We need to attach an external water pump to our rainwater tank, to pump water up to the back of the garden. What is the process/mathematics for determining what the specs of the pump should be? e.g:

- Our land slopes upward from where the tank sits, so the pump head/height thingy is important. How do we work that out?

- We need it for manual plant watering so it needs to create a moderate amount of pressure at full height, as opposed to the very gentle pressure(?) probably required for drip irrigation. Is it the 'litres per hour' (or gallons, if you are still on the old system!) that matters most for this, or what?

- What does the 'power' of a pump really tell me, versus the 'head'?

- Anything else we need to know when choosing which water pump to buy?