---
title: "Rating curve definition and uses"
subject: "Engineering Hydrology"
chapter: 4
key: "semester_5/engineering_hydrology:rating-curve-define-uses"
source: https://www.bctengineer.com/q/semester_5/engineering_hydrology/rating-curve-define-uses
site: BCT Engineering
---

# Rating curve definition and uses

Engineering Hydrology · Chapter 4: Surface Runoff

- Asked in: 2081 Baisakh, 3 marks; 2068 Chaitra, 4 marks (asked 2 times)
- Video: [UNDERSTANDING STREAMFLOW AND RATING CURVES | Hydrology 101 Lesson 10](https://www.youtube.com/watch?v=rbpAvw2g05E) — Clear Creek Solutions
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/engineering_hydrology/rating-curve-define-uses
- Whole chapter: https://www.bctengineer.com/md/semester_5/engineering_hydrology/chapter-4

## Question

Define rating curve and explain its uses in hydrology.

## Short answer

A rating curve is a stage-discharge relation (Q = C(G-a)^b) at a river section. It converts continuous stage records into discharge series for flood forecasting, flow-duration studies, design of structures, and detecting channel changes.

## Answer

#### 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.

```text
  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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is a rating curve?
   A. Relation of rainfall and runoff
   B. Relation between stage and discharge at a gauging section
   C. Plot of velocity vs depth
   D. Flow vs time
   Answer: B. It is the stage-discharge relation, often Q = C(G-a)^b. (Key point: Definition)

2. In Q = C(G-a)^b, what is a?
   A. Slope
   B. Roughness
   C. Gauge reading for zero discharge
   D. Area
   Answer: C. a is the stage at which flow is zero. (Key point: Equation terms)

3. What is the main use of a rating curve?
   A. Estimating evaporation
   B. Measuring sediment
   C. Measuring rainfall
   D. Converting continuous stage records to discharge
   Answer: D. Stage is easy to record continuously, discharge is not. (Key point: Main use)

4. What does a shift in the curve indicate?
   A. Increase in temperature
   B. Scour, deposition or vegetation change at the section
   C. Change in rainfall
   D. Gauge height error only
   Answer: B. Channel changes alter the stage-discharge relation. (Key point: Change detection)

## Related in Engineering Hydrology

- [Rating curve, its uses and drawing method](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Arating-curve-uses-drawing) — Engineering Hydrology, chapter 4
- [Hydrological cycle and uses of engineering hydrology](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Ahydrologic-cycle-uses) — Engineering Hydrology, chapter 1
- [Zero-discharge stage of a smooth rating curve](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Astage-zero-discharge-23-37) — Engineering Hydrology, chapter 4, asked 2×
- [Rating curve equation, zero-discharge stage 161.0 m](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Arating-curve-equation-161-0) — Engineering Hydrology, chapter 4
- [Rating curve equation and parameter estimation](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Arating-curve-equation-parameters) — Engineering Hydrology, chapter 4
- [Rating curve and factors affecting runoff](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Arating-curve-runoff-factors) — Engineering Hydrology, chapter 4

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