---
title: "Steps to develop a rating curve"
subject: "Engineering Hydrology"
chapter: 4
key: "semester_5/engineering_hydrology:steps-rating-curve"
source: https://www.bctengineer.com/q/semester_5/engineering_hydrology/steps-rating-curve
site: BCT Engineering
---

# Steps to develop a rating curve

Engineering Hydrology · Chapter 4: Surface Runoff

- Asked in: 2082 Bhadra, 4 marks
- Video: [Developing stream Rating Curve Equation and curve Using EXCEL [ Step By Step ]](https://www.youtube.com/watch?v=CSPLLGA4c3M) — Hydraulics and Hydrology Tutorials
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/engineering_hydrology/steps-rating-curve
- Whole chapter: https://www.bctengineer.com/md/semester_5/engineering_hydrology/chapter-4

## Question

List out the steps how to develop rating curve of hydrological station.

## Short answer

To develop a rating curve: install a gauge, measure discharge at various stages, plot stage vs discharge, determine zero-discharge stage a via three-point method, linearise log Q vs log(G-a), fit Q=C(G-a)^b by regression, extrapolate for floods, and update with new measurements.

## Answer

Steps to develop the rating curve of a hydrological station:

1. **Select the gauging site** with a stable control, a straight reach and easy access.
2. **Install the gauge** (staff or automatic recorder) and fix its datum with a benchmark.
3. **Measure discharge** at the section by the area-velocity method (current meter) at different stages from low flow to flood, and record the stage at the time of each measurement.
4. **Tabulate** stage ($G$) and discharge ($Q$) pairs, covering the whole range of stages (about 15 to 20 pairs).
5. **Plot $G$ against $Q$** on arithmetic paper and draw a smooth curve.
6. **Find the stage for zero discharge $a$** by the three-point method or trial and error.
7. **Plot $\log Q$ against $\log(G-a)$**; fit a straight line (least squares) to get $b$ and $C$ in $Q=C(G-a)^b$, and find the correlation coefficient.
8. **Extend the curve** to high floods by extrapolation, and check it using the slope-area method for floods.
9. **Verify and update** the curve with new measurements; correct for shifts in the control.

## Check yourself

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

1. What makes a good gauging site?
   A. A meandering reach with a changing bed
   B. A stable control, straight reach and easy access
   C. A reach just below a confluence
   D. A very wide shallow floodplain
   Answer: B. A stable control keeps the stage-discharge relation constant. (Key point: Site selection)

2. How is discharge measured at different stages?
   A. By rainfall measurement
   B. By slope-area at low flow
   C. By the area-velocity method with a current meter, recording stage at each measurement
   D. By calculating from the rating curve itself
   Answer: C. Pairs of stage and measured discharge covering low to flood flows build the curve. (Key point: Discharge measurement)

3. What is done after finding zero-flow stage a?
   A. Skip fitting and use the plotted curve only
   B. Fix b = 2 always
   C. Discard the correlation coefficient
   D. Fit log Q vs log(G-a) by least squares, extend to floods, and update for shifts in control
   Answer: D. The log-log fit gives b and C; the curve is extended, checked and revised as the control shifts. (Key point: Fitting and updating)

## 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×
- [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, its uses and drawing method](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Arating-curve-uses-drawing) — Engineering Hydrology, chapter 4
- [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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