Why herbaceous plants drop their leaves in hot and dry weather while woody plants remain erect

Iwould say overuse ( but not 100% sure ) in the u.s. we tend to overuse water by leaving it running, throwing it away, taking long showers, etc.

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.