To grow an even crop the pressure within your irrigation system must be uniform and within the operating range for your sprinkler type.
Matching sprinklers to their recommended pressure and checking to make sure they have even pressure in the lines are simple things you can do to make sure your irrigation system is operating efficiently.
All sprinklers have recommended operating pressures that determine their flow rate and the distance the stream of water is thrown (the diameter). If a sprinkler is operating outside its recommended pressure range it will not be operating efficiently and is therefore not doing the job you intended it to.
Running sprinklers at low pressure will form larger droplets and decrease the diameter of throw causing dry patches form around the sprinkler giving a “doughnut effect’. Running sprinklers at higher than recommended pressure will cause misting making the stream of water more likely to evaporate or be blown away before it reaches the ground.
Check to make sure your sprinklers are operating within the recommended pressure range. The manufacturer of your sprinkler will have recommended pressures for each jet size. As a rough guide, 2.4 mm - 4.8 mm jets need between 240 and 345 kPa, 4.8mm - 6.3 mm jets need 310 to 415 kPa and 6.3 - 9.5 mm jets need between 345 and 480kPa.
Pressure also affects the volume of water (flow rate) a sprinkler will distribute. Pressure and volume are closely related. A jet operating at lower pressure, will distribute less water than the same jet running at higher pressure. For example a 4 mm jet at 200 kPa will put out 15.5 litres per minute, while the same 4 mm jet running at 300 kPa will put out 18.5 litres per minute. Three litres per minute may not seem much of a difference, but it would mean that one section of your crop is receiving 20% less water. For the uniform application of water you need a pressure difference in your laterals of less than plus or minus five percent of the average pressure.
So, next time you are checking your irrigation, get a pressure gauge and check your sprinkler pressures along the laterals in at least three places to check for any differences. Good places to check for pressure differences are at the valve, the end of the lateral and halfway between.