A storm with 150 mm precipitation produces a direct runoff of 8.7 cm, with incremental hourly rainfall values being 0.6, 1.35, 2.25, 3.45, 2.7, 2.4 1.5 and 0.75 cm/hr. Estimate the ϕ index of the storm.
1
0.925 cm/hr
2
0.825 cm/hr
3
0.625 cm/hr
4
0.725 cm/hr