My wife and I have two large Hollywood Junipers. Each tree is about 22 feet tall and about 12 feet wide. We live in San Francisco, where the annual rainfall is about 20 inches. I’ve been watering each tree about 3 gallons every 3 days. Is that enough?
It should be enough. The extremely limited amount of exposed soil surface that I notice in the photo, will limit applying much more than that.
I know the climate there very well and the day to day variables in fog cover, wind and rains during winter and spring.
I recommend that you adjust your watering schedule and amount of water applied to the climate factors and needs of your magnificent juniper. Hold off on water during rainy periods, not much needed on days where the fog lingers all day, a little more when its windy because that can cause more water loss from foliage. This month and next is when SF can get some dry hot days and the tree may need more water than usual.
By the way, apply water to the other open planting area also, not just the small cut out at the base of the tree. The absorbing root system of a mature tree like this will be extensive and wide-spread, under the pavement and into the surrounding planting beds for a drink.
I'm happy to see that you are not shearing the juniper and are allowing the natural form and beauty of the unique upward twisting canopy.