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:
Good uniformity of manufacturer
Had excellent response to pressure variation
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
Sand media filters are commonly used in agricultural microirrigation systems. They have the advantages of simplicity and large capacities and are favored by many farmers and designers over other filtration hardware when there is a lot of organic matter in the water. The Irrigation Training and Research Center (ITRC) at Cal Poly San Luis looked at sand filters to see if it were possible to use lower-than-accepted backflush pressure and thereby reduce the total pressure required for irrigation systems. By lowering backflush pressure it would be possible to design a system that could run at an overall lower pressure and hence cost. The various components of microirrigation systems run at lower pressures than the backflush pressures recommended for most sand media filters.
The conclusions are:
There are substantial pressure differences amongst different models and designs during backflush and filtration
The main pressure loss is at the backflush valves
If designed right large backflush flow rates can be accomplished at low backflush pressures (this is critical for proper detritus removal).
There are substantial differences among underdrains of various models which affects pressure requirements
No large intimal high pressure was necessary to break up the media bed, a common practice.
Different underdrain designs create different patterns of cleaning the media.
There were substantial differences among models in the amount of sand discharged from the system at backflow rate of 190 GPM. Sand discharge should actually be avoided since it's an indication of preferential flow and poor cleaning.
These are some new ideas, and even though they are meant to reduce pressure and energy use, they are also good management suggestions.
If this strikes your fancy, read more at http://www.itrc.org/reports/mediafilters.htm
A crucial aspect to microsprinkler and dripper performance is maintaining the size of the orifice as it was delivered from the factory. Even small changes in the size of the orifice can have significant effects on the volume of water distributed in the orchard. One of the most common causes of a decrease in orifice size or even clogging is a result of the high lime content of our waters. Calcium carbonate precipitation can readily be observed by the whitish deposits that form on emitters and microsprinklers. With the drought it's important to make that water go farther.
Reasons for carbonate precipitation include the following:
1. Change in the pH of the groundwater. This can occur when groundwater is pumped. Pumping reduces the pressure of the groundwater as it flows into the well. This reduction releases dissolved carbon dioxide gas causing the pH of the groundwater to increase. The pH increase can result in carbonate precipitation.
2. Evaporation of water in the dripper or microsprinkler. Evaporation increases the concentration of chemicals dissolved in the water remaining in the emitter. Because of its low solubility in water, calcium carbonate readily precipitates during evaporation.
3. Increase in water temperature. The solubility of calcium carbonate is reduced as water temperature increases.
4, Injection of certain chemicals, such as bleach or some fertilizers that interact with the water.
The problem of lime precipitation depends primarily on the pH of the water. At pH values less than 6, mostly dissolved carbon dioxide and a small amount of carbonic acid exist in the water. At pH values between about 6.5 and 10, bicarbonate is the dominant species. When water evaporates from the irrigation system the bicarbonate precipitates as lime if there is adequate calcium in the water. The potential for carbonate clogging is highest when bicarbonate concentration in the water exceeds 2 milliequivalents per liter (meq/L) and the pH exceeds 7.5.
This relationship of bicarbonate to water pH indicates that lowering the pH will prevent or reduce carbonate clogging of the system. Lowering the pH, dissolves any existing carbonate precipitation and prevents the formation of lime deposits.
A water's pH is lowered by injecting acids. The common acids, such as sulfuric and hydrochloric (muriatic) have been used, as well as the more expensive citric and nitric acids. An acid/fertilizer compound of a combination of urea/sulfuric acid (N-pHURIC ®) has proven to be useful and much safer than straight acids. This acidifying material is most commonly used for water treatment rather than as a source of nitrogen. If the material is used in any significant amount, its nitrogen contribution to the fertilizer program needs to be considered.
Determining frequency and amounts of acid to prevent clogging can be fairly matter of fact. Depending on the rate at which carbonate precipitation occurs, acid injection may only need to occur intermittently during the irrigation cycle to. It might only require 30-60 minutes to maintain a pH of 4. With more problematic waters, continuous acid injection to maintain a pH between 5 and 7 may be necessary.
The amount of acid needed to lower the pH to a desired level depends on the bicarbonate/carbonate concentration in the water and the target pH. The water can be sent to a lab for determining the acid amount or a trial and error approach can be used. Acid can be added to water in increments while measuring the pH until the desired pH is reached. Water pH can be measured with pH test strips or a hand held pH meter. Test kits are also readily available at swimming pool supply stores.
Other than acid for correcting lime clogging, there are several other amendments being sold on the market. Sodium hexametaphosphate has also been used and works against iron and manganese clogging. The small amount of sodium is not a problem. It is safer than an acid, but costs a bit more.
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.
We are creatures of habit and when we see the effects of a treatment, we can often persist in seeing the same or similar symptoms and assuming the cause is the same. In a recent case, a newly planted ‘Pixie' orchard, planted in August had gone into an old ‘Valencia' ground. The trees went through an adjustment period, but still didn't look sprightly in the fall. The grower applied a hand application of urea on the rootball that within 10 days had caused the trees to go into a salt swoon. Meaning, they got too much fertilizer that burned them. The grower seeing the effect, immediately started the sprinklers, but the damage was done. Several months later the trees had either died or were still lingering, but hanging in there. The trees were still coming out of winter, but the trees hadn't perked up. It was a dry winter and some of the yellowing was due to underirrigation, that overall yellowing from lack of nitrogen and the leaves were curled. But the assumption was still that the trees were recovering from the salt burn from the urea.
Looking more closely at the trees, something else was odd about some of the trees that were continuing to die. The leaves suddenly wilted. Getting down on hands and knees and digging around the roots, there were few roots and ………………………….a tunnel. A gopher had been at this tree and the lack of roots were probably due to Phytophthora root rot. Looking around there were some old ‘Valencias' that had been hit by gophers and there were gophers mounds and runs all over the place.
So, young trees planted in the heat of the summer into root rot ground with gophers waiting in anticipation that had been salt burned in a year with little rainfall. A lot of causes for trees that generally weren't happy – triste, as they say in French.
So what are the lessons here? Avoid old citrus ground when planting with citrus, and if you can't make sure, don't plant in a stressful period. Phytophthora loves stressed trees and adding lack of rainfall an gophers and salt, just heightens the stress. Make sure to get the irrigation right. Don't irrigate them to the schedule of the older trees and start them off on one of the phosphite materials.
Wilted, yellow leaves from lack of water and Phytophthora
Gopher chewing on stem
Gopher run and lack of roots from Phytopthora and gopher
Gopher mounds in planting area
Older Valencias dead and dying from Phytophthora and gophers