Steps to develop a rating curve

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.