Rain fall intensities in mm/hr at half an hour interval during a 4-hour storm were: 5, 9, 20, 13, 6, 8, 16 and 3 mm/hr. If the corresponding observed runoff is 27.45 million m3 from a basin having an area of 1830 Km2? The ϕ-index for storm is
1
6.25 mm/hr
2
6.7 mm/hr
3
7.2 mm/hr
4
7 mm/hr