That heat sounds pretty intense, like here but probably just less humid. These plants might prefer a spot without the mid-day sun if that option exists. Not sure what you meant about 90% of the water going in the ground. Are the plants in containers, or in the ground and you think about 10% of the water runs farther away to the side and not down near the roots of the plant? A gallon a week sounds like very little, this is a thirsty jungle-kind of plant. Applying the water more slowly will allow it to penetrate a smaller area more deeply. If they are in the ground, some mulch would help retain the moisture by preventing it from evaporating so quickly.
