Question
I will be installing a 1000 gallon tank at ground level at back of my house under a deck. I want to pump the rain water from it around to the front where I have large flower gardens. I want to pump to run on demand, i.e. when hose is opened the pump starts and runs until hose is closed. I will be using one hose for hand watering for now and then moving to drip irrigation in the near future. I believe I need a self-priming centrifugal pump. It should be 3/4 hp I think ?? because pump needs to lift the water 10' and along total of 100' of hose. I don't think I need a pressure tank ?? But it seems like a pressure switch might be in order. I searched and can find 100+ different ones and am not comfortable with the right rating for the switch i.e total pressure and differential pressure. If my approach is all wrong, I'm all ears. Thanks

Answer
Good question and great info.
A 3/4 hp should be sufficient for your application. Anything larger could be a problem once you change over to drip. For what you require, a pressure tank and switch will be needed. If you operated without the tank, the pump would start and stop everytime you used any water, even a tablespoon full. Starting and stopping a pump repeatedly is what cause premature failure, it also require much more electricity. The tank provide storage of a few gallons giving you a minute or 2 of water flow before the pump come on.
If at all possible. I would suggest installing the pump so it is at the same level as the bottom of the tank. This serve two purposes. First your not sucking the water up rather pushing it along, this is more efficient. The other, you never have to worry about losing "prime" since the water is forced into the pump, eliminating the need for a "self-priming" pump. This is referred to as a "flooded suction". If your tank doesn't have a port in the lower end, you can install one. It's referred to,(no joke here) a bung port or bung hole. Farmers use them to tap their many tanks they use. A farm store would be a good place to look.
One thing you might consider is some sort of "lower water switch". When the tank is about empty, it stop the pump from running. With a pressure switch system, if the tank runs out of water the pump will continue to run, trying to build pressure. The pump won't last too long without a water supply. I use a simple float switch used in sump pump applications.
A final note, the suction pipe to the pump should be the same size as the inlet of the pump.
Good luck, if you have more questions, feel free to ask.

Matt