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Stage-discharge relations (ratings) are usually developed from a graphical analysis of numerous discharge and stage measurements. Discharge measurements are made on various schedules, at a variety of stages, and sometimes for different purposes. All discharge measurements are compiled and maintained in a data base.

This stage-discharge relationship is called a rating curve. It's developed by making frequent manual discrete discharge measurements at stream gaging stations. The rating curve depends on the physical characteristics of the stream channel and floodplain, and will vary over time at almost every station.

Rating curves are used to calculate or predict a variable that is difficult to measure continuously, from another variable that is easier to determine. The aim is to produce a continuous time-series of discharge, or suspended sediment or solute concentration.

Rating Curves This is equivalent to Q=C(h+a)n in the Flood Estimate Handbook (Volume 3 page 274). This is the format of all rating curves supplied to the project. The equation is presented as it is read and used.

Generally, If Q and h are the discharge and the stage correspondingly, then this relationship can be analytically expressed as Q=f(h), where f(h) is an algebraic function of stage.

Rating curves are usually extended using log–log axes, which are reported to have a number of problems.

Explanation: (i) A rating curve is a graph of discharge versus stage for a given point on a stream, usually at gauging stations. (ii) The rating curve depends on the hydraulic characteristics of the stream channel and floodplain, and will vary over time at almost every station.

A hydrograph is a time series of stream discharge at a specific point in a river. Hydrographs are constructed by continuously measuring stage (depth of water) and developing a rating curve (Figure 1), which is a relationship between stage (D) and discharge (Q) at a specific monitoring location.