Circuiting emitters (and the plants they irrigate) into groups to be supplied by one valve requires that they all have the same daily operating time, operating pressure and irrigation frequency. As in the mix and match exercises certain plants on the project may be grouped by these factors. If other groups can be formed around their factors, each of those groupings will require a separate valve. For best control of the circuit, each valve should have its own separate station on the irrigation controller or timer (no doubling up).
Grouping similar trees or plants first (before trying to mix and match) is the easiest way to
begin circuiting. For example, one group might be fifteen street trees fronting the project, all with the same gallonage and time requirements. Another group might be forty-five nearly identical plants forming a hedge along a property line.
Once you have grouped the various plants on the project, the next step is to determine how many valves it will require to water all the groups. The object is to use as few valves as possible without losing individual control, overtaxing the hydraulics of the system or creating a maintenance man's nightmare by circuiting groups together that are nowhere near each other on the project.
The amount of water available, its pressure and some basic pipe or tubing hydraulics determine, for the most part, how large each of your circuits can be. In this section of the site, some streamlined hydraulic calculation methods will show you how to size drip emitter circuits.
Basic Drip System Hydraulics
To reduce the quantity and complexity of hydraulic calculations for your drip irrigation systems, this section uses a number of charts and tables designed to work with 1/2" (12mm drip tubing or 1/2" PVC pipe).
The charts on 12mm tubing use 225 gallons per hour as the flow limit and the 1/2" PVC chart uses 360 gph as its maximum flow rate. Though these flows could represent 225 and 360 emitters respectively, actual flows are only 3.75 gallons per minute and six gallons per minute. To avoid writing a manual of basic sprinkler hydraulics within this manual, we will assume that the pipe from your water source to which your drip control valves will be connected can supply a minimum flow of six gallons of water per minute at a pressure over 30 psi.
If you use .580"(16mm), tubing for your drip laterals you can use 225 gph as your maximum flow per circuit and use the rule-of-thumb mentioned earlier of 300' as your maximum tube length.Next, look at the total gph requirements for your various plant groups. (See the example below).
For the 26 gallons per day tree used earlier, a 6 gph per tree flow rate was established for the time available.
If there are fifteen such trees on the project, perhaps in a long row along an entrance drive, our circuit total now would be 6 gph x 15 trees =90gph.