Rating curve definition and uses

Definition

A rating curve (stage-discharge curve) is the graph or equation that gives the relation between the stage (water level) and the discharge at a gauging section of a river. It is prepared from simultaneous measurements of stage and discharge, and is usually of the form

$$Q = C(G - a)^b$$

where $G$ is the gauge height, $a$ is the gauge reading for zero discharge, and $C$, $b$ are constants.

  Stage
   |             .'
   |          .'
   |       .'     <- measured points
   |    .'
   |__.'________________ Discharge
   a (zero flow stage)

Uses in hydrology

1. Conversion of continuous stage records into discharge: stage is easy to record continuously, while discharge cannot be measured every day. The rating curve converts the daily stage record to a flow series (hydrograph).
2. Flow-duration and flood-frequency studies: long discharge series are needed for design of hydropower, irrigation and water supply.
3. Flood forecasting and warning: the stage at a gauge can be converted quickly into discharge.
4. Estimating discharge for high flows by extrapolation beyond the measured range.
5. Checking changes in the channel: a shift of the curve shows scour, deposition or vegetation change at the section.
6. Design of structures such as weirs, bridges and intakes needing discharge-stage information.
7. Water-resources management and allocation, where daily flows are required.