The dripline is at the edge of the canopy. Roots extend beyond the dripline.
Deep water your trees to allow all the roots to absorb moisture. The best way to deep water trees is by a soaker hose that slowly applies water to the soil over several hours. You can use a garden hose set to a trickle and moved to at least four different locations under the drip line. Sprinklers may be used to water deeply by watering until water begins to run off, then waiting at least an hour or two to resume watering. This should be repeated until water has penetrated at least one foot in depth. Special care will need to be taken when watering on a slope. Water around and beyond the drip line of mature trees where the roots are, not near the base of the trunk. Irrigation frequency during our rainless months may vary greatly depending on the tree species, daily temperatures, and location in the yard, along with soil texture, structure and depth. Established drought tolerant trees may need occasional watering at one or two month intervals. California native oaks, California laurel, cork oak, Chinese pistache and goldenrain trees can be damaged and short lived with frequent summer watering.
Moisture adapted trees such as birches, redwoods, magnolias and red maples may need regular deep watering throughout their lives to look their best and perform well. These and other species greatly benefit from an occasional deep watering to the depth of at least a foot once or twice a month. Trees in or near lawn areas with frequent shallow watering may develop surface roots. Poorly adapted Monterey pine, Leyland cypress and giant sequoia are prone to insect damage and diseases in hot dry interior areas of the state regardless of how much water they are given. Use a shovel or soil sampling tube to check the depth of moisture to at least a foot.