The rate of precipitation (ROP) of a sprinkler is the amount of water it distributes over a given area and amount of time. It is most commonly expressed in millimetres per hour (mm/hr). We use the ROP to calculate how long to run our irrigation zones in order to apply a specific amount of water to our garden. For example, if our ROP is 20 mm/h and we need to apply 10mm to our garden we would run the system for 30 minutes. Most manufacturers will provide an operating specifications sheet which will give you the ROP of a sprinkler under certain installation conditions. In some cases, you may need to calculate the ROP, especially when your design doesn’t fit the exact criteria in the operating specifications sheet. In this article we will discuss how to calculate ROP from first principles.
The ROP is dependent on 3 criteria:
1. Sprinkler Type
There are many different sprinkler types, including rotary sprinklers, cone sprayers (normal pop-ups and stand pipes), micro misters, and drip. Each of these has its own range of precipitation rates, depending on the choice of nozzle. A typical cone sprinkler has a ROP of about 40mm/h while a micro mister may have a ROP as low as 5mm/h.
Typically, the lower the pressure the higher the ROP. This may sound counter intuitive, but there is a good reason for it. As the pressure drops two things happen to the sprinkler. First of all, it uses less water (its flow rate decreases). Secondly the radius the sprinkler sprays reduces which means a smaller area is being watered. The key here is that the area the sprinkler covers reduces faster than the flow rate reduces. Therefore, we have proportionately more water on a smaller area and the result is a higher ROP.
3. Sprinkler Spacing
In general, the further sprinklers are spaced from each other the lower the ROP (more area being watered with the same amount of water). In addition to this it is important to note whether the irrigation system was designed using square spacing or triangular spacing. Triangular spacing yields a higher ROP than square spacing (for more information on spacing see our article on methods of spacing sprinklers). Once we know which sprinkler type we have, what pressure it is operating at and how far apart sprinklers are spaced from one another we can calculate ROP. The formulas for calculating ROP is as follows: