Flow and pressure are two totally different things but are related when it comes to how they affect the operation of an irrigation system. It is easy to confuse one with the other. Your whole system is built upon these two things. When planning your irrigation system, both flow and pressure are important to know and helpful to understand. We will do our best to explain what each one is and how they relate to each other in the operation of your irrigation system.
Flow is the volume of water moving in the pipe or irrigation over a certain amount of time. Most frequently noted in gallons per minute (GPM) or gallons per hour (GPH) in irrigation. Flow rate is defined as the amount of water that moves through or is emitted from, your water source, irrigation system, or component over a specific period of time.
Knowing the flow rate of your water source is one of the first things we will ask you when you reach out for assistance. This is the maximum amount of water that you can expect to utilize at one time. When designing your irrigation system we recommend not planning to the max unless the water source is solely for your irrigation system. Calculating your flow rate is simple, only requiring a bucket (of known size), a stopwatch or timing device, and an easy to access water spigot. Here is a link to our Flow Rate Calculator and Instructions. Our calculator will give you the flow rate in both GPM and GPH. When planning a sprinkler system, GPM is most frequently used; while in slower drip irrigation, we use GPH most of the time.
You can’t increase the flow of your water source unless you increase the pressure, but you can use control valves to control or reduce flow if necessary.
Pressure, in irrigation terms, is a measure of the amount of force (of water) over a one square inch surface area. In the US, this is generally noted as PSI (pound-force per square inch), which we will use here at Drip Depot. In other areas of the world, the term Bar or Kilopascal (kPa) might be more common. The pressure is what pushes water through the pipes. Whether you have a pressurized piping system like municipal (city) water or a well system or have a tank or cistern using gravity, the only way to increase your water pressure is by mechanical means like a pump, or increased elevation for a tank gravity feed systems. Elevation changes affect the pressure in this way: Every 2.31 ft in elevation = 1 PSI or every 1 foot in elevation change increases/decreases pressure by .433 PSI.
Two different pressures are relevant when planning your irrigation system: 
The static water pressure is water at rest, which means it is measured when no water is flowing. Attach a pressure gauge directly to a water spigot and make sure all other water sources are shut-off, then turn on the spigot and read the gauge. This is the pressure your water system provides. Most residential water pressures are generally in the 40 - 80 PSI range. Under 40 is considered low, so you may need to boost pressure if installing an irrigation system. Over 80 is high, and many household water appliances may be at risk of damage over this range. A pressure reducing valve may be necessary. Static pressure is affected by elevation.
The dynamic water pressure is the pressure of the flowing water, often referred to as working or operating pressure. To measure your dynamic pressure you will connect a pressure gauge at the end of one of your lines, turn on the system and let it fully pressurize, then take the pressure reading with emitters flowing. Dynamic pressure is affected by elevation and friction loss. As water moves through the pipe or line, the turbulence causes friction, which causes loss of pressure. Dynamic or operating pressure is what you control with a pressure regulator in your irrigation system. Operating pressures will vary by the type of emitters used.
One more term that you might come across when working with flow and pressure in your irrigation is velocity. Velocity is the speed of the flow, how fast the water is moving through the pipe. We measure velocity in feet per second (fps). Most irrigation professionals recommend not exceeding 5 fps in irrigation. This is where pipe size comes into play in your planning. Higher velocity means higher friction loss resulting in lower pressure. A common myth is using a smaller pipe size will provide you with more pressure. This myth is false! The pressure pushing the water (flow) through a smaller pipe is increasing the velocity, which in turn causes even more turbulence, thus increasing the friction loss. You end up with less usable pressure.
Laminar flow is smooth flow with minimal friction loss, while turbulent flow has the water molecules bouncing off the pipe wall, thus causing higher friction losses. Most of our irrigation and plumbing systems experience turbulent flow. Long pipe/tubing lengths, fittings, and other components all create turbulence. The four main factors affecting friction loss are:
Water velocity - Water movement in the pipe causes turbulence, which causes pressure loss. Velocity increase when you add flow or use a smaller pipe size at the same flow.
Inside diameter of the pipe - Smaller pipe size with the same flow increases turbulence because there is a higher percentage of water contacting the wall surfaces.
Roughness of interior pipe wall - The rougher surface the more turbulence, resulting in higher friction loss. Pipe wall roughness is rated by what is called the “C” factor. The lower the value, the rougher the surface.
Length of pipe - The longer the pipe length, the greater the cumulative effect is of the first three factors.
To make things easy for us DIY’ers, Friction Loss Charts have been published for pipe and tubing used in plumbing and irrigation systems. The most common computation used in calculating pressure drop in fluid flow is the Hazen-Williams method, and most of the charts are based on that principle. Once you take a look at all the calculations required to figure this stuff out, you will be happy to use the charts. Below are links to some of our manufacturers' Friction Loss Charts for your convenience.
Hunter Industries Friction Loss Chart
Just to recap, the importance of flow and pressure when designing your irrigation system is paramount to the success and efficiency of your system.
Check the flow rate of your water source using the Flow Rate Calculator found here: https://www.dripdepot.com/irrigation-calculators/flow-rate-calculator
Test your water source ‘static’ pressure (no flow) using a simple hose-threaded pressure gauge found here: https://www.dripdepot.com/category/lawn-irrigation-pressure-gauges
Once you have this information, you can start designing your system to meet your needs while staying within the boundaries of what your water source can provide. There is one additional boundary you will want to consider when designing your system, the mainline and sub-main or lateral line sizes. All tubing/pipe sizes have maximum capacity limits to reduce the friction losses we discussed above. See our Tubing Buying Guide and Irrigation Planning Guide and the Friction Loss charts or graphs to help with this.
Determine the flow rate of your system by adding up all the emitters multiplied by their flow rate(s) in your system or largest zone.
Determine the operating pressure (dynamic) your system components will require.
We do hope this helps and welcome any questions or feedback. You can reach us at support@dripdepot.com.