This simple concept should be kept in mind. It helps to keep your plants watered.
It isn't only above ground either, soil holds water and the water still flows.
Use the way water moves downhill to water your garden with as little waste as possible.
Water things higher up first and let the water move itself to lower plants.
Water carries things along with it, so start your watering at the top of a hill to wash nutrients from compost and mulch at the top, to the plants lower down.
If you want to catch water look at which way it flows and dig trenches (swales) or ponds to stop it.