Drought-Resistant Lawn/Turf Recommendation For Area Above Septic
Sorry, I can't help you with the type of grass. But I do have some suggestions that will work with any type.

Don't just leave it dirt. Improve the dirt. Add compost to the soil. If necessary, have the compost roto-tilled in.

Organic matter helps the soil to retain moisture. Periodically aerate the soil (yearly is a good idea if you can do so. A pull-behind aerator is not that expensive if you have a small tractor, and well worth the expense.) After each aeration, add more compost on the surface. Grass with a deep, robust root system can survive a period of no water for longer than otherwise.

You may also find it a good idea to check your soil for problems. Is it heavy clay? Sand? Are there problem trees in the area, like walnut? Is the soil too acidic?