It's a little more complex than that.

To simplify for a moment, let's pretend like you're on city water with a static pressure of 50psi. That means that while no water is flowing through the system, the pressure is 50psi, and it will be 50 psi everywhere in the system. What static pressure represents is the amount of pressure you are starting with.

Dynamic pressure, on the other hand, is the pressure when water is flowing, and the pressure is litterally different everywhere in the system. How do you lose water pressure? As it flows through anything. Objects like water filters, backflow preventers, and irrigation valves act like single point pressure losses. The amount of the pressure loss depends upon flow rate. So a valve might lose 4.7 psi at 5gpm (gallons per minute) and it might lose 5.4 at 10 gpm. The other primary source of pressure loss is the pipes. The pressure loss is usually measured in psi per 100 feet of pipe, and because it depends both on pipe size and flow rate, this type of information has to come from pressure loss charts. And because the inside diameter is different for different types of pipes, you need a chart for the exact type pipe you are using. Changes in elevation will also affect pressure. An elevation gain of 10' will loose about 4psi while an elevation loss of 10' will gain about 4psi.

Finally, for a sprinkler to work, it has to have a certain amount of pressure when the water gets to the sprinkler. The amount of pressure at the head determines the performance of the head.

So the way you have to work it out is layout your system. Then assume a working pressure at each head. Assume something like 30psi. Then for your selected pressure, determine what is the flow rate of the head at that pressure. Then look at all the pipes. Determine what is going to be the flow rate through each pipe. If you are assuming 30psi at every head, that means that the flow through any segment of pipe equals the flow of one head times the number of heads the pipe feeds. Then based on the pressure loss charts for the pipe, you have to figure out how much pressure you expect to lose through each segment. So if you have a 25' section that the charts (for your pipe size and flow rate) indicates a pressure loss of 4psi (per 100'), that segment will lose 1 psi.

So once you know all of your pressure losses, you start with the static pressure, work out all the pressure losses through the system, and see if you don't still have 30psi or more of pressure at the head. If the pressure losses leave less than 30spi at the head, you have to change the design. Try again with a lower working pressure, or increase the size of pipes for less pressure loss. From there, it's pretty much trial and error.

It's a lot of reading, but these principles are better spelled out at

www.irrigationtutorials.com.