1.4 Drip Emitters and Spray stakes

Drip emitters can also be evaluated by the audit method to be discussed in this module. A drip irrigation device has an emitter that may be one of many designs on the market. The emitter device is basically a low flow rate, low pressure device that emits water close to the individual plant.


Emitter application patterns

Many drip emitters typically discharge in drops or a small spray stream onto the substrate near to a plant and then the water enters the substrate. There are many types of drip devices that use this delivery method. A drip application works fine with fine substrate materials or soils because the water spreads out by capillary action to wet the substrate in a container or the soil in a crop row in a field. With course substrates, the water tends to flow downward and not laterally. A spray stake works better on course substrates or sandy soils than a dripper; the water flows out under pressure and is deflected by a flat surface into a spray pattern that spreads the water out over a greater surface area. The spray pattern wets a larger surface area and causes the water to move downward from a large surface area to wet a larger root zone area.


Spray stakes are attached by low diameter tubing to a larger heavy wall pipe or tubing that acts as a lateral line to supply the water. The spray stake generally discharges its water in a part circle pattern, maybe ranging from 60 to 120 degrees so it is placed on one side of a container to cover the top of the container surface. Figure 1.4a shows a spray stake emitter on one side of the top of a container. A dripper, on the other hand, drops the water below the orifice of the emitter, in a small area of the surface.



Figure 1.4a. A spray stake is designed to spray water over the substrate surface for more uniform watering in course substrate.

Emitter Response to Pressure Change

There are several types of design for the flow channel in drip emitters. Of importance here is that some designs are relatively pressure-sensitive, meaning that their flow rate varies significantly with pressure changes. A few designs are “pressure compensating”, meaning that a flow path modification can be accomplished using an elastomeric disc, diaphragm or water passage. This allows the pressure compensating device to deliver the correct flow of water over a fairly wide range of pressures. These emitters may be in thinwall or heavywall tubing at some spacing; the tubing is laid over the top of a row of containers.


It is important to know whether a pressure-sensitive or a pressure-compensating drip emitter is being used, particularly if there is any elevation change or long lateral runs. Figure 1.4b shows the emitter and coil of heavywall tubing used for watering containers.



Figure 1.4b. Pressure compensating emitter in a heavywall tubing.

Figure 1.4c is a typical layout of a drip system and Figure 1.4d shows a dripper line in place watering trees in containers. These illustrations courtesy of Netafim at http://www.netafim-usa-greenhouse.com where more information can be found on their specific products.



Figure 1.4c. Typical layout of a dripperline system on nursery containers.


Figure 1.4d. Dripperline in place over containers of landscape trees.

Other features of drip emitters include devices to close the emitter so it will not drain below a given water pressure. This helps to keep the drip line charged and ready to be used. Another feature prevents the emitter from developing a negative pressure that would pull dirty water into the emitter.

The important point is that drip systems can be very sensitive to water pressure unless a pressure compensating emitter is used.