Record needle drop
Thank you for your question. It is fairly common for all conifers (including Douglas-fir) to exhibit late summer and fall foliage loss (i.e. yellow to reddish-brown foliage). One would usually observe this on older foliage and branches, whereas newer foliage (usually at the branch tips) remains green. Most conifers only retain their foliage for a certain amount of years, and they cast off older foliage (anywhere from 2 to 11 years old depending on the species) at the end of the growing season. This is a completely natural process and does not damage the tree.
That said, it does appear that you have a significant amount of foliage loss. I notice the browning primarily on older branches but certainly on some newer growth, too. Long spells of hot, dry weather in late summer can exacerbate the visibility of foliage loss, which may be the case with your trees considering the past few hot, dry summers in the Willamette Valley.
I suggest keeping an eye on these trees over the next year. They are likely to improve in appearance as they shed yellow needles and restore their moisture levels over the winter. If you continue to see browning throughout the winter and next growing season, the trees may be declining. In that case, consider contacting an International Society of Arboriculture certified arborist to assess the trees.
Brandy, thanks very much. I was watering the trees over summer for a few years, and stopped doing that a couple of years ago, just forgot. Maybe it is time to start putting that soaker hose to work, eh?
Yes, if you have the means to irrigate your trees during dry weather, that would certainly help. Apply water slowly over many hours so it penetrates to the tree roots, or use drip irrigation if you can. You could also apply mulch beneath the trees to retain moisture. There are a few other tips in this publication from the Oregon Department of Forestry that you may find helpful.