It doesn't matter how much rain falls on the surface — it's how much has percolated down to the root system that matters.
Our experience on the Olympic Peninsula (east side) bears this out. Our soil is mostly clay, but with many well-established large conifers and big-leaf maples, there is a rich loamy top layer. Last winter we planted 140+ bare-root natives, and then in April we had almost no rain; we usually get a lot. Our property has a ~60 ft rise, and the water from those rains usually saturates the soil but lasts significantly longer throughout the summer in the lower portion of our land than at the hilltop and on the hillside. We were advised to water the newly planted natives for two or three years throughout the summers (when there is no longer sufficient rain in the PNW = long-term drought), but to do so only occasionally and then deeply! The idea being that we want the roots to seek water deeper to establish healthy, drought-tolerant plants. This has been working quite well; we have only lost about 10 of the 140 plants so far, but we noticed something interesting: we weren't surprised that the willows and cottonwoods and nine-barks we planted in the lowest, generally wettest areas are doing fine, but we
were surprised to find that the few plants we neglected to water at higher elevations (they were out of sight, out of mind) also are doing very well.
Our thinking about water has shifted from "give them water" to "let them find water," which seems to be more abundant down a foot or two than we expected, even in a year with very little spring rain.
Who'da thought?