Phi-index from mass curve, 3.5 cm runoff

Hourly rainfall from the mass curve (difference of successive values):

Total rainfall $P = 7.75$ cm; runoff $R = 3.5$ cm; total loss $= 7.75 - 3.5 = 4.25$ cm.

Hour 1 has the smallest rain (0.5 cm). Try $\phi > 0.5$ cm/h, so hour 1 gives no excess and the other 5 hours do:

$$(7.75 - 0.50) - 5\phi = 3.5 \;\Rightarrow\; \phi = \frac{7.25-3.5}{5} = 0.75\ \text{cm/h}$$

Check: $0.75 > 0.50$ (hour 1 below $\phi$, correct) and $0.75 < 0.95$ (all other hours above $\phi$, correct).

Answer: $\phi$-index = 0.75 cm/h.