tof,

There are many more details to be asked before a sensible answer can be given. I can give you basic physics information and some observations of what info you need to provide. Okay here is my $0.02.
Bougainvilleas are drought loving plants and are good in dry climates once established. Unless you live in a ultra dry climate only a temporary system is needed. Why the objective of filling a 50-gallon drum and gravity feed if you have to fill it from a pressure source or pump. If you have enough rain to keep a 50-Gallon drum filled periodically you don't need the drum or the catchment system. Okay here is the physics. It will take approximately 80 psi to reach and fill the tank at 40 ft height above you water source which is the street level. Horizontal distance matters only it is 500 linear feet or more away. A 1" PVC pipe will give you ample pipe size to minimize the friction loss for the distance if less than 500 linear feet away. 80 psi is high for a house or even street pressure and it would fill the tank very slowly if at all. You would need to take the water from your house mainline with a tee connection to get the most pressure for your system. You can experiment with setting a pipeline up and see how far up the hill the water reaches then you would have to a booster pump to get it the rest of the way. You don't need a tank for this system either and with just a timer you can control the watering better. What is the flow rate and pressure from you source? Do you know how to find out? How far away is the irrigated area? How many plants do you need to cover and total area? Is your objective an ascetic component? JMHO Aloha

@Lehua,

This bougainvillea between boulders thing is just a thought. The location is zoned 8b, so not ideal for Bougainvilleas, most likely end up as annual or one big winter away from replanting. That being said, the location does have 9 months of hot and dry weather that is perfect, I think they need more water before they get established. And the slope is south facing so plenty of sun and heat.

The slope full of boulders have some pines etc in between them, I think a minimum of 150-180ft of pipes or tubes would be needed to get to the top, Running a rigid 1" pipe on top of the boulder would probably be too ugly.

Water for this development came from a water tower about 1/2 mile away on a hill side above the development. I wonder one doesn't care about the water pressure, ie the 50g storage tank idea, water could be available at the 50g courtesy of physics and without help from a pump ?

tof,

If you have adequate pressure from the community tank (your area is below the Comm tank. You don't need 50g tank just a timer and automatic valve at the bottom of your pipe with a backflow preventer. Just make sure you connect to your house mainline before your house pressure regulator. You can bury the pipe if you don't want to see it or paint it to blend in if it is temporary. You can put in the 50g tank if you want but you have to watch while it fills or time it for the water volume or have level limit floats in the tank. JMHO Aloha