[Reproduced from the journal Irrigazette, no. 54, September-October 1999, published by EURL, Aix-en-Provence, France]

The perfect emission device does not exist. The user’s choice will be the result of a well-thought-out compromise between technical and economical requirements. The device or devices which are best technically adapted are those which offer a superior compromise between their particular performance features and the characteristics of the environment in which they will be placed. This includes the nature of the soil and type of crop, agricultural practices, water quality, topography of the terrain, and the shapes and dimensions of the areas to be equipped.

Nowadays, low volume irrigation installations are mostly equipped with emitters and drip tape, yet it is possible to use other systems such as microsprays or microsprinklers. The choice of a low volume irrigation system can be reasonably based, in part, on knowledge of the various equipment and systems, and on the analysis and consideration of certain parameters such as the type of crop, topography, water resources, and nature of the field to be irrigated.

Microsprays are small, non-rotating sprinklers with flow rates ranging from 20 to 150 L/h (20 or 40 L/h are the most frequently used flow rates in France) and a radius of 1 to 2 metres. The shape of the wetted area varies depending on the type of spray used. These units are relatively vulnerable to clogging and require fine filtration from 120 to 150 microns and are mostly used in tree crops.

Microsprinklers are small, rotating sprinklers, with flow rates which are higher (100 to 300 L/h) and a throw of 3 to 6 metres. Less likely to clog, they still require fine filtration and are used in tree crops and in greenhouses. The wetted areas range from 1 to 6 metres in diameter. Microsprays and microsprinklers are necessary when lateral water infiltration is inadequate or when water is not available at all times (irrigation interval).

Emitters are less vulnerable to clogging. In terms of hydraulics, most emitters presently available commercially have long, tortuous flow paths. The water in the dripper follows a fine labyrinth with a zigzag path, which creates a pressure loss. The cross-sectional area in which the water flows is thereby larger, and these emitters are less likely to clog than those with a laminate flow. The usual flow rates are from 2 L/h to 4 L/h.

• The nonpressure compensating emitters’ flow rate varies directly with the pressure. This limits the lengths of the run and use in hilly terrain. They are generally less likely to clog and a filtration of 120 to 150 microns is generally sufficient.
• The pressure compensating emitters’ flow rate remains constant when the pressure varies within a certain range. They are useful for very long runs or on hilly terrain since they limit the number of main or secondary lines. They are generally a little more likely to clog and a filtration of 100 to 120 microns is recommended.

The second most widely used water distribution device in annual crops is drip tape. Some disposable units are used only during one irrigation season and others, which have a longer life span, are reusable. Today, there are about ten different models on the market. The tapes are made of extruded or welded polyethylene film with a long labyrinth path in zigzag form. The flow rates and the operating pressures are low, and are in the range of 1 L/hr at 0.6 bar pressure. However, the water outlets are close together and generally wet the ground in a continuous line. The thickness of the file from which these tapes is made is given in thousandths of an inch. Some manufacturers offer various wall thickness (4, 6, 8,15 and 20 thousandths of an inch).

In tree crops, the 2 L/h emitters are used with two lines watering a single row in order to develop the root system so that the trees resist uprooting from the wind. The spacing of the emitters is set at about 1 metre. Emitters with a flow rate of 4 L/h are used in tree crops. Their spacing varies according to the spacing of trees and the types of soil from 0.8 to 1.5 m. The online and inline emitters are presently the most used. In-line emitters are widely dominant. The tubes equipped this way are easier to manipulate, and can be moved and reeled without much trouble. The online emitters are used in greenhouses and in tree crops. In greenhouses, they are placed directly on the ground close to the crop or mounted on the tubing, and maintained close to the plant by a metal tie-down.

In tree crops, the use of these movable drip systems permits the owner to postpone making all of the investments at one time by moving the drip lines as the trees are planted and grow.

In vegetable row crops where the plant density is high, the emitters are set close together and only low flow rates of 2 L/h are used. A wetted areas is thereby created which is favourable to crop root development and the flow rates are reduced.

Tapes, especially the thin-walled, are also intended for vegetable farming. The tapes can be placed under plastic covering, or they can be buried at shallow depths to keep them from being blown away by the wind. The manufacturers generally advise laying the tape with the outlets facing up. The recommended filtration for these tapes is 80 microns. There is also another type of sturdier tape that can last for several years. These tapes occupy an intermediate rank between the conventional laterals consisting of pipe and emitters, and disposable tapes. Like the former, they include well-identified in-line emitters and like the latter the tubing is flexible but also thicker.

[pages 5-7 from the journal Irrigazette, no. 54, September-October 1999, published by EURL, Aix-en-Provence, France]