Trees do not survive on groundwater, but on capillary hang water. The deepest roots ever found are 84 meters. Many times groundwater in deserts is deeper than 84 meters. The capillary hang water is the water that penetrates in the soil during the annual rain. For example:

Suppose you have 300 mm of rain. Of this 300 mm, 100 mm  does not penetrate, because the soil is very hard or because there is a steep slope. Only 200 mm of the precipitation enters. Then you can plant trees:

  1. That resist the climate (cold, hot, strong winds);
  2. That can survive with 200 mm.

So it is important to understand that:  

  1. The annual rainfall says nothing about the quantity that enters the soil
  2. Trees do not survive from groundwater but from capillary hang water caused by the penetrated quantity of rainfall.