Micro-Irrigation versus Flood Irrigation
Micro-irrigation is gaining popularity amongst western growers; like flood irrigation, this system has its own drawbacks and benefits.
What is a micro-irrigation system?
Micro-irrigation systems are pressurized irrigation systems that precisely deliver relatively low flows of water. Micro-irrigation can be broken down into two main categories—drip irrigation and micro-sprinklers—each with a wide range of variations in form and design.
As the name implies with drip irrigation, each emitter deposits the water directly onto (surface drip) or into (buried or subsurface drip) the orchard floor at a very low flow rate, often only 0.5 to 2.0 gallons per hour. With these systems, the irrigation lines (i.e., the “laterals”) in the orchard are typically half- to 1-inch black polyethylene tubes that run parallel to the tree rows. For buried drip, producers may position these tubes between the tree rows without interfering with equipment and orchard operations. As for surface drip, they are typically located in or near the tree rows.
For micro-sprinklers, on the other hand, the emitters spray, mist, or shoot water through the air to wet a much larger area of the orchard floor than drip irrigation. Sometimes with micro-sprinklers the wetted areas from adjacent emitters overlap with one another in the tree row, but typically in a pecan orchard, these emitters do not overlap between the tree rows as they often do with full-sized sprinkler systems.
The flow rate on micro-sprinklers generally ranges from 2 to 15 gallons per hour. The irrigation lines in the orchard for this type of system may be buried or above ground, but the emitters are always above ground. Buried lines are generally PVC pipes with risers for the emitters coming above the soil surface, in line with the trees in the tree row. Above-ground lateral lines, though, are usually polyethylene tubes (like drip irrigation) running parallel to the tree row right next to the trees with one-eighth-inch black polyethylene or vinyl tubes connecting the lateral line to the emitter elevated above the ground on a stake.
Besides emitters, there are a number of other components to a micro-irrigation system that we won’t cover in detail here. These parts include a pump, filtration, flow control, flow-meter, timer, and chemical injector.
What are the advantages of a micro-irrigation system?
The list of advantages for micro-irrigation is long. Still, the most important advantage of drip and micro-sprinkler systems is that they afford much greater control to the grower over irrigation water timing, amount, and placement than flood irrigation does. This kind of control can translate into crop production benefits because it ensures that:
- more of the water applied in the orchard is actually used by the trees in transpiration, and less water is lost to evaporation and deep percolation.
- fluctuations in tree water status between irrigation events are smaller.
- there are fewer times when the trees’ entire rootzones are in fully water-logged, anoxic soil conditions.
- the trees across the orchard receive a more uniform volume of water each time they are irrigated.
The importance of this last point is probably the most underappreciated. In basin flood irrigation, the facts that water must run laterally across the surface of a field, that soil textures (and therefore soil water holding capacity) is usually variable in areas, and that orchard floor surfaces are not always 100 percent smooth or properly leveled mean it is very difficult to supply uniform water volumes to all of the trees in a flood-irrigated orchard block. Because of these factors, some trees receive more water than others each time they are irrigated.
Another advantage of micro-irrigation includes the ability to deliver fertilizers through injection into the irrigation system. Fertigation is also possible with flood irrigation, but as with the water, a micro-irrigation system gives a grower a greater amount of control over the placement of nutrients, ensuring that much more of the nutrient inputs applied are actually taken up by the trees and used by the crop.
At New Mexico State University, we currently have an ongoing USDA grant-funded graduate student research project investigating the potential to use novel approaches in drip irrigation. At this time, we are studying the potential to use a technique called partial rootzone drying to further enhance production potential by tapping into the tree’s own physiological responses to drying soils. In the future, we hope that some of what we are learning in this research will provide pecan producers with even more options in micro-irrigation.
What are the drawbacks to micro-irrigation compared to flood?
There must be some advantages of flood irrigation over drip irrigation. Right? Yes, there certainly are, but they vary a great deal from one situation to the next. Here are some of the major challenges for micro-irrigation that need to be accounted for.
- If you already have a flood irrigation system in place, as is the case for many growers, the costs of adding (some producers like the flexibility of having both kinds of irrigation systems) or converting to a micro-irrigation system can be a significant financial hurdle. Depending on the situation and what is needed, the per-acre costs can be well over $1,000 per acre to install a micro-irrigation system.
- Micro-irrigation requires ready availability of water. In areas where the irrigation water comes as surface water flows from an irrigation district, it can be challenging to ensure that water is available at the time it is needed on such a frequent basis. Sometimes on-farm reservoirs to store irrigation water allow growers to have a constantly available supply. In almost all cases in the western region in the U.S., on-farm wells are necessary in order to have the ready supply of irrigation water for micro-irrigation.
- Very clean irrigation water is needed to do micro-irrigation well. Particulates (e.g., sand and silt), mineral deposits (e.g., calcium carbonate), and biologicals (e.g., algae) can all clog micro-irrigation emitters, especially drip emitters. These sorts of clogging issues really aren’t a problem in flood irrigation (although just about everybody who’s done flood irrigation can tell some exciting stories about clogged ditches, turnouts, pipes, and valves). To prevent clogging, micro-irrigation requires a good water source and excellent filtration, which can also be a quite expensive investment. Furthermore, chemical treatments are sometimes needed to keep the emitters from clogging.
- Managing soil salinity can be more challenging with micro-irrigation than with flood irrigation. Irrigation water always has dissolved salts in it, and if those salts are not pushed down below the rootzone by leaching, then they accumulate in the rootzone soil and eventually create problems for the trees. Assuming there aren’t any barriers to drainage in the soil, leaching salts with flood irrigation can be a pretty straightforward process. Leaching for micro-irrigation is the same process as it is for flood irrigation (i.e., applying more water than the trees use), but the fact that such low flow rates are involved makes it a much slower process. Since micro-irrigation does not wet the entire orchard floor, salts in the water can travel laterally instead of downward during an irrigation—and can even move upward(!) with sub-surface drip systems. These laterally moving salts may only be leached downward when there’s precipitation or if a secondary flood irrigation is applied.
How is scheduling irrigations different for micro-irrigation systems?
Irrigation scheduling for a flood system usually involves letting the soil dry down to some pre-determined threshold that can be determined by the water budget method that estimates orchard evapotranspiration losses or by monitoring soil moisture levels over time. So, with a flood system in a given orchard, the time interval between irrigation events will vary depending on the stage of tree development in the season and the weather. The time interval between irrigations is longer in the spring when the canopies are just beginning to leaf out and the temperatures are cool than in the mid-season when the trees are in full leaf and the weather is hot.
With micro-irrigation, a common technique is to hold the time interval between irrigations constant throughout the season. Different growers may choose an irrigation interval of perhaps anywhere from one to seven days between irrigations. Instead of changing that interval, they use this approach and vary the length of the irrigation runtime, and thus, the volume of water applied as the evapotranspiration changes in the season. The goal with this scheduling approach is, with each irrigation, to replace the water used by orchard evapotranspiration since the last round. (As was described in my May 2020 Pecan South article on flood irrigation scheduling, the evapotranspiration numbers may be calculated based on weather station data.)
Although basin flood irrigation remains popular throughout the western pecan region, micro-irrigation systems offer several advantages that attract growers. When deciding between these two systems, growers must weigh the drawbacks of cost, challenges with soil salinity, and water quality requirements against micro-irrigation’s gift of greater control.