and the corresponding discharge is essential at all river gauging stations when continuous-flow data are required from the continuous stage record. This calibration of the gauging station is dependent on the nature of the channel section and of the length of channel between the site of the staff gauge and discharge measuring cross-section. Conditions in a natural river are rarely stable for any length of time and thus the stage-discharge relationship must be checked regularly and, certainly after flood flows, new discharge measurements should be made throughout the range of stages. In most organizations
responsible for hydrometry, maintaining an up-to-date relationship is a continuous function of the hydrologist.
The stage-discharge relationship can be represented in three ways: as a graphical plot of stage versus discharge (the rating curve); in a tabular form (rating table); and as a mathematical equation, discharge, Q, in terms of stage, H (rating equation).
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