Drip Irrigation Need of the Time

Introduction
Drip irrigation is the most efficient method of irrigating. While sprinkler systems are around 75-85% efficient, drip systems typically are 90% or higher. What that means is much less wasted water! For this reason drip is the preferred method of irrigation in the desert regions of the United States. But drip irrigation has other benefits which make it useful almost anywhere. It is easy to install, easy to design, can be very inexpensive, and can reduce disease problems associated with high levels of moisture on some plants. If you want to grow a rain forest, however, drip might not be the best choice!

Drip irrigation (sometimes called trickle irrigation) works by applying water slowly, directly to the soil. The high efficiency of drip irrigation results from two primary factors. The first is that the water soaks into the soil before it can evaporate or run off. The second is that the water is only applied where it is needed, (at the plant's roots) rather than sprayed everywhere. While drip systems are simple and pretty forgiving of errors in design and installation, there are some guidelines that if followed, will make for a much better drip system. The purpose of this tutorial is to guide you toward materials and methods that will increase the benefits of your new drip system, while steering you away from some common misconceptions and practices that can cause you trouble.

Why would you want to consider drip irrigation?
Drip irrigation can be a great aid to the efficient use of water. A well designed drip irrigation system or subsurface drip irrigation system will lose practically no water to runoff, deep percolation or evaporation. Irrigation scheduling can be precisely managed to meet crop demands, holding the promise of increased crop yields and quality.
Drip irrigation will decrease water contact with crop leaves, stems, and fruit. Thus conditions may be less favorable for the onset of diseases. Often growers or irrigation professionals refer to "subsurface drip irrigation" or SDI. When the drip tube can be buried below the soil surface, it is less vulnerable to damage during cultivation or weeding. Water use can be managed to be very efficient with SDI because irrigations can avoid water losses to evaporation, runoff, and wetting the soil below the root zone.

Agricultural chemicals can be used more efficiently with drip irrigation. Since only the crop root zone is irrigated, nitrogen already in the soil is less subject to leaching losses. Fertilizer N that is added can be used more efficiently. Where insecticides are labeled for application through drip irrigation, less insecticide may be required to control pests.

With all the potential benefits of drip irrigation, conversion to drip irrigation can increase production costs, especially where another pre-existing irrigation system is already in place. Ultimately, there must be an economic advantage to the growers for them to consider drip irrigation.

Advantages of drip irrigation

1. Drip is adaptable to fields with odd shapes or uneven topography. Drip irrigation can work well where other irrigation systems are inefficient because parts of the field have excessive infiltration, water puddling, or runoff.

2. Drip irrigation can be helpful if water is scarce or expensive. Drip irrigation has become common where water is very scarce or where water is very expensive to pump. Precise water application is possible with drip irrigation. Irrigation with drip can be more efficient because evaporation is reduced, runoff is reduced or eliminated, deep percolation is reduced, and irrigation uniformity is improved so it is no longer necessary to "over water" parts of a field to adequately irrigate the more difficult parts.

3. Precise application of nutrients is possible using drip irrigation. Fertilizer costs and nitrate losses can be reduced. Nutrient applications can be better timed to plants' needs.

4. Drip irrigation systems can be designed and managed so that the wheel rows are sufficiently dry so that tractor operations can occur at any time at the convenience of the producer. Timely applications of herbicides, insecticides, and fungicides is possible.

5. Proven yield and crop quality responses to drip irrigation have been observed in onion, broccoli, cauliflower, lettuce, melon, tomato.

6. A drip irrigation system can be automated. For an example of automated drip irrigation.

Disadvantages of drip irrigation

1. Drip irrigation systems typically cost $500 to $1,200 per acre. Part of the system cost is a capital investment useful for several years and part of the cost is annual. Systems can easily be over designed. Growers without experience may want to start with a relatively simple system on a modest acreage and gain experience.

2. Drip tape has to be managed to avoid leaking or plugging. Drip emitters can easily be plugged by silt or other particles not filtered out of the irrigation water. Emitter plugging also occurs by algae growing in the tape and chemical deposits at the emitter. Tape depth will have to be carefully chosen for compatibility with other operations such as cultivation and weeding.

3. The weed control program may need to be redesigned. Compatibility with weed control programs can be a problem if herbicides need rainfall or sprinkler irrigation for activation. But, drip irrigation can enhance weed control by keeping much of the soil surface dry.

4. Drip tape disposal or reuse needs to be planned and will cause extra clean up costs after harvest.

Components and design of a drip irrigation system

A wide range of components and system design options is available. The Digital Drip Directory provides lists of equipment and their suppliers. Tape, depth of tape placement in the soil, the distance between tapes, emitter spacing and flow, and irrigation management must all be carefully chosen to meet crop water requirements and the soil's properties. Emitter spacing depends on the crop root system and soil properties. Seedling plants such as onions have relatively small root systems, especially at the beginning of the season. The wetting pattern of water in the soil from the drip irrigation tape must reach those roots.

1. Drip tape varies greatly in its specifications depending on the manufacturer and its use (Table 1).

Table 1. Drip Tape Specifications by Manufacturer (adapted from Hansen, et al., 2000)
 

2. Distribution system, valves, and pumps must match the supply requirements of the tape.

3. Design must take into consideration the contour: elevation, pressure, and flow requirements. Design for water distribution uniformity by carefully considering the tape, irrigation lengths, topography, and the needs for periodic flushing of the drip tape. Vacuum relief valves need to be designed into the system.

4. Power and water source limitations need to be considered. Water should be analyzed by a laboratory that is qualified to evaluate emitter plugging hazards.

5. Water quality may provide limitations and increase system costs. Filters must match worst case scenarios.

6. Injectors for chemigation should be included.

7. Flow meters are necessary to confirm system performance


Management of drip irrigation

1. Plan for seed emergence. The drip tape needs to be sufficiently near the surface to germinate the seed if necessary, or a portable sprinkler system needs to be available. For example, SDI with the tape tube 4 to 5 inches deep has been used successfully to germinate onion seeds in silt loam soil. Deep placement of drip tape at 12 inches failed to uniformly germinate onions.

2. Water requirements for irrigation are reduced because water can be applied vastly more efficiently with a drip irrigation system. For example, with furrow irrigation systems, typically 4 acre-feet/acre or more of water is applied to onion fields in the Treasure Valley of eastern Oregon and southwestern Idaho. Depending on the year, summer rainfall, and the soil, 14 to 28 acre-inch/acre of water has been needed to raise onions under drip irrigation in the Treasure Valley. Drip irrigation with more water than a plant's requirement will result in the loss of most of the drip irrigation benefits. The soil will be excessively wet promoting disease, weed growth, and nitrate leaching. 

3. Use both measurements of soil water and estimates of crop water use called "crop evapotranspiration" or "ETc". Irrigate only to replace the soil moisture deficit in the top 12 inches of soil. It is usually not necessary to exceed ETc . For our local crop production area, the daily crop evapotranspiration estimates at Ontario are made automatically on a daily chart by the AgriMet station.

4. Chlorine or other chemicals need to be added periodically to the drip line to kill bacteria and algae in the drip lines. Acid may be needed periodically to dissolve calcium carbonates.

5. Filters must be managed. In spite of filtration, drip tape must be flushed, with a frequency that is dependent on the amount and kinds of sedimentation in the tape.

6. If irrigation is managed closely to meet plant water needs, nitrogen fertilizer requirements are reduced because nitrate leaching is reduced or nearly eliminated. Total nitrogen requirements are reduced using drip irrigation and less nitrogen should be applied in each application. Our experience is that if fertilizer nitrogen applications are not reduced, an onion crop under drip irrigation will become excessively leafy. The leaves can inhibit curing and increase harvest costs and increase harvest losses.

7. Fertilizers containing sulfate, phosphate, calcium, or anhydrous or aqua ammonium can lead to solid chemical precipitation inside the drip tape. The precipitates can block emitters. Seek chemical analyses of your irrigation water and competent technical advice before injecting chemical fertilizers into drip tape.

8. Root intrusion needs to be controlled for some crops.

9. Rodents must be controlled, especially where drip tape is buried to provide irrigation for a number of years.

 

The Basic Parts of a Drip System:

Valve-
Use any valve you want! They can be automatic or manual. If you use an anti-siphon valve it has a built-in backflow preventer which saves money! (But be sure to read up on backflow preventers first, as anti-siphon valves won't work in some places. For more information on valves.



Backflow Preventer-
You need to use a backflow preventer on ALL drip systems. No exceptions! For more information on backflow preventers.



Pressure Regulator-
Most drip systems will need a pressure regulator. You need one if your water pressure is over 2,8 bars (40 PSI). If in doubt, install one. The regulator can go before or after the valve. Traditionally it is installed after the valve.



Filter-
You must use a filter. Drip emitters have very small openings that are easily clogged. City water is not free from stuff that will clog your emitters! Use a 150 mesh screen or one with a higher mesh number like 200 mesh. The filter may be installed before the valve or pressure regulator, but the inexpensive plastic filters often sold for drip systems should be installed after the pressure regulator. I like to use a top quality filter and install it right at the water source so it protects the valves and the pressure regulator too. Most valve failures result from sand or rust particles clogging of the tiny passages inside the valve! Use a filter that is the same size as, or larger than, the valve.



Emitters-
Most emitters emit 4 liters/hour (4,0 l/hr) of water. That's about 1 gallon per hour (1 gph). I prefer a lower flow rate than that, and use mostly 2,0 l/hr (0.6 gph, often referred to as "1/2 gallon per hour") emitters on my drip system designs. Use pressure compensating emitters if you are irrigating a hilly area. There are many different types and brands available.

Multi-Outlet emitters are very popular. They also suck! (That's a personal opinion, based on years of observation.) Use single outlet emitters for a less troublesome drip system.



Mainline-
The mainline is the pipe that goes from the water source to the valves. In the illustration of a very simple drip system above the gray colored vertical pipe under the valve is a mainline. The mainline may be made of galvanized steel, copper, SCH 40 PVC, SCH 80 PVC, Cl 315 PVC, Heavy Wall Polyethylene (SDR 7 or SDR 9). Do not use polyethylene for the mainline unless approved by your local building officials. Polyethylene has a low burst pressure and should only be used for mainlines where local conditions are appropriate.


Sub-Main-
The sub-main is the pipe that goes from the valves to the connection point of the drip tube. Many small drip systems do not have a sub-main, in those systems the drip tube connects directly to the valve The illustration of a very simple drip system above shows a system without a sub-main. Sub-mains are usually Cl 200 PVC pipe or standard weight polyethylene (poly) pipe. You use a sub-main when multiple drip tubes are needed.



Drip Tubing-
Drip tubing is a thin wall polyethylene tube, and is generally produced in metric sizes. Common sizes are 12 mm (0.455" or 3/8"), 16mm (0.620" or 1/2"), 18mm (0.720" or 1/2"), and 24mm (0.940" or 3/4"). Do you see the problem? Two sizes are commonly referred to as "1/2 inch" in the USA! The fittings for these two are not interchangeable. So make sure you know what you're getting when you buy it! Do not bury drip tubing underground- gophers and moles love to chew buried tubing!



Adapters and fittings-
Used to attach the drip tube to the other parts. Important- make sure the fittings are the exact right size! Using fittings made for a different tubing size will result in the tube blowing out of the fitting. 9 times out of 10, when a tube blows out of a fitting it is because the fitting is the wrong size.



End Cap-
The end cap is important. Without it the water all runs out the end of the drip tube.
 

Additional Resources:

Drip Irrigation for Row Crops. 1994. Chanson, Schwankl, Grattan, and Prichard, University of California, Davis. Order from Cooperative Extension office, Department of LAWR, 113 Veihmeyer Hall, University of California, Davis, CA 95616, telephone (530) 752-1130.

B.C. Trickle Irrigation Manual. 1999. Van der Gulik, B.C. Ministry of Agriculture and Food Resource Management Branch. Order from Irrigation Association of British Columbia, 2300 Woodstock Drive, Abbotsford, B.C., Canada, V3G 2E5, telephone (604) 859-8222.

Fertigation, 1995, Burt, O'Connor, and Ruehr, California Polytechnic State University. Order from The Irrigation Training and Research Center, California Polytechnic State University (Cal Poly), San Luis Obispo, CA 93407, telephone (805) 756-2434.

Microirrigation Management and Maintenance. 1998. Hassan, Farouk A.. Fresno, CA, Agro Industrial Management, 1998. The book is available from Farouk A. Hassan, Ph.D.
Irrigation & Soils Consultant, Agro Industrial Management, P. O. Box 5632, Fresno, California 93755, U.S.A. Phone: (209)224-1618, Fax: (209) 348-0721,

References:

• Irrigationtutorials.com (Jess Stryker's )
• Malheur Experiment Station (Clinton C. Shock)
  Oregon State University

Edit by M. Irfan

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