14.4.4: Water Applicators

For microirrigation, adequate pressure must be maintained in the pipelines to overcome friction losses and elevation differences to distribute water throughout the field. Once the point of application is reached, the difference in pressure inside the lateral and the atmosphere must be dissipated by a water applicator device.

There are three common types of applicators: emitters, line-source tubing (drip tape and porous tubing), and sprayers. Many different emitter designs have been devised and manufactured with the requirements that the emitters be inexpensive and reliable. Emitters are designed to dissipate pressure while discharging small uniform flows of water at a constant rate. They are often classified according to the mechanism used to dissipate pressure. Long-path emitters have a long capillary-size tube or channel to dissipate pressure. Orifice emitters rely on an individual opening or a series of openings. Vortex emitters dissipate pressure by creating a whirling or circular motion that tends to form a cavity or vacuum in the center of the swirling.

Many emitters are designed with additional features. Some are designed to provide a flushing flow of water to clean the discharge opening every time the system is turned on. Continuous-flushing emitters permit the passage of relatively large particles while operating. Another special feature is pressure-compensating emitters which discharge water at a constant rate over a wide range of pressure. Some emitters have multiple outlets and supply water through small diameter auxiliary tubing at various points. Examples of various types of emitters are illustrated in Figure 14.8.

Figure 14.8. Example of emitters with various designs and features for line-source microirrigation laterals (images courtesy of Toro).

There are three types of line-source tubing, all of which are normally less than 1-inch in diameter. The wall thickness of tubing is available from 0.004 to 0.025 inches. The thin wall tubing is frequently discarded after each crop. The most common wall thickness is 0.008 to 0.010 inches. Recommended operating pressures depend on wall thickness. Several manufacturers recommend a maximum continuous operating pressure of 15 pounds per square inch (psi) for tube walls that are 0.015 inches thick, and 8 to 12 psi for thinner walls. Porous wall tubing is constructed of porous material with pores of capillary size that ooze water when under pressure. Single chamber tubing has orifices punched through the hose wall or emitters fabricated or inserted at fixed intervals along the hose. Double chamber tubing has both a main and an auxiliary passage. Widely spaced inner orifices are punched through the wall common to both passages. Typically, 3 to 6 exit orifices are punched at short intervals in the outer wall of the auxiliary passage for each inner orifice. Sketches of line-source tubing are provided in Figure 14.8. Early developments included double-chamber tubing and porous-wall tubing (illustrated in Figure 14.9).

Figure 14.9. Porous-wall tubing, an example of line-source tubing for microirrigation systems.

Sprayers are designed to discharge a small spray of water to cover an area of 10 to 100 ft2 . Aerosol emitters, foggers, spitters, misters, microsprayers, or miniature sprayers are examples of spray devices. Ideally, sprayers apply a relatively uniform depth of water throughout its wetted area and should have a low water trajectory angle.

The construction and materials used for water applicators are very important because they are exposed to sunlight, chemicals in and applied with the irrigation water, extremes in environmental conditions, and physical abuse. Emitter performance is a dominant factor in the uniformity of water applications and the life expectancy of the system.