Adjust the rotor-type sprinklers. For rotors the most common adjustment error is to try to create even coverage by breaking up the water stream using the radius adjustment screw. On a typical rotor the radius adjustment screw is located on top of the sprinkler, just in front of the nozzle. When turned the screw drops down into the water stream causing the stream to deform. This deflects the water stream and reduces the distance it shoots from the sprinkler.
Most newer rotors give the best, most uniform, coverage when the screw is not touching the water stream at all. This is not true of older rotors (at least 20 years old) and really cheap rotors that don’t have engineered nozzles. Often these inexpensive rotors require the radius adjustment screw be used to break up the stream. Most turbine/gear-driven rotors have engineered nozzles and don’t need any adjustment to break up the water stream. Don’t worry if it looks to you like most of the water is falling at the edge of the pattern, this is normal. Most people can’t see the small droplets of water that fall in the area closer to the sprinkler head, so it looks to them like the sprinkler is out of adjustment.
Turn the adjustment screw clockwise until it is touching the water stream (you will notice the stream change shape when the screw contacts it.) Now turn the screw counter-clockwise just enough that it is not touching the stream. This is the proper default position, unless the sprinkler is spraying too far you should leave it in this default position. Warning: the adjustment screw also holds the nozzle in place on most gear-drive rotors. Stop turning the screw when it no longer touches the water stream. If you turn the screw too far, or remove it, the nozzle will fly out and you may never find it! OK, now if the stream of water is going too far, you can turn the screw clockwise until the distance is reduced to keep it within the irrigated area. Use the radius adjustment only if you need to actually reduce the radius so the water doesn’t spray on something like a sidewalk or the house.
Check the yard in a week or so. Dry areas located midway between the rotor heads usually indicate the adjustment screw is too far into the water stream, or the sprinklers are spaced too far apart (check the manufacturer’s recommended spacing.) If a yellow “donut” shaped dry area develops around the sprinkler this indicates that you either have one of those cheap rotors without engineered nozzles, or the rotor sprinkler does not have enough water pressure. Typically lack of water pressure is because the pipe that leads to the rotor is too small, or there are too many sprinklers on the valve circuit. This is a design problem, but it is a bit late for easily correcting it now. If donuts appear, try adjusting the radius screw so the end of the screw just slightly touches the water stream. That should cause more water to fall in the dry donut area. You may need to experiment with different settings until you get the best possible results.
If you don’t want to wait for dry spots to appear in your lawn, there is a short-cut way to check the adjustment and coverage uniformity of your sprinklers. Place identical-size cups, more or less evenly spaced, throughout the area watered. The cups should be between 5 and 10 feet (1,5m to 3m) from each other. Keep the cups at least 3 feet (1m) from any sprinkler heads, if they are closer than that to a sprinkler you will get inaccurate results, also keep the cups at least 12″ from the edge of sidewalks, curbs, patios, or other paved surfaces. Disposable plastic or foam cups work good but tend to get knocked over by the water stream. Here’s a trick- place a stake made from a piece of stiff wire (ie; a wire coat-hanger) next to each cup and use a couple of rubber bands or masking tape to strap the cup to the stake. Run the sprinklers for a few minutes so that the cups are at least 1/4 full, if you have rotors the slowest moving one should make at least 5 passes over the cups. Now compare how much water is in the cups. When the sprinklers are properly adjusted each cup should have about the same amount of water in it. Be warned, it is not always possible to get it perfect if there is an underlying design problem with the sprinkler system. If you want to try to fix the design problem, try creating a new design following the method in the Sprinkler Design Tutorial. Then compare the optimal design you just created with what is actually installed. You may see some possible ideas for improving the system.