There are three factors which affect the pressure in your irrigation or sprinkler system. These factors are the performance capability (or total flow rate), the diameter and total length of hose, and any elevation change between your water source and the sprinkler.
Performance Capability or Total Flow Rate
Performance capability is the first thing to consider when finding how far you can pump water and have pressure to run your sprinkler(s). How much water can your pump move and at what pressure? You should be able to find out from the manufacturer of the pump if you’re unsure.
What is the total flow rate or gpm needed to run the sprinkler? How much pressure (or psi) can your pump produce at that flow rate? Knowing this will give you the base pressure with which you are starting. From there you can calculate the other factors in that cause pressure loss.
Hose Diameter and Length
To choose the proper diameter of the hose, you need to know total flow rate (gpm) that will flow through the hose in order to calculate “friction loss”.
Friction loss is the loss of pressure that occurs in hose flow because of the fluid's friction near the surface of the hose.
The amount of friction loss increases the more gallons per minute you push through a fixed-size hose. To reduce friction loss, you will need to increase your hose diameter to accommodate the desired flow rate and distance your water will be traveling to reach your sprinkler.
Friction loss increases as hose length increases. There is a friction loss chart with the measurements calculated for you and are measured per 100 feet of hose.
Using this calculation, you multiply the friction loss factor by how many 100-feet of hose you will need from your pump to sprinkler. This is how you find your total friction loss through your hose.
You must know the complete elevation change from your water source to your sprinkler or sprinklers. Elevation can change your pressure both positively or negatively. To push water uphill it will require pressure and if water goes downhill then you will gain pressure.
An easy calculation to know is that for every 10 feet of rise you lose -4.33 psi. For every 10 feet of fall in elevation, you will gain +4.33 psi.
Once you know your total pressure loss, you subtract it from your starting pressure. If you have enough remaining pressure and flow rate, you’re your pump will power your sprinklers with any given set-up.
*Note: You can increase hose diameter, if needed, to reduce pressure loss and adjust your numbers accordingly.