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.

• 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. 

  • 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.
    1. 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. 

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.

(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

"We don't need to irrigate, it's winter."  This is a commonly held idea, and many years it is true.  Adequately timed rains will often meet the needs of avocado trees during the winter period, and in times like last year, even satisfy much of the spring requirement.  And the calls are coming in – “What’s wrong with my trees, they have all these brown leaves?”.  This from San Diego to San Luis Obispo.

In a low rainfall year, irrigation can be as necessary as at other times of the year.  This is because a subtropical evergreen like avocado continues to use water regardless of rainfall patterns.  At the time of writing this article in March, we have had a scant 4 inches in Ventura and this is on top of a low rainfall year in 2011-12.  Rain is necessary to leach the salts that have accumulated from the last irrigation season.

The driving forces for plant water use are light intensity, wind and relative humidity, as well as temperature.  Remember how cold, dry winds can dry your skin or freeze-dry backpack food.  Even during the winter, the trees are quite capable of losing large amounts of water with clear skies and cold winds.   

Dry Santa Ana conditions are also more common in winter than in the past.  This winter, a time of drought, I went out to see an orchard to evaluate it for pruning.  On arrival, my first concern was for the water stress in the trees.  The grower, however, was unconcerned.  The trees had been dutifully irrigated the previous Friday.  But over the weekend, a Santa Ana had blown for three days and completely dried the soil in the top 10 inches.  Digging around the roots convinced the grower of water stress.  Do not take irrigation for granted.

Contributing to the problem is the determination of what amount of rainfall is effective.  Effective rainfall is defined as the amount of water that is retained in the root zone after rain.  Avocados, especially on shallow soils, do not have much of a root zone.  Most soils can be expected to hold about 2 inches of available water in the top 2 feet, less the more sandy, more the more heavy. 

If rainfall exceeds the holding capacity within the root zone, it is lost to the plant.  Just imagine if all the year's expected rain fell during one storm.  It would not be long before irrigation would be required with no more rain coming.  The extra water may, however, perform the all-necessary function of leaching accumulated salts from the root zone.  When the rain gauge says that 2 inches fell, it is quite possible that all that rain will not be available to the tree.  This also goes for the quarter inch storms we get that do not even make it through the leaf litter.  It is not effective rainfall, even though it may wash the persea mite off the leaves.

One of the best ways to assess the effectiveness of rainfall within the root zone is with tensiometers.  These trusty instruments are most commonly used to schedule irrigations.  A good rainfall should return the 8- and 18-inch depth gauges to close to 0 cbars.  This will tell you whether the rain thoroughly wetted the root zone.  It will not tell you how much may have passed through the root zone, however.

If you are using soil sampling to assess the depth of rain infiltration, simply squeezing a handful of soil can help.  Regardless of soil texture, a wetted soil will form a ball or cast when thoroughly wetted.  Water moves as a front through the soil.  After a rain, take soil samples with depth to find where the potential to form a ball abruptly ends.  This will tell you the depth of effective rain.

How well a soil holds together can also be an indication of when to irrigate.  Even a sandy loam texture will retain a ball that does not hold together well when there is still adequate moisture for the tree.  The possibility of forming a ball decreases with water content.  When visible cracking of a soil ball is obvious, it is time to irrigate.

Winter irrigation is something we do not commonly perform, but in low rainfall years it is an activity we need to consider, especially for controlling the salts that accumulate from our previous irrigation season.

Salt damage due to lack of leaching

Irrigation Calculator developed by Reuben Hofshi, Shanti Hofshi and Ben Faber.