- Author: Ben Faber
In a recent meeting the topic of where to go for irrigation information came up. Well there's no substitute for attending a class in irrigation, such as offered at Cal Poly SLO (http://www.itrc.org/classes/iseclass.htm ,
but here's some written sources to get you started thinking.
- Author: Ben Faber
So after five years of drought a grower told me he finally gets it. Farming avocados in Goleta with limited well water and poor quality and expensive, rationed delivery, he has finally cut out trees that were not performing well. These were wind sept trees, areas with root and crown rot. Tree were either stumped or removed altogether. The idea is to focus on those trees that are productive, and are they ever. They are getting the water they need and their schedule has been changed. Previously irrigation had been on a fixed schedule of 2 weeks and got 24 hours per set. The schedule was dictated by the time it took to get around to all 300 acres. Now trees are irrigated one to two times per week with shorter sets, from 5-8 hours depending on the time it takes to get 18 inches of penetration in sandy ground. Yields per acre have significantly increased, largely because non-productive areas have been eliminated and the remaining trees are getting what they need.
He is also anticipating irrigation needs – projecting a schedule.
The driving forces for water loss in avocados in decreasing effect are sunlight (day length, cloud cover), wind, humidity, and temperature. More light, more transpiration. More wind, more water loss, lower humidity, more water loss. And least of all if temperature which is what we normally respond to, but which the tree responds the least. Usually, though the most desiccating conditions occur during periods of high, dry winds that blow out of the Great Basin – the dread Santa Anas or in the case of Santa Barbara the Sundowners. When they start blowing, it's hard to play catchup. It takes a while for water to infiltrate and for the roots to start taking it up and filling all the drained leaves and stems. Now the grower more carefully watches the forecasts and makes sure to get water on before the high demand conditions arrive. With multiple blocks he wants to get all of them wetted to a normal irrigation depth and then it's time to start the cycle over again. He keep an eye on the nearest CIMIS station to see how much the water demand is increasing and adjusts the irrigation frequency. Importantly, h keep your eye on how fast the soil in the root zone is drying out and then makes even smaller and more frequent applications.
When it comes to making the water meet the needs of the trees, he is really customizing each irrigation.
- 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
B. Crop coefficient (kc)
C. Calculated ETc for the week (=AxB)
D. Sprinkler application rate
E. Hours of irrigation required (=C/D)
(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
- Author: Gary Bender
Quite frankly, in a county where water is costing $700 to $1000 per acre foot, we though this practice would have been a common practice. Added to this is the increasing pressure to reduce nitrate leaching into creeks and ground water, where there is a serious problem developing. The natural response when water prices are high is to reduce water use, but we have seen groves where even a 10% reduction in water reduces the yield by 50%, and we have also seen quite a few growers irrigating too much with the belief that a couple of extra feet of water per acre will more than pay the cost of water in increased yield. Clearly we need to apply enough water to make the trees produce a profitable yield, How does a farmer accomplish this?
I believe every grower should be using tensiometers or some other kind of soil moisture monitoring equipment to determine when to water, and using CIMIS to determine how much to water. There, just simply, is no an easier, or a better method.
Some growers said that tensiometers don’t work. Well, they work just fine if they are installed correctly and serviced periodically. If the soil gets too dry (the reading goes above 80 cb) the device breaks suction from the soil, and they don’t work until they are removed, filled, pumped and re-installed. As for gypsum blocks, they work just fine also, but are not very accurate under wet conditions. Both work a lot better than just guessing. There are newer electronic devices that work very well if calibrated with the soil moisture, but they don’t work very well in rocky soil (rocks don’t hold water).
This assignment is to help you figure out the water use in your grove. The following is a step by step procedure that is not difficult. Several of our grove managers use this on a weekly basis to calculate the water requirement in each of their groves. We have one grower who has this task assigned to his child in the third grade…Really, this is not that difficult!
This assignment will demonstrate how to use CIMIS to calculate the irrigation requirement for an avocado grove in Escondido. ETo is called the reference evapotranspiration (defined as the water use for eight inch tall grass), and all crops in California are related to this water use by adjusting ETo with a “crop coefficient”. In this example you will see that the crop coefficient for avocado in November is 0.55. ETo data is gathered from the automated weather stations that are part of the CIMIS network in California. The irrigation calculator you will be using multiplies the ETo number by the crop coefficient and gives you Etc, the water use by the crop in question. This comes from the station in “inches” of water loss, and the calculator changes this into gallons per tree per day. The calculator then tells you how much water to apply to the avocados to replace the water they used during the last seven days.
Go the website www.avocado.org
Click on California Industry (on the top right side of the page)
Click on Growers
Click on Water
Click on Irrigation Calculator
Start with Evapotranspiration (ETo).
Click on Go To CIMIS
Use the drop down box and Click on San Diego
Click on Submit
Click on Daily Data
- “Select a Time Period”, in this example we will select the previous week; select November 15 through November 21
- In “Select Variables”, leave everything selected with the green checkmark.
- Leave “English Units” selected.
- Click “Retrieve Data”
Write down ETo for the last week. In this case it will be: 0.12, 0.11, 0.11, 0.10, 0.12, 0.12 and 0.10.
Add these up, and you get 0.78 (this is your ETo for the past week). Minimize this window.
You are now back to the Irrigation Calculator on the Avocado website.
- Evapotranspiration, delete the 0.22 and fill in your 0.78
- Under “Crop Coefficient”, just click on November in the drop down box.
- Leave “Distribution Uniformity” at 0.85.
- Leave trees at 109 per acre.
- Leave sprinkler output at 17 gal/hr. (of course, you can change this to match your sprinkler output, but for the sake of this example, leave this at 17).
- Click on Calculate.
You should get 138 gallons (this is the amount of water used by one tree in the last seven days) and a watering run time of 8 hrs and 8 minutes.
As I mentioned earlier, you should have tensiometers (soil moisture meters) set at the 8 inch depth (avocado) or 12 inch depth (citrus) to tell you “when” to water. In avocados, I like to irrigate when the shallow tensiometer reads 20-25 cb, and in citrus when the tensiometer reads 35 – 40 cb. You cannot rely on irrigating every seven days because the tensiometer may tell you the soil is getting dry by the fourth day. This often happens in the summer.
To review, CIMIS tells you how much to water, the tensiometer tells you when to water. Now, in actual use, you may find that, in a windy area or on the south side of a slope, your trees may need more water. Merely add a 10% increase to the run time, and keep making minor adjustments until you get this right for your grove. Or, if you have root rot, you may want to water 10% to 30% less water.
By the way, if you are using this calculator for citrus, merely put 0.65 into the crop coefficient for each month, and you can use the same calculator. Some people believe the crop coefficient in the avocado calculator might be too low. Both Ben Faber and I believe the coefficient should be 0.80, but we don’t exactly have good data to support this…just experience. At any rate, the calculator will put you in the ballpark…and it is a lot better than “guessing”.
Give this a try, and Good Luck!
Irrigation Calculator developed by Reuben Hofshi, Shanti Hofshi and Ben Faber.
- Author: Ben Faber
Irrigation efficiency requires not only uniform irrigation, but also the proper timing and amount of applied water. It is important that the irrigator know the system water application rate, either in inches per day, inches per hour, or gallons per hour.
Irrigation scheduling which determines the time and amount of water to be applied can be accomplished through a variety of methods, including measuring soil moisture, determining plant moisture status and determining evapotranspirational loss (ET crop or ETc). Evapotranspiration values are a measure of the actual amount of water well watered plants would use. This information is available in many areas of California from newspapers, irrigation districts, and over the Department of Water Resources CIMIS network California Irrigation Management Information System, or CIMIS Help Line (800) 922-4647).
Evapotranspiration varies seasonally and from year to year for a given location. DWR has developed a map of the average daily ET for various zones in California. These zones are distinctive because total sunlight, wind, relative humidity and temperature are the parameters that drive water loss and differ in each zone. Where the Central Valley becomes hot and cloudless in the summer, along the coast the intensity of the marine layer and its effect on sunshine differs from year to year.
Scheduling, as opposed to a fixed amount applied at a fixed time, is especially important in Southern California coastal valleys. Although the average annual irrigation requirement is about 2 feet of applied water per year (2 acre-feet per acre or 651,702 gallons per acre), this value varies tremendously from year to year, from as little as 18 inches to as much as 3 feet.
One of the most important variables in the quantity of applied water is the length of the rainfall season and the effectiveness of the rainfall. The rainfall season determines the length of the irrigation season and effective rainfall determines how much the plant can use. Effective rainfall is defined as the amount of rainfall, which is retained in the root zone of the tree. For example, consider a rooting depth of 2 feet and each foot holds 1 inch of available water. If you have just irrigated or if it rained 2 inches yesterday and it rains 2 inches today, none of today's rain is effective since the soil was already moist. It did leach salts out, however. Rain events of less than 0.25 inches are also not considered effective.
Determining an irrigation schedule based on tree water requirement falls into three broad categories of technology - plantbased, soil-based and weather-based. Many of these technologies are proven and have been in use for years. Others are more experimental and have not been fully tested. In several cases improved electronics and digitalization have put a new spin on older technologies. A method of determining when to irrigate should be learned by all growers and often a combination of techniques can be employed.
Plant-based Scheduling Methods
The plant is the ideal subject to measure, since it is integrating all the various factors driving water loss as well as soil moisture and any stresses such as soil salinity and plant health. To be a useful tool in irrigation scheduling, plant-based measuring devices must provide indicators of stress before that stress reaches levels that result in yield decreases. The methods include:
- Pressure chamber (pressure bomb or Schollander pressure chamber) measures plant water tension by applying a comparable air pressure to a leaf or stem. The amount of pressure required to equilibrate with the plant sap indicates how much stress the plant is under.
- Trunk diameter fluctuations (shrink/swell), measured continuously with linear variable displacement transducers (LVDTs), can be used to calculate parameters that are directly related to tree stress.
- Stem flow gauge estimates transpiration by placing a heat source on the trunk of the tree and then measuring the temperature differential along a trunk.
- Porometer measures the ability of a leaf to transpire, so when the leaf is under water stress then less water is transpired.
- Infrared thermometry measures the canopy temperature as affected by the rate of transpiration, so as the plant goes under water stress, the leaves gets warmer.
- Visual symptoms (wilting, leaf curling) are the cheapest method, but the most expensive in the long run.
- While these techniques can be valuable for scientific use, there has been little adoption in commercial agriculture. With the exception of the pressure chamber and LVDTs, this is due to the aforementioned problem of being able to identify mild water stress. Another reason for their lack of use by commercial agriculture, specifically subtropicals, is that there are logistical problems with mature trees, such as with the stem flow gauge and infrared thermometry. At this time, the pressure chamber is the state of the art in measuring tree water stress in subtropicals while recent research indicates that the LVDTs show promise for automating irrigation scheduling.
Soil-based Scheduling Methods
A rule of thumb is that irrigation timing should occur when about 50% of the water available to the plant has been depleted from the soil. The 50% figure is arbitrary; it allows a buffer of water in the soil in case the weather suddenly turns hot and windy.
Of course a sandy soil will hold less water than a clay soil, so irrigation will be more frequent. A common perception is that it takes more water to grow plants in sandy soil than clay soil. The total amount required for the whole year by the tree will not be changed by the soil type. This is because it is the sun, wind, temperature and humidity, which decides how much water the tree, will need. The soil is only the reservoir.
To check the water content in the soil, take a trowel, shovel, or soil tube and dig down 8 to 16 inches. A soil that has about 50% available water remaining will feel as follows:
- coarse - appears almost dry, will form a ball that does not hold shape;
- loamy - forms a ball, somewhat moldable, will form a weak ribbon when squeezed between fingers, dark color;
- clayey - forms a good ball, makes a ribbon an inch or so long, dark color, slightly sticky.
Irrigation timing can be determined and also mechanized with the use of a tensiometer. These water filled tubes with a pressure gauge accurately reflect the amount of energy a plant needs to extract water from the soil. The pressure gauge measures "tension values" in centibar units (cbars). When the gauge reads 30 cbars, it is a good time to irrigate.
Placement of the tensiometers requires that they be within the root zone, between the emitter and the tree trunk. Having two tensiometers next to each can be helpful in deciding both when to turn the system on and when to turn it off. A tensiometer at a one-foot depth tells when the water should be turned on and a tensiometer at three feet tells when to turn the system off. Placing a plastic milk crate over the device will prevent pickers from kicking them over.
There are other devices on the market for measuring soil moisture. Gypsum blocks are very effective. Although the part in the ground is inexpensive, the reading device costs in the $250 range. This cost means a large enough acreage is required to spread out the cost of the system.
There are portable meters on the market for measuring soil moisture. These meters rely on an electrical current carried by water in the soil. Even the cheap $10 ones can give a rough estimate of the soil water content. None are very effective in rocky ground, because their sensitive tips break easily.
The amount of water to apply at an irrigation depends on the amount of water held within the root zone. A loamy soil where a microsprinkler with a 20-foot diameter throw has wetted a twofoot depth will hold about 200 gallons of water at 50% of the soils water holding capacity. Exceeding this amount of water will help leach salts; but if far in excess, additional water is only pushing existing water out of the root zone.
It is best to follow one or two irrigation cycles to find out how long to run the system to achieve a certain depth of infiltration. This can be done with a shovel or more easily with a pointed rod or tensiometers. Water moves in a wetting front, and the wetted soil will allow the rod to be pushed in to the depth of dry soil. The system should be run to find out how long it takes water to infiltrate to a depth of two and three feet. That information will indicate how long to run the system when irrigating.
Applying water to achieve a two to three foot depth may take several hours. If run-off occurs, the system may be turned off for a few hours, then turned on again to get the total run time required to infiltrate to a given depth. If run-off is severe, use emitters with a smaller flow rate.
Soil-based methods monitor some aspect of soil moisture which, depending on the method, requires some correlation to plant water use. Some of the methods are well understood and inexpensive, others are expensive, inaccurate, inappropriate or not well researched. Some of the techniques allow multiple site readings while others require a device to be left in place. Some measure soil water directly, like oven-drying and others measure some other parameter with is associated with water content, such as electrical conductance. Some are affected by salts or soil iron content and others have limited value in the desired soil moisture range. Some, like tensiometers and gypsum blocks, give a reading from a porous material, which comes to equilibrium with soil moisture, while many others use the soil directly as the measured media. This is an important distinction since discontinuities in the soil caused by rocks or gopher holes can affect readings when the soil is used to carry a signal. Also, times have changed and some of the old techniques have been improved. For example, gravimetric oven-drying can now be done by microwave, considerably speeding up the process. Tensiometers and gypsum blocks can now be found with digital readouts and connections to data loggers, which make data easier to manage. There are quite a number of devices on the market and the following chart will shed some light on their differences.
As with any tool, the value of these devices increases with use and familiarity. Even though several of these are listed as stationary devices, by placing them in representative positions in the orchard, they can accurately reflect the rest of the orchard. Several of the devices are listed in the table as being both stationary and portable; this is because there are various models that can act one way or the other. The "Ease of Use" category in the table indicates not just the ease of reading the device, but also the maintenance required for it.
Weather-based Methods of Irrigation Scheduling
Another scheduling technique that has become popular is the use of weather data that has been converted to a crop water use value. This value is the estimated amount of water an orchard would use. The value is often referred to as the evapotranspiration (ET) of the crop. ET is the amount of water that can be lost by a well-watered crop either through the leaves (transpiration) or evaporation from the surface of the soil. By applying the ET amount at an irrigation, the trees are kept at optimum moisture content. The technique is often called the water budget method or checkbook scheduling.
The CIMIS network of over 50 weather stations calculates reference evapotranspiration (ETo). This value is an estimate of the amount of water lost from a well-watered field of grass. Grass is the standard or reference for all other crops. ETo is modified for the specific crop with a crop coefficient (kc). The formula for converting ETo to crop ET is: ETo X kc = ETcrop.
For a full-grown subtropical orchard a kc of 0.65 is used in most of the State, but in the desert growing areas, 0.56 is used. With smaller trees, a smaller kc is used. When trees are young and intercept little energy to drive water loss, a kc of 0.05 works well. As the trees increase in size to where their shade covers about 65% the soil surface, the kc is gradually increased each year. With rapidly growing trees, the kc increase is usually about 10 % each year, until about year 8 when the 65% figure is reached. A correction factor needs to be incorporated for the irrigation system distribution uniformity, as well.
If the orchard is cover cropped for part or all of the year, the period during which the cover is present needs to be recognized in the water use calculation. A soil that is covered by a cover crop and trees uses water just like a mature orchard. Therefore, if the young orchard is covered by a perennial cover crop a kc of 0.65 is used regardless of tree size. If a winter annual cover is used, that uses only rainfall for its growth, correction is not usually necessary in a high rainfall year. But in low rainfall years, the water requirements of the cover need to be recognized in the irrigation program.
Reference evapotranspiration values are available from many irrigation districts, CIMIS, several weekly journals and magazines. In Ventura County, the values are available through County Flood Control, and in San Diego County, they are available from the Resource Conservation Districts.
One of the drawbacks of the centralized weather stations is that in hilly terrain with different sun exposures, the station values can be quite different from the water loss at a grove. When using evapotranspiration figures it is always important to back up the estimates with field checks in the grove. An alternative to using the centralized weather stations is establishing one of your own. These electronic stations cost in the range of $5,000 and require regular maintenance as well.
A simpler weather station can be developed with an evaporation pan or an atmometer (atmosphere meter). Both of these devices actually measure the loss of water due to evaporation and since the physics of evaporation and transpiration are very similar, the values can easily be used in a water budget.
The major drawback to the evaporation pan is the maintenance required to keep birds, coyotes, and bees from causing inaccurate readings. Algae also needs to be kept free of the pool. An atmometer is a closed system with a ceramic head, much like a tensiometer. As water is drawn out of a reservoir, a sight tube shows how much water has been evaporated. The atmometer is more expensive (~$300) than a pan, but it is much easier to maintain.
Regardless of what scheduling technique or combination of techniques is used, a thorough evaluation of the system needs to be performed so that a known amount of water is being applied. Until volume and distribution of water are known, it makes little sense to schedule applications.