Have you seen anything about how to adjust watering needs to account for rainfall? Was there anything in the source that you are citing that plots how the watering needs trend over time? I would imagine 6 gallons per week, and 16 gallons per week should be netted against any rainfall during the period?
You are correct that you certainly can adjust for rainfall, as long as you have a reliable measure near your beds. I don't think that this is really necessary though, as hop farms irrigate to around 50-60" per season. There is a sliding scale between too little water (<25" per season) and too much water (>60" per season). I'd say, as long as you are not getting 30" of rainfall between May and Aug, you can just irrigate 30" per season regardless of rainfall. If you do happen to get a good rainfall (2" plus) and want to skip an irrigation event or two, that will also be fine. You can find a close estimate of
average monthly rainfall for your area here.
For reference, the math is:
- 1" of rain = 0.6234 gal/sq. ft.
If you assume a normal size crown has a root area of 3'x3' (9 sq. ft.):
- 1" of rainfall/hop crown = 0.6234 gal/sq ft x 9 sq ft = 5.6 gal/crown/1" of rainfall
If you figure, 120 days (17 weeks) between May and August, that's:
- 30" rainfall/season ÷ 17 weeks/season = 1.75" rain/week/crown (on average)
- 1.75" rain/week x 9 gal/1"/crown = 15.75 gal/week/crown (on average)
Turns out that is pretty darn close to the MSU recommended 16 gal/week/crown.
Now, the majority of that "rain" needs to happen
between mid-June and harvest time. So, If you are using a drip system, I'd start with 5 gal per week and then ramp up to 10 gal and then finally to 16gal per week during mid-June until harvest. Of course, if you are outside the "normal" range for hops, you have to adjust your irrigation accordingly. Also, if you are in clayey soil, you can probably adjust down and if you are in sandy soil, you'll probably have to adjust up.
Hope that helps,