I have completed my first try at tweaking the sprinkler head types, nozzles, and layout to get the system down to below 16GPM and improve distribution. I haven't measured actual pressures at each sprayhead, so I am assuming (for now) the pressures listed in the table below the graphic. The system is fed starting at head #1 but I don't know the piping layout. Once I get a pressure gauge rigged to measure sprayheads, I can adjust accordingly.
The original setup had rotor heads #1 through #7, all PGP's with Red-7 nozzles, most cranked down with the radius adjustment. With the installed nozzles that original system would have needed more GPM than available, so I don't know what the actual distribution was. All I know is the irrigated part of the back yard is 3100 sq ft and at 16GPM it would have taken 90 minutes to get .75" of water over that area assuming even distribution. The installer set that zone to 50 minutes, so I was only putting about .42 inches of water down overall, with some areas getting less than half of that. It is no wonder half of the lawn kept dying.
My diagram shows the setup after my planned changes. Any area shown with a pebbled texture is a shrubbery area. The two brown squares are a shed and patio. What I plan to do is move heads #2 and #5 each two feet closer to the back fence and change them out for PGJ's with minor radius reductions, then add head #8, a PGP, to provide overlap for sprinkler #7. All heads get smaller nozzles than the original setup had. The lawn areas near the shrubbery around the back of the house do get coverage from shrub nozzles which are not shown on the diagram. Since head precipitation charts assume proper head layouts, I had to compute coverage by measuring the GPM and sq ft of each head. I then looked at the coverage diagram and added up the inches per minute for each area based on how many heads were hitting that area and the inches per minute of each of those heads. The distribution runs from .006 to .009 inches per minute for most areas, with a few hot spots getting up to .014.
My planned changes should vastly improve the distribution though it is far from ideal. Other than dig up the system and start over, I don't know how to make it better. I welcome your input on my proposed system changes. I haven't started digging or bought parts yet, so feel free to suggest any level of changes.
By the way, after quotes for replacing the dead sod (front yard has issues too, but I am not working on that yet) versus replacing the entire lawn, I have decided to stop fighting Floratam turf and have ordered a new Empire Zoysia lawn which is claimed to be a better grass for Central Florida (more drought and chinch bug resistant) so I have about a week to dig and change the irrigation system before the new grass gets put down.
Sorry for the quality of the graphic. I did my drawing in MS Word and had to do a screen capture to get it as a graphic file. Each head is where the # is shown on the diagram. Head #4 (yellow) is the hardest to see the pattern for because it is blocked by others. It's pattern is close to a half circle which extends all the way to the shrubbery in the top right corner. Head #2 is shown in red with most of its half circle pattern covered by head #1.