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Microirrigation System Flushing

Procedure and Requirements

Farouk A. Hassan, Ph.D.


Microirrigation systems convey water in pipes and tubing from the pump (water source) to the plant with virtually no losses in transmission. Special emitters and lateral lines are used to deliver water, fertilizers, and agricultural chemicals in a localized manner close to the plant at much smaller application rates than those of conventional surface and sprinkler irrigation methods. This makes it possible to apply water and fertilizer by microirrigation more frequently and maintain favorable moisture and nutritional conditions in the root zone. Microirrigation is steadily gaining acreage in California, Florida, and other states, also in Mexico, South America, Africa and in many parts of the world.

Advantages and Limitations of Microirrigation

A well designed, managed and maintained microirrigation system can enhance the efficiency of utilization of irrigation water, fertilizers and agricultural chemicals. It can also minimize losses by runoff, deep percolation, evaporation, and wind drift. It conserves energy and preserves the environment. Due to its above mentioned advantages, microirrigation is usually preferred over conventional irrigation for growing crops on slopes, on soils of low infiltration (e.g., clay soils), of low water-holding capacity (e.g., sandy soils), and of shallow profile. However, microirrigation is not without limitations. It is a capital intensive and management intensive system and it is not a cure-all solution. The key to successful microirrigation is system maintenance and management.

Maintenance of Microirrigation System

The main objective of converting to microirrigation is to improve crop yield and production efficiency while water saving is always contemplated. Microirrigation system maintenance is essential for achieving the above mentioned objective. System maintenance aims at preserving design emission uniformity (EU) and efficiency. This can be achieved by operating the system according to design pressures and flow rates while making regular field inspections, performing necessary preventive measures in a timely manner, and promptly remedying observed malfunctions. Regular monitoring of the system performance, including pump discharge rate, pressure and water quality, enables the system operator to anticipate rather than react to problems. This strategy keeps the system running well, avoids malfunction surprises, and minimizes down time.

Emitter Clogging and Flushing

One of the major problems of practicing microirrigation is emitter clogging. Regular system flushing is required for minimizing the clogging of emitters. It is also an integral part of any sound maintenance program.

Flushing Requirements

Sediments can be flushed out from pipes or tubing with a minimum flow velocity of one foot per second (ft/sec), which is referred to as the "scour velocity". In a standard half-inch lateral line, the one ft/sec velocity is equivalent to flow rate of about one gallon per minute (gpm) at the downstream end. Each main line, submain, and manifold should be equipped with a flushing valve on each side of the line. The size of the flushing valve on a line should not be less than half the size of the largest pipe on that line; for example if a line is tapered from 4 to 6 inches diameter, the size of the flushing valve at the end of this line should not be less than 3 inches. However, no flushing valve should be smaller than 1 inch. Figure 8 or similar fittings should be used at the end of lateral lines and should be accessible at the ground surface for flushing.

Flushing procedure

a) Flushing should start with the pipes of largest diameter going down in size, beginning with the mainlines and proceeding to the submains, manifolds and laterals. It may take about 30 to 60 seconds before high sediment water is flushed and then clean water should run for 2 to 3 additional minutes before flushing valves are closed. A general recommendation is to flush the underground lines 4 to 5 times during the irrigation season and laterals should be flushed weekly. More frequent lateral flushing may be necessary depending on the type and quantity of contaminant observed. The laterals should also be flushed following fertilizers or chemicals injection and any periodic chlorine injection. Flushing of a repaired system prior to resuming irrigation is highly recommended.

b) When flushing laterals, open the end of the line while the system is running and allow water to flow until it runs clear. Collect some of the dirty water in a glass jar, and examine it carefully. Take notes of the nature and color of the impurities in the water. If there is a significant amount of contaminant in the flush water, find out what it is. Does it appear to be bacterial slime, is there evidence of iron precipitation, is there any soil material or sand from the media filter. The presence of such contaminant in the water flushed out of laterals could be an indication of a serious clogging problem in the making even if the problem is not so evident. The first step toward alleviating a clogging problem is identifying the contaminant and determining the cause of the problem.

Identifying the Contaminant

Put samples of the water flushed out of laterals into two small jars or test tubes. Treat one with few drops of chlorine and the other with drops of hydrochloric acid. Note the changes: chlorine will attack organic matter, while acid will dissolve many inorganic precipitates. Soil and sand particles will not be affected by acid or chlorine.

Inorganic Precipitates

The presence of the white calcium carbonate (lime) precipitate in the flushed water could be attributed to high bicarbonate concentration in the irrigation water and high pH (8 or higher). Acid injection is the primary treatment for this ailment. Reddish stain and rust particles is an indication of precipitation of iron oxide due to high iron content in the water (0.3 ppm or higher). Iron separation from the water can be achieved by aeration and sedimentation in a settling basin.

Organic Material

High bacterial count with moderate iron concentration could cause the appearance of reddish sludge in the flushed water. White cotton-like slime is usually associated with the activity of sulfur bacteria and high concentration of hydrogen sulfide in the water (0.2 ppm or higher). Chlorine injection is commonly used for controlling microbial growth in the system.

Soil Particles or Sand Media

Soil sediments that appear in the flushed water are usually cemented together by microbial slime. High load of fine particles (100 ppm or higher) of clay and silt could be separated by sedimentation in a settling basin prior to pumping the water into the system. The appearance of sand media in the flushed water calls for checking if the right media is used, also examining the underdrains for any corrosion or damage.

Evidently periodic evaluation of the quality of irrigation water by laboratory analysis is essential for anticipating and identifying potential clogging problems.

Conclusion

The importance of preventive maintenance cannot be overstressed except to say that there are two kinds of maintenance: preventive and after-the-fact, the former saves money and the latter costs money. Regular flushing of microirrigation system is an indispensable part of any successful preventive maintenance program.


Farouk A. Hassan is an Irrigation & Soils Consultant with
Agro Industrial Management
P.O.Box 5632
Fresno, California 93755

Phone: (209) 224-1618, Fax: (209) 348-0721, E-mail: weaim@aol.com