- Author: Ben Faber
Chemicals are often injected through irrigation systems, hence the name chemigation. When applied to fertilizers, it is called fertigation. Surprisingly many growers still apply fertilizers by hand, however the predominant use of micro-irrigation systems in avocados readily lends itself to fertigation. This compatibility is due to the frequency of operation and the control the operator has over the system. If the uniformity of the system is high, applying fertilizers through the irrigation system can improve fertilizer distribution, allow for flexibility in fertilizer timing, reduce labor in spreading materials, allow for less fertilizer to be used and can lower costs relative to hand applications of fertilizers.
Chemigation can still cause environmental damage, particularly when the chemicals injected move readily with the irrigation water. Over-irrigation resulting in deep percolation can contaminate groundwater when a mobile chemical is injected. Contamination can occur if: 1) the irrigation water pumping plant shuts down while the injection equipment continues to operate, causing contamination of the water source or unnecessary amounts of fertilizer to be injected into the irrigation system, or 2) the injection equipment stops while the irrigation system continues to operate, causing the irrigation water to flow into the chemical supply tank and overflow onto the ground. To prevent these problems, backflow devices and check valves can be installed. Local regulations should be followed in selecting these devices.
Many different materials may be injected, including organic fertilizers, dry fertilizers and liquids. The major fertilizer injected is nitrogen, but potassium and micronutrients, as well as water treatments, such as urea-sulfuric acid are also injected. The important point to remember is that the materials must be soluble. Fertilizers delivered as a solution can be injected directly into the system, while those in a dry form must be mixed with water to form a solution.
Fertilizer material differs widely in water solubility, with solubility depending on the physical properties of the fertilizer as well as on irrigation water quality. Agricultural grade fertilizers and amendments are often coated to inhibit moisture absorption and to assist in material flow through machinery. These coatings and other foreign materials can cause clogging problems in the mixing tank, as well as the irrigation system. The foreign material exists in the tank as sediment or as a scum on the surface. To prevent problems, stock tanks should be agitated until the material is dissolved. To further prevent problems, the solution should be filtered between the stock tank and the injection point, and the injection point should be upstream of the irrigation system filters.
Irrigators wishing to inject chemicals have a variety of injection equipment from which to choose, including differential pressure or batch tanks, venturi devices and positive displacement pumps. The most expensive are displacement pumps that are powered by electricity or water. They put out precise amounts of material. Venturi and batch tanks are much less expensive and are relatively simple to operate. Their major drawback is that they require a pressure loss to force the material into the irrigation system. A 10 - 30 psi pressure differential is often required and in some places this differential is not possible. In that case a fractional-horsepower pump can be used to provide the pressure differential. Neither of these devices is as precise as the displacement pumps. However, fertigation does not require a fixed concentration of solution, only a known amount of applied material. And these pressure differential applicators do the job very well.
- Author: Larry Schwankl, Freddie Lamm, Dana Porter
Maintenance of Microirrigation Systems
Microirrigation systems include microsprinklers for tree crops, drip emitters for trees, vines, and some row crops, and drip tape for row and field crops. Microirrigation systems apply water to the soil through emitters that are installed along drip lines and contain very small flow passages. Microirrigation systems can apply water and fertilizers more uniformly than other irrigation methods. This uniformity results in potentially higher yields, higher revenue, and reduced irrigation operating costs.
Uniformity, a performance characteristic of irrigation systems, is a measure of the evenness of the applied water throughout the irrigation system. Distribution uniformity (DU), sometimes called emission uniformity (EU), is an index that describes how evenly or uniformly water is applied throughout the field. A uniformity of 100% means the same amount of water was applied everywhere. Unfortunately, all irrigation systems apply water at a uniformity of less than 100%, and thus some parts of a field receive more water than others. Field evaluations have shown that microirrigation systems have the potential for higher uniformity than other irrigation methods. However, clogging reduces the uniformity of applied water in microirrigation systems, thus increasing the relative differences in applied water throughout a field.
The small flow passages in the emitters and microsprinklers make microirrigation systems highly susceptible to clogging. Clogging reduces the uniformity of the applied water and decreases the amount of applied water. Clogging also decreases the amount of salt leaching around the lateral line in saline soils.
The objective of this web site is to provide information to irrigators about the causes of clogging and the methods for preventing or correcting clogging problems in microirrigation systems. Among the topics covered are the sources of clogging, chlorination, preventing chemical precipitation, filtration, flushing, and monitoring microirrigation systems.
This web site is divided into sections to allow the users to more quickly access the information they want. For example, if you already know you have a clogging problem and you want to solve it, go to the section Solutions to Existing Clogging Problems - "I have a problem and I want to solve it".
- Author: Neil O'Connell
By the beginning of the irrigation season, the entire root zone is usually wetted by winter rainfall. Under low volume irrigation during the irrigation season only fifty percent or less of the root zone is wetted with each irrigation on most soil types. Soils with slow infiltration do not allow enough water to penetrate into the root zone to meet the plant’s water requirement. During an irrigation the water puddles while the soil beneath remains dry. Less than ten percent of the soil in the root zone may be wetted during an irrigation when water infiltration is a problem. Water storage in such a small volume of soil may amount to only two to three days of evapotranspiration. The tree may be under stress even though the amount of applied water exceeds the amount lost by evapotranspiration (ET). An infiltration problem is often associated with irrigation water low in salt and/or soils with inherently slow infiltration rates. Soil particles contain sites occupied by electrically charged ions such as calcium, sodium, and magnesium. In an optimum situation, a sufficiently high percentage of these sites are occupied by calcium which results in an aggregating or clumping effect among soil particles allowing water to penetrate. When the percentage of sites occupied by calcium is low and sodium predominates there is a repelling or dispersion of particles and water penetration is reduced. With increasing numbers of the exchange sites occupied by sodium ions the soil particles swell and repel each other creating a dispersion or loss of aggregation resulting in single particles. As this happens the porosity (or pore space) is reduced and the ability of water to enter is reduced. On the other hand as the exchange sites become more occupied by calcium the particles move closer together and aggregate or clump resulting in an increase in pore space. Therefore, soils that have a high percentage of the exchange sites occupied by sodium ions are dispersed and deflocculated and resist the entry of water while those with a high percentage of calcium ions are flocculated and favor water infiltration. With the use of low salt water over time, such as snow melt water, calcium may be removed from the soil particles exchange sites and these sites may then become occupied by another ion such as sodium.
Research addressing this problem of low infiltration was conducted in citrus under low volume irrigation by University of California researchers Peacock, Pehrson and Wildman. The soils type, at the experimental site of mature navel oranges, was a San Joaquin sandy loam characterized by a low infiltration rate. Canal water with a low salt content was used for irrigation. The trees were irrigated with a drip system every week day. Treatments began in June when soils typically begin to exhibit a reduced infiltration rate and were continued until mid-August but measurements continued until September. Simple devices for measuring the infiltration rate, called infiltrometers, were made from 12 inch PVC pipe and installed in the orchard. Chemical treatments and water were applied and rates of water infiltration were measured within these infiltrometers. Gypsum was applied weekly to the soil surface to maintain a slight excess continually on the soil surface and watered in resulting in gypsum application with each irrigation. Calcium nitrate and CAN-17 were each injected into the irrigation water. Calcium nitrate was introduced into the irrigation water at the rate of ten pounds per acre per irrigation. Calcium nitrate was applied daily, biweekly and in a single application. CAN -17 was applied daily, biweekly and in a single application. With these injections into the irrigation water, calcium was being introduced into the water at the rate of 3 milliequivalents per liter. Adding calcium continuously to irrigation water doubled infiltration rates over that of untreated low-salt water. It took 2-3 weeks before a treatment difference could be measured. However, the occasional additions of calcium nitrate or CAN-17 were not effective in maintaining infiltration rates. There were concerns that nitrogen application from these treatments could result in the nitrogen level in the tree being in excess of the tree’s nutritional requirements. Following this research equipment was made available on a commercial basis for regulated injection of materials into low volume irrigation systems.
- Author: Gary Bender
- Author: David Shaw
OK! Let's Strategize. There are four steps for everybody to consider, it doesn't matter if you have a backyard lawn and landscape or if you have 700 acres of avocados.
1. Maintenance: Irrigation System and Cultural Practices
2. Improve Irrigation Scheduling
3. Deficit Irrigation
4. Reduce Irrigated Area
1. Maintenace
a. Irrigation System.
- Fix leaks. Unfortunately, there are almost always leaks for all kinds of reasons. Pickers step on sprinklers, squirrels eat through polytube, branches drop on valves, coyote puppies like to chew….the system should be checked during every irrigation
- Drain the lines. At the beginning of each year every lateral line should be opened in order to drain the fine silt that builds up.
- Maintain or increase the uniformity of irrigation so that each tree or each area gets about the same amount of water. Common problems include different sized sprinklers on the same line or pressure differences in the lines. Where there are elevation changes, every line should have a pressure regulator, they come pre-set to 30 psi. Having all of your lines set up with pressure regulators is the only way you can get an even distribution of water to all of the trees, and it solves the problem of too much pressure at the bottom of the grove and not enough at the top.
- Clean the filters often. You don't have a filter because you think that the district water has already been filtered? Hah! What happens if there is a break in the line in the street and the line fills with dirt during the repairs? All of your sprinklers will soon be filled with dirt.
- Is water flow being reduced at the end of the lateral line? It could be because scaffold roots are growing old enough to pinch off the buried line. The only cure is to replace the line.
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b. Cultural Management.
- Control the weeds because weeds can use a lot of water.
- Mulch? Mulching is good for increasing biological activity in the soil and reducing stress on the trees, but the mulch will not save a lot of water if you are irrigating often….the large evaporative surface in mulches causes a lot of water to evaporate if the mulch surface is kept wet through frequent irrigation. Mulches are more helpful in reducing water use if the trees are young and a lot of soil is exposed to direct sunlight.
2. Improve the Irrigation Scheduling.
- CIMIS will calculate the amount of water to apply in your grove based on last week’s water evapotranspiration (ET). You can get to CIMIS by using several methods; for avocado growers the best method is to use the irrigation calculator on the www.avocado.org website. If you need further instruction on this, you can call our office and ask for the Avocado Irrigation Calculator Step by Step paper. You need to know the application rater of your mini-sprinklers and the distribution uniformity of your grove’s irrigation system.
- CIMIS tells you how much water to apply, but you need tensiometers, soil probes or shovels to tell you when to water.
- “Smart Controllers” have been used successfully in landscape and we have used one very successfully in an avocado irrigation trial The one we used allowed us to enter the crop coefficient for avocado into the device, and daily ET information would come in via a cell phone connection. When the required ET (multiplied automatically by the crop coefficient) reached the critical level, the irrigation system would come on, and then shut down when the required amount had been applied. Increased precision can be obtained by fine tuning these devices with the irrigation system precipitation (application) rate.
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3. Deficit Irrigation.
- Deficit irrigation is the practice of applying less water than the ET of the crop or plant materials. Deficit irrigation is useful for conserving water in woody landscape ornamentals and drought tolerant plants where crop yield is not an issue. Water conserved in these areas may be re-allocated to other areas on the farm or landscape.
- There hasn’t been enough research on deficit irrigation of avocado for us to comment. We suspect, however, that deficit irrigation will simply lead to dropped fruit and reduced yield.
- Stumping the avocado tree could be considered a form of deficit irrigation. In this case, the tree should be stumped in the spring, painted with white water-based paint to reflect heat, and the sprinkler can be capped for at least 2 months. As the tree starts to re-grow, some water should be added back, probably about 10-20% of the normal water use of a mature tree.
- Regulated Deficit Irrigation for Citrus is an important method for saving water, and in some cases will reduce puff and crease of the peel. In one orange trial done by Dr. David Goldhammer in the San Joaquin Valley, an application of 25% of ETc from mid-May to Mid July saved about 25% of applied water for the year and reduced crease by 67%, without appreciably reducing yield.
- 4.
- 3. Reduce Irrigated Area.
- Taking trees out of production. Trees that are chronically diseased and do not produce fruit (or the fruit is poor quality) should be taken out of production during this period. Also consider: trees in frosty areas, trees in wind-blown areas, trees near eucalyptus and other large trees that steal the water from the fruit trees.
- Changing crops. You may want to take out those Valencias during this period and replant to something that brings in more money, like seedless, easy-peeling mandarins. The young trees will be using a lot less water.
- Fallow Opportunities. You may decide to do some soil preparation, tillage or cultivation, or even soil solarization of non-irrigated areas.
We have found that this four step process is a logical way to achieve water cutbacks with least impact. It is possible to achieve a ten percent reduction in water by only improving irrigation system uniformity and scheduling procedures. Often, these two measures also result in better crop performance and reduced runoff. Reducing irrigated area or taking areas out of production should be a last resort and a well thought out decision. Plan for the future, hopefully water will be more available in future years.
- Author: Mark Battany
Efficient and precise irrigation management is becoming increasingly important inCaliforniaagriculture, both for maximizing crop quality and for conserving water. The most advanced irrigation scheduling strategy is based on local measurements of reference evapotranspiration (ETo), which is converted to crop evapotranspiration (ETc) with an appropriate crop coefficient (kc).
To be able to use this method, an irrigation manager needs to have locally accurate ETo values throughout the growing season. However, the highly variable microclimates that characterize many farming areas often make it difficult to use data from distant weather stations; therefore an accurate local measurement may often be preferable to relying on a regional value.
One inexpensive option for measuring ETo locally is to use a simple atmometer (Fig. 1). Atmometers are water-filled devices, in which the actual evaporation of water is measured over time. In their simplest form, the atmometer is outfitted with a graduated sight glass on the water supply tank which allows the user to easily measure the evaporation that occurred over a given period. In practice, this type of atmometer is most suited for making readings at multiple day intervals, for example once per week, or on days when irrigation is applied.
The performance of atmometers versus more expensive weather stations was evaluated on theCentralCoastin 2003. In this study, atmometers were placed adjacent to seven weather stations throughout the area, and weekly values for both methods were compared (Fig. 2). The results indicate that the atmometers and weather stations have very comparable ETo readings, with the atmometers indicating somewhat lower ETo values under conditions of lower evapotranspiration.
Like any technique, using atmometers has advantages and disadvantages. Advantages include their very low cost and ease of operation, with no computer or power required. Disadvantages include the potential for damage by freezing weather, the need to refill the water supply (every three to six weeks), and the need to read the gauge manually. Also, if they are installed in a large open area, birds may tend to perch on the evaporating surface and foul it with their droppings; for this reason several wires are installed on top of the device to
discourage birds from perching there. In general, atmometers function quite reliably with few problems.
Converting atmometer ETo readings to the amount of irrigation run time required to replenish the soil moisture lost to evapotranspiration is fairly straightforward. A relatively simple example for a sprinkler-irrigated field is presented below in Table 1.
Table 1. Example conversion of ETo to irrigation run times for a sprinkler irrigated field |
||
|
|
|
A. Measured atmometer ETo for one week |
2 |
inches |
B. Crop coefficient (kc) |
0.8 |
|
C. Calculated ETc for the week (=AxB) |
1.6 |
inches |
D. Sprinkler application rate |
0.13 |
in/hr |
E. Hours of irrigation required (=C/D) |
12.3 |
hours |
(Note: To convert Gallons to Inches: Gallons ÷ Area (square feet) ÷ 0.6234 = Inches
To convert Inches to Gallons: Inches * Area (square feet) ÷ 1.604 = Gallons)
Atmometer installed on a fence post
atmometer
atmometer ET