Yes, when it is hot outside a mature tree can evapo-transpire over 100 gallons of water a day!  Trees have amazing water holding capacities but when they are loosing so many gallons we need to supplement their water to prevent the devastating and sometimes fatal effects of drought damage.  When trees live in lawns it is very stressful for them and they compete with the turf for water, air and space to grow.  When you water a tree it is different than watering your lawn, which in our culture is done way too often and not thoroughly enough.  Trees need to be watered very heavily and then allowed to dry out in between waterings. (ref: Watering Abstract)