The dripline is at the edge of the canopy. Roots extend beyond the dripline.
Water the Roots
The roots of a mature tree extend underground even wider than the branches extend above ground. Slowly soak the root zone beginning at the drip line (the soil beneath the edge of the leaves) and extending outward. Soak the soil so the water reaches the roots 12-18 inches below the soil.
Do not apply water at the base of your mature tree. This will not effectively provide water to the tree's roots.
You can use a soaker hose spiraled throughout the root zone, drip emitters, or an oscillating sprinkler on a low setting and moved to various areas within the root zone. Allow the water to soak into the soil for several hours.
Check the Soil
To determine how often your tree needs water, learn about your soil type and check your soil moisture. Use a screwdriver, hand trowel, or soil probe to test the soil 6-12 inches below the surface. If it is wet and sticky, allow it to dry for several days before adding water. If it is dry and crumbly, apply water slowly to soak the soil 12-18 inches below the surface.
Know your Tree Type
Some established, drought tolerant trees like California native oaks, California laurel, cork oak, Chinese pistache, and goldenrain trees can be damaged with frequent summer watering. These trees may need one or two thorough soaks during periods of dry weather.
Moisture adapted trees such as birches, redwoods, magnolias and red maples will likely need regular, deep watering throughout their lives and especially during dry weather.
Trees in or near lawn areas which have become accustomed to frequent, shallow watering from sprinklers may develop surface roots and will have higher water needs or require a period of transitional watering techniques to wean them from regular sprinkler water.
Watch this Demonstration