Given Data:
The diameter is, d = 18 feet
The height of the cylinder is 4.5feet.
The height of water is, 6inc
The radius of the cylinder can be calculated as,

The height of the cylinder is 4.5 ft, but the water is filled 6 inc from the top.
So, we have, 6 inc = 0.5 ft
Therefore the height of the water is,
h = 4.5 ft - 0.5 ft =4 ft.
Now, the water is in cylinderical shape, so the volume can be calculated as,

Thus the volume is 1017 ft^3.