The majority of a mature tree’s roots exist within the top 12 inch layer of soil and extend several feet past the outer limits of the branches. Ideally, a mature tree would receive 2-3 inches of rainfall every few weeks, and a tree can more easily absorb moisture when periods of rainfall are long but infrequent. It is important to note that Turfgrass and landscape plants will out-compete mature trees when watering is shallow which is why a typical irrigation system is not adequate for proper watering of trees.
If you suspect that your tree has not gotten enough rainfall, or the top 6 inches of soil is noticeably dry, then supplemental watering is recommended. To ensure a deep watering, you will want to apply water slowly over a long period of time, and while any type of sprinkler head will work, a soaker hose will be more efficient and economical.