@Modawg2k For some reason that spreadsheet isn't editable by me, but I think I know what it's doing. @emil describes the equation in this thread.
So the equation summarized there is key:
When calculating your nozzle output (precipitation rate, or "PR"), you'll need to factor ALL heads/emitters. Here's the equation:
PR = (96.25(A*B))/(C)
96.25 = constant that converts gallons per minute (GPM) to inches per hour. It is derived from 60 minutes per hour divided by 7.48 gallons per cubic foot. times 12 inches per foot.
A = #heads -or- #emitters
B = flow rate (GPM) per head -or- emitter
C = zone square footage
I'm an engineer, so I went into way too much detail and calculated the units for the 96.25 constant, which are 96.25 (min*in*ft*ft)/(gal*hr). I have 1GPH emitters for shrubs, but the PR equation above is for GPM, so I have to use the value of (1/60) gal/min. So
PR (in/hr) * area (ft*ft) = 1/60 (gal/min) * 96.25 (min*in*ft*ft)/(gal*hr) = 1.6 ft*ft*in/hr
Assuming a 1ft x 1ft area, the PR = 1.6 (in/hr)
A while back I dialed in my settings according to the 'Water Use it Wisely' watering guide. I plugged into their site that I wanted the watering time for a 3' shrub canopy, and it suggested 8 hours. From the Rachio app. I eventually got to 7.5 hrs by creating a custom nozzle and using a PR of 0.2 in/hr. Now, going back to the last equation,
PR (in/hr) = 0.2 in/hr = [1.6 ft*ft*in/hr] / area
area = 1.6/0.2 (ft*ft) = 8 sq. ft
So when I dialed down my PR to get the 7.5 hrs I was targeting, I was inadvertently ended up making an assumption of an 8 sq.ft area in this equation. Does that make sense ? Now remember that I assumed a 3' diameter canopy for the shrubs in the "Water Use it Wisely" site. A 3' diameter circle translates to an area of 7.1 sq.ft. That's eerily close to the 8 sq. ft number that I ended up at in the estimate above, especially given that I my duration is 7.5 hrs instead of 8 hrs.
I could very well have made some basic mistake somewhere here. If not, it seems to me that:
- The area to use for your PR estimate should be an average of the canopy sizes in your yard. Perhaps the zone area would then be the average canopy size * number of shrubs/emitters.
- I question the use of 1 sq.ft. for the area estimate, but as I mentioned I might have made a very basic mistake. I'd love to hear from @emil or @franz, or any of the other experts.
Sorry for all of the math, but I needed to do it to make sense of it all!