A rating curve is established by measuring river discharge at various water levels—without needing a weir or Venturi channel—using tools like a current meter or ADCP, and correlating these values to create a reliable stage-discharge relationship.
In the world of hydrology, rating curves are essential tools. They allow us to estimate a river’s discharge (flow) based on simple water level measurements. This is invaluable for water resource management, flood forecasting, and climate adaptation.
But a common question arises:
“Do I need to build a weir or install a Venturi channel to establish a reliable rating curve?”
The short answer: No. You don’t need expensive engineering works to monitor a river reliably.
With basic field tools, consistent measurements, and a good site, you can create a solid, science-backed rating curve that supports hydrological forecasting, planning, and risk reduction. This blog post is thought about as a guide to help you setting up your first flow durating curve without having any construction work to do.
So set’s explore how to create an accurate rating curve in a natural river, using simple tools and methods.
A rating curve shows the relationship between water level and discharge. It’s usually built by measuring river flow at several water heights, then fitting a mathematical function to interpolate or extrapolate flow values.
Here’s a typical example:
| Water Level (m) | Discharge (m³/s) |
|---|---|
| 0.20 | 0.5 |
| 0.35 | 1.2 |
| 0.50 | 2.4 |
| 0.70 | 4.8 |
| 1.00 | 9.5 |
| 1.20 | 12.7 |
This relationship is often modeled by a power function:
Q = a × (h – h₀)^b,
where Q is the discharge, h is the water height, and a, b, h₀ are site-specific parameters.
More here if you’re interested about how river flow and water levels interact.
Many hydrologists assume you need a weir or a Venturi flume to standardize the measurement section. While these help stabilize the relationship between water level and flow, they are not essential.
In fact, hundreds of rating curves are used today in natural, unmodified rivers — especially in remote or protected areas, or where budgets are tight.
What matters more is:
Choosing a stable, regular river cross-section,
Taking accurate and frequent discharge measurements,
Ensuring the riverbed isn’t shifting too often (due to sedimentation or erosion).
France – Durance River (Embrun):
No weir, just a wide, stable alpine section. Data used for hydropower optimization.
Nepal – Trishuli River (Betrawati):
Mountain river with natural rocky bed. Staff gauge + manual gauging for flood forecasting.
Senegal – Anambé River (SAED):
Agricultural monitoring. Natural channel, light maintenance, radar level sensor only.
1. Site Selection
Pick a section that’s:
Straight and uniform,
Free of obstacles or sharp bends,
Stable across seasons.
2. Install a Water Level Sensor
Use a:
Staff gauge for low-cost manual checks,
Or pressure/radar sensor for automated logging (every 15–60 min).
3. Perform Gauging (Discharge Measurements)
Use:
Current meters (mechanical or electromagnetic),
Or portable ADCP for fast or deep rivers.
Take at least 5–10 measurements across various flow conditions.
4. Build the Curve
Plot discharge vs. height. Fit a mathematical curve (usually power law).
Validate it regularly.
5. Maintain the Site
Inspect for sediment or changes in bed geometry,
Clean sensors, verify battery or data logging,
Re-gauge if flood or drought events occur.
Below is a simplified diagram showing the two key components of field-based rating curve creation:
A rating curve (left),
A typical field installation in a natural section (right):
Key components:
Staff gauge for manual level readings,
Water level sensor (pressure or radar),
Current meter or ADCP for flow measurements,
A well-defined and accessible river section.
