FAQ - Answer
During years of drought tree growth is usually arrested, if the drought persists for a prolonged period of time the encumbered water-related stress can lead to other secondary problems of pests and disease affecting both individual trees and stands of trees.
Tree selection - As you know some trees are more “drought resistant” than others. Usually this means they have some physiological characteristics that allow them to conserve water better than others. This can be observed by early leaf drop, leaf stomata being closed during daylight hours to reduce transpiration or even an evolutionary trait of going into summer dormancy.
Irrigation – is usually important in tree seedling establishment for the first summer or two. Most woody plants are relatively able to survive under “normal” precipitation regimes once established.
Landscape zones – if I understand the concept you are providing it usually means not mixes plants that have high irrigation needs with those who are irrigation sensitive. In the case of oaks, it is usually not recommended to plant turf grass around oaks since one (the grass) needs frequent summer irrigation while the tree (oak) may not need any.
I think you are mixes two concepts here. The “role” irrigation plays is it 1) establishes plants; or 2) keeps them alive. The conflict of water use between agriculture and urban needs has little to do with plant biology and is function of local, regional or statewide policy. In the case of limited or expensive water “a farmer” would have to make a personal choice on how extensive of planting they are able to consider on a yearly basis. In other words if a farmer wanted to plant 10 acres of trees they may have to plan to do the planting over period of time to accommodate both availability and costs. For example perhaps planting the 10 acres at a rate of 2 acres/year over the course of 5 years could be considered to meet these conflicting values.