Deep Percolation—One of Irrigation Water’s Forgotten Fates
Fate #1) The applied irrigation water could be taken up by the pecan trees’ roots or by the roots of other desirable plants in the orchard (e.g., cover crops or intercrops). In that event, the vast majority of the water will rather quickly move up the plant stems through the xylem tubes and be transpired to the atmosphere as water vapor out of the microscopic stomatal openings on the leaves. A tiny percentage of that water might be stored in the plant tissues or be used up in biochemical reactions (like photosynthesis) inside the plant cells, but again, the vast majority is transpired out the leaves.
Fate #2) The irrigation water could be taken by weeds’ roots, reaching the same ultimate end as with uptake by the trees’ roots or other desirable plants’ roots: transpiration as water vapor.
Fate #3) The irrigation water could go to the atmosphere as evaporation from open surfaces of running or standing water in the orchard, from water droplets that are passing through the air, or from wet soil or plant surfaces. Evaporation is not very different from transpiration in that it involves water vapor leaving the orchard to the atmosphere, except that evaporation does not involve any root uptake or transport through the plant. When scientists measure evaporation, it’s pretty difficult to actually distinguish between it and transpiration, so these are usually lumped together as one term called evapotranspiration or ET.
Fate #4) If so much irrigation water is applied and infiltrates into the orchard soil such that the volume of water in the orchard rootzone (the part of the soil profile where the pecan roots are reasonably abundant and active) surpasses its maximum water holding capacity, then irrigation water will normally move downward into the soil layers below the rootzone and be lost from the orchard as deep percolation. That’s a complicated way of saying it, but it’s actually pretty simple. Deep percolation is just the loss of water that occurs when it moves down through the soil past the reach of the pecan trees’ roots before they have a chance to take it up.
Fate #5) Finally, the applied irrigation water could be lost to runoff, a lateral flow of water out of the orchard. This kind of water loss should be pretty low in most irrigated orchards. Still, if it happens, the water lost as runoff could ultimately go to any one of the other fates described above, or it could eventually flow down a river to the ocean.
Most of the time, an important goal of pecan orchard irrigators is to try to get absolutely as much of the irrigation water applied as possible to go to “Fate #1”—to be taken up by the crop—and absolutely as little water as possible to go to those other four fates. If we succeed at that, we usually might consider our irrigation water use to be “efficient.” Fates #2, #3, and #5 are familiar to most growers, but Fate #4 can often be forgotten. Yet, we should all spend a little time thinking more deeply (so to speak) about this fourth fate, deep percolation.
In basin flood irrigated orchards, deep percolation can be a very challenging thing to manage for multiple reasons, including that flood irrigation systems deliver water on one end of the field and the water flows across the field the whole time that an irrigation event lasts. This means that the “head” of the field (the side where the water is delivered) has water percolating down for the whole irrigation event, while the “tail” of the field has water percolating down only once the water flow makes it to the end of the irrigation run. That means there is inherent spatial variability in the depth of percolation associated with the irrigation system itself, with the head of the field typically seeing water reaching much lower depths in the soil profile than the tail end of the field. More than that, natural variation in soil textures, differences in soil permeability to water, or even any kind of rut, groove, or imperfection in the orchard floor surface can also mean that the depth of water percolation after a flood irrigation event is rarely very uniform across a whole orchard block.
There are several management practices that can allow pecan growers using basin flood irrigation to improve the uniformity (and therefore management control) of irrigation percolation depth. First, growers using flood irrigation should avoid very long irrigation runs. The idea here is that if we are trying to better manage deep percolation in a flood-irrigated pecan orchard, the irrigation run should not be so long that excessive deep percolation losses occur before the water can approach the end of the irrigation run and the irrigation water is shut off. That means it will be better to have shorter irrigation runs, especially for flood-irrigated orchards with coarse-textured soils (having high infiltration rates and low water holding capacity). This design is often a prior-to-planting orchard question that involves balancing many other practical, management, harvest, and cost factors, along with even just the shape and location of the field. However, orchard managers using flood irrigation systems will generally find controlling deep percolation to be a simpler task with shorter irrigation runs than with longer ones.
The second management practice is the slope of the field—controlling deep percolation will be improved for flood-irrigated orchards that are laser leveled to level or, if other situations require it, to a less than 1% grade. It will be necessary to periodically re-level an orchard, but the needed frequency of re-leveling can vary quite a bit and involves carefully observing the orchard for changes in the water distribution patterns. Orchard floor management, especially weed control and soil surface smoothness, can affect the speed that water moves across the orchard; keeping weeds to a minimum and other hindrances to water movement down the field will improve your control of deep percolation.
And, finally, higher flow irrigation turnouts that can more quickly deliver larger volumes of water can help improve your control of deep percolation—faster deliveries of water at the head of the field are generally better for this purpose so long as it doesn’t cause erosion.
Conventional basin flood irrigation is not the only way to water a pecan orchard. One approach that is an interesting variation on flood irrigation was developed some years ago, primarily with agronomic or vegetable crops in mind; this approach is called surge irrigation. The concept behind this is flood irrigation water is applied as a series of relatively short cycles that are each separated in time by a break during which the water is turned off, rather than as a single continuous flow of water as is done in conventional flood irrigation. This approach does seem to reduce the difference in percolation depth across the beginning and end of the field and, therefore, gives the grower greater control over deep percolation. While surge irrigation has been tried with orchard crops, it has never really caught on in pecans, probably due to the complexity and expense involved.
Pressurized irrigation systems like sprinklers, micro-sprinklers, and drip irrigation in pecan orchards are becoming much more popular alternatives to basin flood irrigation. The advantage these kinds of irrigation systems bring to controlling that deep percolation component of the orchard water balance is twofold. First, pressurized irrigation systems potentially have much higher uniformity of application than any kind of flood irrigation system. Second, pressurized irrigation systems allow for much more precise control of the irrigation water application volumes and timing than flood irrigation.
All of that brings us to a final discussion about whether deep percolation is necessarily bad. As mentioned above, reducing deep percolation losses of irrigation water is one of the things we need to focus on in order to maximize our irrigation water use efficiency. We all agree on that. Moreover, deep percolation at certain times of the growing season results in leaching losses of nitrogen fertilizers in the form of nitrate. That’s an expensive loss of an extremely valuable production input and also contributes to groundwater pollution. But do all these things really mean that pecan farmers should make zero deep percolation their goal? The answer is definitely no.
There are at least two major reasons why deep percolation actually serves important beneficial roles in pecan orchards if carefully managed. If the irrigation water originates from surface water sources, the first beneficial role of carefully managed deep percolation is that it can contribute to aquifer recharge.
Carefully managed deep percolation also offers a second beneficial role: leaching. While leaching of nitrate is, of course, a very bad thing and should be avoided as much as possible, irrigation with little to no deep percolation will over time result in the accumulation of excessive deleterious salts in the soil. Deleterious for tree health and deleterious for nut production. It is critical for a “leaching fraction” to be applied so that soil salinity levels do not rise over time in the tree root zone. Often this involves applying 15% or more additional irrigation water volume than the actual consumptive water use of the trees. You can get a sense of whether sufficient leaching is occurring by regularly monitoring the Electrical Conductivity of Saturated Paste Extract (ECe) of soil samples. If that number is going up over time or is already above 2 deciSiemens per meter, then you can be confident that it will be beneficial to increase the volume of your leaching fraction. You can also calculate the leaching requirement if you have the electrical conductivity data for your irrigation water. Both New Mexico State University and the University of Arizona have Extension Publications that may be helpful for determining the leaching requirement to manage salinity in your orchard.
All of the approaches described above help pecan growers better harness the beneficial aspects of deep percolation in their orchards for salinity management and recharge while avoiding the negative aspects such as nitrate leaching and reduced irrigation water use efficiency. However, we still have a lot to learn about it. At NMSU, we currently have a USDA-funded grant called SWF (Secure Water Future*), with Dr. Sam Fernald as the NMSU lead investigator. As part of that project, we have several NMSU graduate students conducting research to better refine our understanding of how irrigation systems and irrigation scheduling can be used to better manage deep percolation. In the years to come, be on the lookout for updates on this research!