Water depth of 10 cm corresponds to \( y = -6 \) (since maximum depth is 16 cm below ground level at \( y = 0 \)):

\[ 16 \sec\left(\frac{\pi x}{36}\right) – 32 = -6 \quad (M1) \]

\[ \sec\left(\frac{\pi x}{36}\right) = \frac{26}{16} = \frac{13}{8} \]

\[ \cos\left(\frac{\pi x}{36}\right) = \frac{8}{13} \quad (A1) \]

\[ \frac{\pi x}{36} = \pm \arccos \frac{8}{13} \quad (M1) \]

\[ x = \pm \frac{36}{\pi} \arccos \frac{8}{13} \]

Width of water surface is the distance between \( x \)-values:

\[ \frac{36}{\pi} \arccos \frac{8}{13} – \left(-\frac{36}{\pi} \arccos \frac{8}{13}\right) = \frac{72}{\pi} \arccos \frac{8}{13} \quad (A1) \]

[4 marks]