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

# Rating curve, its uses and drawing method

Engineering Hydrology · Chapter 4: Surface Runoff

- Asked in: 2070 Chaitra, 1+2+2 marks
- Video: [Stage Discharge Rating Curve : Stream Flow Measurement : WRE](https://www.youtube.com/watch?v=RkorkZaEbL8) — The Classroom
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/engineering_hydrology/rating-curve-uses-drawing
- Whole chapter: https://www.bctengineer.com/md/semester_5/engineering_hydrology/chapter-4

## Question

What is meant by rating curve? Write the uses of rating curve. Also explain the method of drawing the rating curve in a particular section of a river.

## Short answer

A rating curve is a stage-discharge relation used to convert continuous stage records into discharge, for flood forecasting and extending flow data. It is drawn by measuring discharge at various stages, plotting stage vs. discharge, fitting Q = C(G-a)^b on log-log paper, and verifying with new gaugings.

## Answer

#### Rating curve
A **rating curve** (stage-discharge curve) is the graph or equation showing the relation between the water level (stage) and the discharge at a river section.

#### Uses
1. Converts continuous stage records into discharge, giving daily, monthly and annual flows.
2. Used for flood forecasting and design of hydraulic structures (weirs, intakes, bridges).
3. Extends the discharge record to high flows by extrapolation, and detects changes in the channel (shift).

#### Drawing the rating curve
1. Choose a section with a stable control, install a gauge and fix the datum.
2. Measure discharge by current meter at different stages (low to flood), recording the stage each time.
3. Plot stage (vertical axis) against discharge (horizontal axis) on arithmetic paper and draw a smooth curve through the points.
4. Fit $Q = C(G-a)^b$: find $a$ (stage for zero discharge) by trial or by the three-point method, then plot $Q$ against $(G-a)$ on log-log paper. The points should lie on a straight line with slope $b$ and intercept $\log C$ (at $G-a=1$).
5. Check the fit by correlation, and extend the line beyond the highest measurement by extrapolation.
6. Verify regularly with new gaugings; revise the curve if points shift.

## Check yourself

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

1. A rating curve shows the relation between
   A. Time and stage only
   B. Velocity and depth only
   C. Rainfall and runoff
   D. Stage and discharge at a section
   Answer: D. It is the stage-discharge relation. (Key point: Definition)

2. A key use of the rating curve is
   A. Converting continuous stage records to discharge
   B. Finding soil moisture
   C. Measuring rainfall
   D. Measuring evaporation
   Answer: A. It gives daily, monthly and annual flows from stage records. (Key point: Use)

3. When plotting Q against (G - a) on log-log paper, the slope is
   A. C
   B. b
   C. G
   D. a
   Answer: B. A straight line with slope b and intercept log C (at G - a = 1). (Key point: Log-log plotting)

## Related in Engineering Hydrology

- [Rating curve definition and uses](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Arating-curve-define-uses) — 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
- [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 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
- [Steps to develop a rating curve](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Asteps-rating-curve) — Engineering Hydrology, chapter 4

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