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Takes a lot of rain to penetrate deeply. And soils that have been occupied by conifers for a time may become water repellant (hydrophobic).

Re:
" soils that have been occupied by conifers for a time may become water repellant ."

How? and/or why?

how does it change the soil structure? or what does it do?

I've seen waxes given off by the trees mentioned. Otherwise when the conifers are still present a planting area can be dusty because they may use their crown structures to divert water away from the trunks, resulting in persistent dryness beneath their canopies.

But local soils can be dry to some depth in October just because it stops raining in July and there isn't enough precipitation to compensate again until the hammer falls in November - the stormiest month of the year. If a body doesn't water enough to compensate and the planting is not made on a retentive soil or a site that tends to remain more moist for other reasons then by this time of the year they will be finding some quite dry dirt when they dig around.

I'm still finding dry soil several inches down under large Doug Firs, even under several inches of woodchips out in the open.
Really dry fine soil repels water like flour. I remember as a kid north of Spokane we used to disturb the top layer of soil between the rows in the vegetable garden and use the dust as mulch. It broke up the continuity of capillary action and conserved water. Kept the weeds down too. We had hay and straw in the barn but never thought to use that. My Grandfather was in charge and I was kid slave pushing the cultivator for what seemed, at the time, for miles. ;-) The garden was as big as a city lot, and then some.
I wish I had that rock free volcanic soil over here.
Mike

Maybe Saint Helens or Rainier will erupt again soon.

another theory: the clay layer might have been hydrophobic during the heavy rains, and just drained the water away.

I would be tempted to look at a Hebe in that position. It would appreciate the drip irrigation. Personally, I would try to get one rated zone 7 or so (I get conservative with hardiness the closer I get to a house). I won't make any claims re the deer, but Hebes are on a lot of pacNW deer resistant lists.

Interesting conversation. We get the least rain of anyone in our zone during the summertime and with the huge Doug Firs ringing our property, there is always competition for the moisture. Eeldip, your suggestion of a hebe is interesting. I had one this summer that the deer left alone, a variegated one. Previously, they had eaten them to the ground. I continue to learn.

Deer feeding variable, the real test (for the deer and unprotected plants) is when there is a hard winter and they are having trouble finding food.

Without effective fencing plantings are constantly subject to whatever deer and rabbits may happen to be doing at the time.

Yes, deer in different areas have different appetites. Last winter they ate everything on the "don't like" list. I have now wrapped the whole border in black plastic mesh. It is impossible for us to fence and there is no hunting in our area, so I continue the war.

If it was a sloped area, probably most of the water ran off and went somewhere else. Unless ground is perfectly flat the water will find a PATH OF LEAST RESISTANCE. this is a universal law .

Feeding behavior varies over time as well as between individuals. Winter weather is probably the strongest general influence, as most of the foliage disappears then but they still have to eat.