So, a great question with absolutely no black and white answer!

Inches are used instead of gallons because it applies to a plant of any size, where the only variable is the area of soil (root mass) being watered. You can easily convert from FEET to gallons if you know the area, using the formula:

Area in square feet X 7.48 X Feet of water = Gallons of water

For example, for a tree with a root mass covering 100 square feet (10 x 10 foot area), 1 inch of water per week would equal: 100 X 7.48 X 1/12 ft water = 62 gallons (approx.).

Of course the age of the tree is not so much a factor as the size. A 3-5 year old apple tree is still pretty small and has fairly limited water requirements according to the above example.

Some thoughts on location, etc... First of all, I'm surprised that you're ready to discount the precip in sunset 2 or 3. Sunset 13 would be a different story, but your zone suggests you are at elevation so you must receive some measurable precip. I'm in zone 3 (average 14 inches precip annually) and have several big old established fruit trees that barely need any watering at all. So watering does not necessarily mean turning on the faucet. It can be done entirely with rainwater of course.

Irrigation - The California Backyard Orchard has an example of a two year old semi-dwarf needs 2.5 gallon per day using emitters and 20%more using sprinklers. (Another site says figure 6000 gal per year. No more info.) So I'm guessing I must budget for somewhere between 20-60 gallon per week. (I'm talking budget because the wife wants to get trees this winter and the cost of water three years from now will probably be the to limit how many.)

Next question. We get snow, sometimes traffic blocking snow in the winter. (Last year total precipitation was 9 in. Mostly in Dec. This year so far 3 inches. Mostly in Jan. Which months do I need to irrigate?

Remember to mulch also.It will help a lot to conserve water. Brady