How would you experiment to find out how much water the plant type needs for optimal growth?

You would know by having several plants and water them with different measures of water to see how they need and see which ones would not grow and the ones that should grow.

By water evaporation from the leaves is created a negative pressure at the top of the tree that promotes the ascension  of the water through channels that are called xylem. there are also two molecule forces that are involved: the intra-water molecules cohesion (water molecules stick to each other) and the adhesion when the water molecules stick to the channels they are running in. this is what enables water to go from the roots to the top of a tall tree.