science

Rating curve

Rating curve is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Rating curve rather than just read about it. In short: In hydrology, a rating curve is a graph of discharge versus stage for a given point on a stream, usually at gauging stations, where the stream discharge is measured across the stream channel with a flow meter. Numerous measurements of stream discharge are made over a range of stream stages.

Key takeaways

Rating curve belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Rating curve to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Rating curve from memory before moving on to harder problems.

Reference excerpt

In hydrology, a rating curve is a graph of discharge versus stage for a given point on a stream, usually at gauging stations, where the stream discharge is measured across the stream channel with a flow meter. Numerous measurements of stream discharge are made over a range of stream stages. The rating curve is usually plotted as discharge on x-axis versus stage (surface elevation) on y-axis. The development of a rating curve involves two steps. In the first step the relationship between stage and discharge is established by measuring the stage and corresponding discharge in the river. And in the second part, stage of river is measured and discharge is calculated by using the relationship established in the first part. Stage is measured by reading a gauge installed in the river. If the stage-discharge relationship does not change with time, it is called permanent control. If the relationship does change, it is called shifting control. Shifting control is usually due to erosion or deposition of sediment at the stage measurement site. Bedrock-bottomed parts of rivers or concrete/metal weirs or structures are often, though not always, permanent controls. If G represents stage for discharge Q, then the relationship between G and Q can possibly be approximated with an equation:

Q = C r ( G − a ) β {\displaystyle Q=C_{r}(G-a)^{\beta }}

where C r {\displaystyle C_{r}} and β {\displaystyle \beta } are rating curve constants, and a {\displaystyle a} is a constant which represents the gauge reading corresponding to zero discharge. The constant a {\displaystyle a} can be measured when a stream is flowing under "section control" as the surveyed gauge height of the lowest point of the section control feature. When a stream is flowing under "channel control" conditions, the parameter a {\displaystyle a} does not have a physical analogue and must be estimated by following standard methods given in literature. The parameter β {\displaystyle \beta } is typically in the range of 2.0 to 3.0 when a stream is flowing under section control, and in the range of 1.0 to 2.0 when a stream is flowing under channel control. A stream will typically transition from section control at lower gauge heights to channel control at higher gauge heights. The transition from section control to channel control can often be inferred by a change in the slope of a rating curve when plotted on log-log graph paper.

References

Worked examples

Example 1 — a first encounter with Rating curve

Start with the simplest possible case. Write down what Rating curve claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Rating curve before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Rating curve ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Rating curve

In research: Rating curve appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Rating curve in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Rating curve is common in secondary-school and first-year university syllabi. It links to neighbouring topics Hydrology, Hydrology stubs, so understanding it makes those chapters shorter.
In everyday life: Look for Rating curve outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Rating curve in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Rating curve means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Rating curve out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Rating curve in simple terms?

Why does Rating curve matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Rating curve?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Rating curve.