The total rainfall in a catchment of area $1000 \: km^2$, during a $6$ h storm, is $19$ cm. The surface runoff due to this storm computed from triangular direct runoff hydrograph is $1 \times 10^8 \: m^3$. The $\phi_{\text{index}}$ for this storm (in cm/h, up to one decimal place) is ____