Several factors and options determine how long to operate the drip irrigation system for each watering cycle.
The factors include:
The daily water requirement for each plant (gallons)
The number of emitters per plant
The flow rates available per emitter
The time available for irrigation.
The Water Requirement
From our earlier work with the system we should know the gallons per day requirement for each size plant on the project.
The Number of Emitters Per Plant
From the emitter formula calculations we should also know how many emitters we will be using for each size plant.
The Flow Rates Available Per Emitter
Flow rates for drip emitters are rated in gallons of water discharged per hour. The emitter's performance chart in the manufacturer's equipment catalog will tell you what flow rates to expect when various water pressures are applied to its inlet.
The most common flow rates are .5 gph, 1 gph and 2 gph. The flow rate is usually listed for a given inlet pressure like 20 psi (pounds per square inch). For standard emitters this flow rate increases as inlet pressure increases. For example, a 1 gph emitter may be rated for that discharge at 20 psi, but at 35 psi its flow rate may be over 1.5 gph.
Pressure compensating emitters on the other hand, maintain a fairly constant flow rate even though subjected to varying inlet pressures. A pressure compensating device inside the emitter closes down a water -port as pressure resisting an increase in discharge. Pressure compensating emitters have a distinct advantage over standard emitters on hilly terrain where
elevation changes cause changes in line pressure to the devices.
When selecting a flow rate keep in mind as a cautionary note that a high flow rate emitter can cause puddling or water runoff on some fine or tight soils that have very low infiltration rates.
Time Available For Irrigation
Today's landscape water management systems average one or two drip irrigation circuits per project. The emitter circuits have to share the time available for watering with impact sprinklers, bubblers,, spray heads and rotor pop-ups on the site also. Usually, a single, electronic controller handles all the irrigation system control requirements with multiple programs and wide ranges of operating time. This may limit the time available for the longer application of water required for drip. Fortunately, while much of the overhead watering is done in the evening or early morning to reduce evaporation and wind drift, drip irrigation can continue into the day. Shrub beds, hedges, and other plant material can be drip irrigated even when nearby lawns or ball field areas are subject to daytime foot traffic.
Now that you are acquainted with the factors on systems running time and emitter flow rates, work through the steps of this typical example.
A twenty-five foot diameter orange tree like the one previously mentioned in the section on emitter spacing has five emitters and let's say a daily water requirement of 68 gallons per day for its climate zone.