- Author: Allison Rowe and Ben Faber
How to irrigate is probably the most common question in irrigated agriculture, even with 10,000 years of cultivation knowledge to guide us. The complexities of irrigation and the unique situation for each grower makes this question so difficult. Not enough water, and plants have diminished growth or the propensity for disease and disorder 1. Too much water leads to root disease and nutrient problems 2. So, it can't be too much or too little, but just right. There are times when citrus can handle a little more water stress than other times, which can lead to water savings 3, especially in a drought year or in areas where water costs are crucial. Salinity further compounds the question of irrigation where striking a balance determines the health of your tree. Staying in tune with your orchard and using appropriate methods to measure water need, water use, environmental water demand, and soil water-holding capacity will help inform irrigation management decisions.
There are all kinds of ways of estimating tree water need 4 , a valuable piece of information for irrigation decision making. An inexpensive and often overlooked method of estimating tree water requirements is grower observation in the orchard to assess leaf color, leaf size, the look of the leaves, and canopy fullness. Pure observation and knowledge of your trees yields a lot of valuable information regarding irrigation management. Beyond observation, a direct measure of the tree with a porometer, pressure gauge (bomb), sap flow meter, dendrometer or other device gives an absolute or relative number of tree performance. Technological advances, such as telemetry and imaging with drones or satellites, holds promise, but are still being perfected for general irrigation use. In general, technological devices yield informative data, but tend to be expensive, delicate, and require manual monitoring to account for tree-to-tree variation in the orchard.
Soil moisture sensors can be an effective method of evaluating water use by the tree. The most basic way to measure soil moisture is with a human powered shovel or soil tube 5. While it requires an operator who knows what they are doing, the technique is easily learned and repeatable. A human and shovel can move around an orchard checking out different suspicious spots that are not easily done with fixed-in-place sensors. Installation of soil moisture sensors systems range in cost and capabilities, yet provide specific data on water use. Integrating certain systems into communication relay systems allow for the monitoring of multiple sites at once. Some sensors can measure soil salinity, as well as soil moisture, to give a sense of whether the water in the soil will be useable by the tree. If soil moisture sensors are used, correct placement of where roots are taking up water is imperative to get an accurate assessment of water uptake. Overall, it is critical to keep the entire orchard in mind and understand that fixed sensors only take a specific location's reading.
Another great technique to inform irrigation scheduling is an estimate of the demand that drives water use. An evapotranspiration estimate either by CIMIS, a private weather station with ET-calculation or atmometer gives not only an amount to apply but also when to apply that amount based on the water holding capacity of the soil and the rooting depth of the crop. Soil moisture holding volume can be complicated, but can be estimated from the NRCS table in the previous paragraph5 or from tables in the Web Soil Survey 7.
Simply running an irrigation system for a specific amount of time and probing for depth of water penetration and extent of wetted area is the best way to get an estimate of soil moisture holding capacity. This knowledge is needed in order to decide whether the active rooting volume is getting wetted sufficiently or too much is being applied. Emitters are rated by gallons per hour, but that 1 gph, 5 gph, 20 gph emitter output might differ according to water pressure that can vary over an irrigation period. On the flip side, monitoring soil moisture depletion over time can give an approximation of how depletion compares to ET estimates. Soil moisture depletion can be measured by soil moisture sensors or by shovel and feel. This estimate of applied water compared to output and ET only needs to be done once at a given growth stage of the orchard. If the orchards is young, it will need to be done each year as the trees fill out. An estimate of canopy growth can also be used to better approximate young orchard ET.
All of these methods suppose that a grower has the capability to irrigate when, where and for how long they need to. If water delivery is on a fixed schedule and the amount of water can be controlled it is valuable to understand specific water needs. Knowing the rated applied amount of an emitter is important, but that amount shouldn't be assumed, especially considering natural wear and tear, damage from harvest, poor filtration, clogging, or damage by wildlife. Maintenance to insure good distribution uniformity is critical to the operation and the correct application of water to trees and for the maintenance of tree health. Low-pressure systems are wonderful but they should be evaluated on a yearly basis and tuned up in preparation for every irrigation season. Many growing areas have mobile irrigation labs that will evaluate system performance and make recommendations for improvement.
All said, knowing the orchard and evaluating tree health will inform irrigation management decisions. Applying technology where technology is appropriate will help. Using it to help advise irrigation decisions is valuable, but new tools will not always be the answer.
It's important to know what is being applied.
Trust but verify.
- Author: Ben Faber
In order to properly irrigate any crop, you need to know how much water you are putting onto the crop. A grower with a small acreage recently asked how to irrigate avocados, and I said amongst other things that it is not only necessary to know how much water the emitters are spraying, but where that water is going. Each manufacturer rates output at a certain flow at a certain pressure. So at 20 psi a rated emitter will put out something like 8, 9, 10, 14 gpm, whatever. If pressure changes, the output changes and some emitters respond to pressure change more than others.
As elevation and distance from the inflow valve changes, pressure changes. That can be corrected by using pressure compensation in-line or at the emitter or both. So even if pressure changes, there will be a known output if the manufacturer has done right by the product. Then it is up to the grower to make sure that clogging, filter cleaning and other maintenance practices are followed.
So now the grower is left with where the water is going. When an orchard is about 8 years old in a conventional planting, the roots of all the trees are starting to get entangled amongst themselves. It becomes one big root system. So you may not think that it matters what pattern the water takes. It all gets to the trees, right? No. Because most microsprinklers as far as I know put out a pattern that puts less water in the pattern than in other parts of the pattern. Typically the water decreases with distance from the microspinkler. Lots of water near the emitter that also leaches accumulated salts from the profile, but with distance, the amount decreases and leaching decreases. The wetted pattern might look like a nice circle, but salt will accumulate with depth in that pattern because there is not sufficient water arriving at that point to drive the water below the root zone. Rain makes up for this faulty wetted pattern by leaching those accumulated salts from those underirrigated areas. A grower could compensate for lack of rain by running the system for a much longer time to make sure those poorly leached areas get a good wetting. But in a drought and with high water costs growers hesitate to put on more than they think might be necessary.
Once salt damage has occurred, though, it is not going to go away. Eventually the tree will shed those leaves and the tree will become less productive. Therefore it is important to know how much of that water is not adequately leaching the salts that are naturally occurring in the irrigation water. So our grower followed a little trial that is very commonly done in the turf trade. He put out catch cans (tuna cans with lids removed) and looked at the spray pattern of the microspinklers. He ran the system for increasing amounts of time and was able to get an idea of how much water could infiltrate at one spot.
He could then place the emitters in a pattern that might more effectively leach salts. First of all he knew that he didn't want the spray pattern to hit the tree trunks to avoid crown rot. So he could then move the emitters further away from the trunks. Also the most efficient roots for water uptake are not right at the trunk. That's where coarser roots are and much of the water will bypass them. Where the fine roots are, are further out from the trunk. So he could then move them out a little further from the trunk. At this point there comes some overlapping of the emitter spray patterns so there will be increased leaching occurring where the overlap occurs.
You can probably see where this is going. The optimum distance for a relatively mature orchard is when the emitters are midway in between trunks and there is some overlap of spray patterns. It might be necessary to on higher output emitters to make sure there is some overlap within reason. This would make the pattern of output more or less even across the wetted area. It will never be perfectly even, though, because that is not how microspinklers were designed. The grower now has a better handle on where that water was going.
1) Spray pattern with distance from the emitter (top graph).
2) Salt accumulation with depth and distance in that spray pattern (bottom graph).