Zero-discharge stage of a smooth rating curve

The stage column shows 23.37 m between 21.95 m and 23.00 m, which breaks the increasing order. It is taken as a misprint for 22.37 m (Q = 400 m$^3$/s). The last stage is 23.90 m.

Three-point method: choose $Q_1$ and $Q_3$ at the ends, with $Q_2=\sqrt{Q_1Q_3}$:

$$Q_1=100,\; G_1=20.80;\qquad Q_3=1000,\; G_3=23.90;\qquad Q_2=\sqrt{100\times1000}=316.2$$

Interpolating for $G_2$ between $(21.95,300)$ and $(22.37,400)$:

$$G_2=21.95+\frac{316.2-300}{100}(0.42)=22.018\ \text{m}$$ $$a=\frac{G_1G_3-G_2^2}{G_1+G_3-2G_2}=\frac{20.80\times23.90-22.018^2}{20.80+23.90-2\times22.018}=\frac{497.12-484.79}{0.664}=18.57\ \text{m}$$

Check by trial on log-log paper: for the straightest line of $\log Q$ against $\log(G-a)$ the best-fitting trial value is $a\approx18.7$ m (correlation 0.9997).

Answer: Stage for zero discharge $\approx 18.6$ m.