A 3 hour storm produced a flood hydrograph and the observations were at 3 hour intervals starting from zero hour. The observed discharge in cumecs are 4, 9, 12, 18, 20, 16, 20, 10, 8, 6, 4. Assume constant base flow 4 cumecs. Determine unit hydrograph ordinates. The catchment area is 50 sq km.

$$\text{Direct runoff (D.R.O)}=\frac{0.36\sum O\times t}{A}=\frac{0.36\times83\times3}{50}=1.79\ cm$$