The mass curve of a rainfall event of $100$ $\mathrm{min}$ duration over a catchment is given in the table.
If the initial loss is $0.6$ $\mathrm{cm}$ and $\phi$-index is $0.6 \mathrm{~cm} /\text{hour}$, the total surface runoff from the catchment is $\_\_\_\_$ $\mathrm{cm}$ (rounded off to one decimal place).
$$\begin{array}{|l|l|l|l|l|l|l|}
\hline
\begin{array}{l}
\text{Time from start of} \\
\text{rainfall (min)}
\end{array}
& 0 & 20 & 40 & 60 & 80 & 100 \\
\hline
\text{Cumulative rainfall (cm)} & 0 & 0.5 & 1.2 & 2.6 & 3.3 & 3.5 \\
\hline
\end{array}$$
