@ghctim, good question. Flow rate, head placement and nozzle selection (for each head) all play a role in calculating the precipitation rate – which is the amount of water put down by a sprinkler zone in a certain amount of time. It’s measured in inches per hour. The reason we care about it is because we want all of the heads on the same zone to have ‘matched’ precipitation rates. You want an area to be evenly watered, not soggy in one spot and dry in another. Some beds or turf areas are irregular in shape, varying in distances to be covered. Varying radii of sprinkler heads are therefore required within the same zone.
I often get asked, “Could these two types of heads work together (in harmony) on the same zone?” The answer is sadly no – they were never meant to be together. Why? They have different precipitation rates. Rotors have a much lower precip rate (I’m going to start saying ‘precip’…that’s how us sprinkler guys talk), which means it takes a lot longer for a rotor to put down the same amount of water as a spray head would in the same area. So if you have rotors covering a large section of lawn coming on with sprays covering a smaller section- one section will always be too dry or too wet, depending on the run time. That’s why well designed systems have the same type of heads on individual zones.
Zones with spray heads can be even trickier to configure if you’re using variable arc nozzles, or VANs. The arc (part of a circle…think of a pie wedge ;)) can be adjusted from just a sliver of a few degrees to almost a full circle. These nozzles work good for angles less than 90 degrees, and curved edges that are a few degrees short or more than common fixed patterns. The problem with VANs is their precip rates are higher than regular nozzles. More than double in the case of an 8′ half circle. According to Rainbird flow charts, their regular 8H nozzle has a flow of a half gallon per minute at 30 psi (pounds per square inch; the measurement of operating water pressure at the nozzle), and a precip rate of 1.56 in/hr. The 8′ VAN nozzle set to half circle at the same pressure is 3.58 in/hr! This would be handy if you needed extra water in a spot, but would make it soggy if you didn’t.
Sorry to ramble on, but I thought it’s important to address the many variables to a sprinkler zone that impact it’s watering schedule. As you can see, there’s many moving parts to get it fine tuned.
Hope this helps.