Posts Tagged: distribution uniformity
Irrigation Emitters - Do They Operate as Advertized?
The Irrigation Training & Research Center (ITRC) of Cal Poly San Luis Obispo tested 28 different pressure-compensating models of microirrigation emitting devices from a total of nine manufacturers in order to compare independent laboratory testing with manufacturer specifications.
The test results indicate that:
The majority of ~0.5 gallon-per-hour emitters (drippers), regardless of manufacturer exhibited:
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Good uniformity of manufacturer
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Had excellent response to pressure variation
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Had consistent flow rates within the nominal operating pressure range
But that the percentage of well-performing products decreased as the designed flow rate increased. Many of the emitters designated as microsprinklers or sprayers, although pressure compensating did not compensate at the normal operating pressures. Often the pressure compensating feature did not start performing until much higher pressures were achieved. Often this occurred when clogging occurred and this clogging often occurred where the pressure diaphragm was located and was not performing. Sediment would get in back of the diaphragm. Effectively the emitters were not pressure compensating. The testing procedure of numerous medium and high flow models also found individual pieces were found to be defective. These faulty emitters had a measurable effect on the evaluation for those models.
Read more at: http://www.itrc.org/reports/pdf/emitters.pdf
An example of the comparisons that ITRC canbee seen here of their results, compared to the manufacturers' values:
emitter performance from ITRC
Avocados in June
No matter where they grow in California, June is a month when avocados are being watered on a regular schedule. How regular that schedule is should be carefully reviewed by the irrigator. In 1991-'92, right along the coast in a Ventura irrigation plot, we applied 32" of water, but in '92-'93 we put on only 26". Same trees, nearly the same size, but a 23% difference in applied amounts dictated by differences in water demand due to different weather. The irrigation schedule we use is driven by tensiometers and a CIMIS weather station. The station generates reference evapotranspiration values which tell us how much water to apply at an irrigation, and the tensiometers are used to verify whether the trees are doing well by the schedule. Irrigation on a fixed schedule, such as once a week for 24 hours, is going to guarantee that on average you will be either under or over irrigating at each irrigation. Using some soil-based measure, such as a soil probe or tensiometer can assure an irrigator that trees are getting the appropriate amount of water when they need it. If you haven't done so, the irrigation calculator available at the Avovcadosource.com website can be quite useful in guiding an irrigation schedule - http://www.avocadosource.com/tools/IrrigationCalculator.asp - check it out. You also need to correct for salinity accumulation.
In orchards which have not closed canopy yet, weeding is an ongoing activity. In a research plot, we are using tensiometers to monitor the effects of weeds, bare soil and chipped yard waste mulch around trees. In weedy plots soil moisture profile rapidly show 30-40 cbars of tension at 6", whereas the bare and mulched plots can go much longer before showing 40 cbars. Centibars is a measure of moisture tension, the higher the value, the drier the soil. As trees get older they make their own leaf mulch and shade which limit weed growth. There is no question that a cover crop can improve soil conditions through reduced erosion, improved water infiltration, and possible reduced disease and pest problems. These soil improvements tend to improve tree growth and orchard productivity. But, if water is the primary issue, weeds and a cover crop can add considerably to water use in an orchard, especially a young one. Weed control through the use of mulches and herbicides can effectively reduce the water requirements of trees.
June is still a good time to replant an orchard. The soils are warm enough to give the trees a good start and there is enough fine weather left for them to establish before winter comes. Late plantings (September, October) are discouraged because the root-shocked plant sits in a cold, wet soil through the winter and becomes a prime candidate for root diseases. Especially in a replant situation, it is a good idea to start them off with a fungicide with one of the phosphonate products, to give them some protection until they get established. The best time for to apply the material to do its job on older trees is when there is a good root flush of growth which occurs after the leaf flush in spring and fall.
When replanting, try as much as possible to avoid interplanting between older trees. The different water requirements of the young and old trees is such that one or both will be stressed because they need different schedules - less but more frequent for the young trees. Attempts can be made to put together a system where the older trees remain on the 10 or 15 gpm mircosprinkler and the young trees are put on a 1 gpm dripper. This still cannot be an ideal situation, since the needs for application frequency are still different between the small and big trees. The best thing to do is to clear out trees within an irrigation block and replant, or replumb a block with a new valve so that small new block can be irrigated differently from the older trees. Where clonal rootstocks fail in a root rot replant situation, it is invariably where water control is lacking or poor.
As we all know, this has been a long dry spell in the avocado growing areas along the coast. With the levels of salt in our waters, it's important to have some kind of a leaching program to ensure that salts do not accumulate in the root zone. Each winter, rain leaches the accumulated salts from the previous irrigation season and starts the orchard off to a good start. These years it hasn't happened and one of the things that can affect the trees is a stress. This is a salt stress that is most pronounced at the end of irrigation lines and where low pressure results in low output, often at the top of the hill. Any irrigation system that has poor distribution uniformity is going to have areas where less water than average is applied.
One of the responses of the trees to salt stress is to exhibit cankers in the branches. These can be silver dollar-sized cankers running in a line up the branch or as diffuse white spots in the branches. The first symptom is related to bacterial canker and the second is to black streak. These are not killer diseases, but they are reflective of the tree being under stress. As soon as the irrigation schedule is corrected, these symptoms can clear up in several months. If the schedule is not corrected the tress will begin losing leaves and sunburn can result. The symptoms of these two problems can be viewed at the UC Integrated Pest Management website - http://www.ipm.ucdavis.edu/PMG/r8100611.html and http://www.ipm.ucdavis.edu/PMG/r8100311.html.
Again these are primarily stress-related diseases and the way to correct the situation is to improve irrigation distribution uniformity and the irrigation schedule. If it goes on too long it can cause problems especially in young trees. When you boil down farming to the basics, the most important activity in the orchard is ensuring proper irrigation.
avocado irrigation
Fertigating through drip does not always result in even nitrogen applications
An important benefit of drip irrigation is the ability to apply fertilizer through the irrigation water, permitting growers to spoon-feed nutrients, such as nitrogen (N), to their crops. By avoiding applications of large amounts of N fertilizer when the crop is small and uptake rates are low, losses of nitrogen by leaching can be minimized. Also, unlike furrow and overhead sprinklers, drip can deliver fertilizer in the zone where roots are most concentrated.
While drip fertigation offers several advantages for managing nitrogen fertilizer during the season, success depends on the management of the drip system and using best practices for fertigation. Drip systems with poor distribution uniformity may likely cause fertilizer to be unevenly distributed within a field. Also, the strategy of injecting fertilizer into a drip system can affect the distribution of fertilizer to the crop. Proper fertigation requires injecting at a steady rate and at a location that provides sufficient mixing of fertilizer with irrigation water. To assure that the fertilizer uniformly distributes within the field after an injection, sufficient irrigation time with clean water is needed so that all of the fertilizer is flushed out of the drip tape before the irrigation ends.
For drip to be economical for vegetable growers on the central coast, most farming operations retrieve drip tape after each crop is harvested and repair and reuse the tape for 8 to 12 crops. Breaks and leaks in the tape are repaired using a splicing machine (Figure 1). Growers have expressed concern that fertigating through their drip systems is not resulting in even applications of N fertilizer after the tape has been reused for multiple crops. Splicing machines often do not fully repair leaks in tape, and emitters tend to plug over time unless the tape was adequately maintained by flushing and chemical treatment.
In response to grower concerns, we evaluated the uniformity of applied water and nitrogen fertilizer for surface placed drip in 11 commercial lettuce fields during the fall of 2012 and during the spring of 2013.
Fig. 1. Splicing machines are used to repair leaks and breaks in drip tape
Procedures
All fields were planted with romaine or iceberg lettuce varieties on 40-inch or 80-inch wide beds. At each site irrigation, pressure, and fertilizer uniformity were evaluated during a single irrigation event. Field sizes ranged from 8 to 20 acres, and the maximum row lengths ranged from 600 to 1340 ft. Drip tape at all field sites was 7/8 inch diameter, medium flow tape (0.34 gpm/100 ft), but varied by manufacturer and age. The location where fertilizer was injected into the irrigation system, and start and end time of the fertigation, as well as the duration of the irrigation, were recorded. Before irrigating, couplers fitted with ¼ gallon per hour pressure compensating emitters that were spliced in to the drip tape at 24 locations within the field, representing the head, tail and middle areas. Water from these emitters was collected into 5 gallon containers during the entire irrigation (Figure 2) and analyzed for NO3-N and NH4-N. The discharge rate of 4 emitters and pressure of the tape was measured near each of the 24 fertilizer sampling locations (total of 96 emitters). Mass (lbs) of N applied at each of the 24 collection locations within a field was estimated by multiplying the measured discharge rate of the drip tape by the irrigation time and by the concentration of N in the collected water. Uniformity of applied water, tape pressure, and fertilizer was calculated by comparing the lowest 25% of measurements to the average of all 24 measurements. In addition to evaluating fertilizer distribution uniformity, we evaluated the time for fertilizer to travel to the furthest distance from the injection point by injecting food dye for a 5 minute period into the irrigation system and monitoring the water for color at the furthest point from the injection location.
Fig. 2. Low flow (1/4 gph) pressure compensating drip emitters were used to collect samples of irrigation water during the entire irrigation cycle.
Results
Distribution uniformity of applied water for the 11 fields averaged 73% and ranged from 38% to 88% (Table 1). The industry standard for irrigation uniformity of surface drip is 85%. Fertilizer application uniformity averaged 67% and ranged from 46% to 82%. The distribution uniformity of the drip systems of 7 fields evaluated was greater than 74% (avg = 82%) and fertilizer uniformity was greater than 72% (avg = 77%) (Figure 3).
One of the causes for poor distribution uniformity of some drip systems may have been related to pressure. Pressure uniformity averaged 80% and ranged from 43% to 99% (Table 1). Average pressures in the drip tape ranged from 3.5 to 13.8 psi (Table 2). Where the system pressure averaged 4.3 psi, the tape discharge rate was 30% less than the manufacturer's rating. Irrigation distribution uniformity decreased substantially when the average field pressure was less than 5 psi (Figure 4). Additionally, a substantial percentage of emitters of some drip systems were plugged (Table 2) which would reduce irrigation system uniformity. Leaks were evaluated in 5 fields and ranged from 1 to 5 leaks per 1000 ft of tape (Table 2). Significant leaks can potentially reduce drip uniformity by lowering the downstream pressure. Other limitations to good drip uniformity included mixing different types of tape in the same field, fluctuating pressure during the irrigation, and row lengths longer than 800 ft.
Field 8 had a high uniformity of pressure and irrigation distribution but a low fertilizer uniformity. We speculate that the fertilizer which was injected at a “T” connecting the valve in the field with the submain did not have sufficient time to mix with the irrigation water before the flow split into opposite directions. Hence, the average concentration of N on one side of the field was approximately half the concentration measured on the other side of the field. The distribution uniformity of fertilizer on individual sides of the fields was greater than 87%.
With the exception of field 8, fertilizer distribution uniformity was closely related with irrigation system uniformity (Figure 4). Fields with the lowest fertilizer uniformity were operated at the lowest average pressure and/or had the highest level of plugged emitters (Table 2).
Fertilizer was injected at the well in 4 of the fields and at the submain valve in the other 7 fields (Table 3). Injections were made simultaneously using 2 valves at 3 of the fields. Fertilizer was injected during an average of less than 30 minute period often at the beginning of the irrigation (Table 3). The time required for the fertilizer to travel to the furthest point of the irrigation system averaged 42 minutes but ranged from as short as 22 minutes to as long as 1 hour. Field size, row length, and injection location appeared to affect the travel time of the fertilizer. The average time for flushing the fertilizer was 3.75 hours, which was ample time to allow the fertilizer to completely flush from the system. The irrigation industry recommends that for long irrigations (> 4 hours), fertilizer should be applied in the middle of the irrigation cycle. Only at field 10 was the fertilizer applied during the middle of the irrigation. The long flush time after injecting could potential leach nitrate forms of fertilizer below the root zone of the crop. On average, half of the applied fertilizer N measured in the collection buckets was in the nitrate form.
Conclusions
This survey of commercial lettuce fields demonstrated that N fertilizer applied by drip has an average distribution uniformity of 77% when the injection is properly made and the drip system is operated and maintained to achieve an average distribution uniformity of 82%. The results also showed that N fertilizer applied by drip is frequently distributed to fields unevenly due to poor uniformity of the drip systems, or because proper injection procedures were not followed. Operation procedures observed at these sites would suggest that irrigators may need training to better understand the principles of fertigation so that fertilizer is applied at the highest uniformity possible, and in a manner that will prevent leaching losses of nitrate.
Acknowledgements
We thank the California Leafy Green Research Board for funding this project and the many growers that cooperated with the field trials.
Table 1. Summary of irrigation, fertilizer, and pressure uniformity of drip irrigated lettuce fields.
Table 2. Drip tape characteristics at commercial lettuce sites.
Table 3. Irrigation summary for drip irrigated lettuce fields.
Fig. 3. Relationship between distribution uniformity of retrievable drip systems and fertigation uniformity. Each symbol denotes a commercial lettuce field evaluated during the study.
Fig. 4. Effect of tape pressure on the distribution uniformity of retrievable drip systems. Each symbol denotes a commercial lettuce field.
Add new plantings to keep track of your crops
Now that you have a ranch established in CropManage, you are ready to add plantings to the ranch....