Carefully chosen and placed trees have a huge effect on your landscape. Beyond their aesthetic value (it is estimated that each mature tree adds roughly $1,000 to your property value), trees can buffer you and your house from extreme temperatures, which will make you more comfortable and lower your utility bills.
When rain is scarce,
concentrate on watering your trees, not your lawn. This is true even though your lawn will look like it needs it more. Here's
an excellent article from UGA that explains how trees cool their surroundings, and also cool themselves through transpiration. They cannot effectively do this without a consistent and adequate water supply.