A 3-hour storm on a small drainage basin produced rainfall intensities of 3.5 cm/hr 4.2 cm/hr and 2.9 cm/hr in successive hours. If the surface runoff due to storm is 3 cm, then the value of ϕ-index will be:
1
2.212 cm/hr
2
2.331 cm/hr
3
2.412cm/hr
4
2.533 cm/hr