Solution:

Given data:
Rainfall in 4 successive 12 hours period = 40,80,90 and 30mm
Infiltration index = 5mm/hour

To calculate:
Total surface runoff

Calculation:
1. Calculation of potential infiltration (PI) for each time period:
a. For the first 12 hours, PI = 5mm/hour x 12 hours = 60 mm
b. For the next 12 hours, PI = 5mm/hour x 12 hours = 60 mm
c. For the third 12 hours, PI = 5mm/hour x 12 hours = 60 mm
d. For the last 12 hours, PI = 5mm/hour x 12 hours = 60 mm

2. Calculation of excess rainfall (ER) for each time period:
a. For the first 12 hours, ER = 40 mm - 60 mm = -20 mm (No runoff)
b. For the next 12 hours, ER = 80 mm - 60 mm = 20 mm
c. For the third 12 hours, ER = 90 mm - 60 mm = 30 mm
d. For the last 12 hours, ER = 30 mm - 60 mm = -30 mm (No runoff)

3. Calculation of cumulative excess rainfall (CER) for each time period:
a. For the first 12 hours, CER = -20 mm (No excess rainfall)
b. For the next 12 hours, CER = -20 mm + 20 mm = 0 mm
c. For the third 12 hours, CER = 0 mm + 30 mm = 30 mm
d. For the last 12 hours, CER = 30 mm - 30 mm = 0 mm

4. Calculation of total surface runoff:
Total surface runoff = Sum of excess rainfall over all time periods
Total surface runoff = -20 mm + 20 mm + 30 mm - 30 mm = 0 mm

Therefore, the total surface runoff is 0 mm.

Conclusion:
The total surface runoff is zero because the excess rainfall during the first and last 12-hour periods was less than the potential infiltration. During the second and third 12-hour periods, there was excess rainfall, but it was compensated for by the deficit in the first and last 12-hour periods.