Center Pivot Technologies
Center Pivot irrigation systems are used widely, especially in the Texas High Plains where most of the systems are low pressure systems, including Low Energy Precision Application (LEPA); Low Elevation Spray Application (LESA); Mid-Elevation Spray Application (MESA) and Low Pressure In-Canopy (LPIC).
Low pressure center pivots are descriptions and their acronyms are the following:
Low Energy Precision Application or LEPA: This type also applies as much to a type of management philosophy as well as the actual hardware. It can operate in a spray or chemigation mode, and includes a surface tillage system that enhances surface storage. LEPA also delivers water directly to the ground in an amount designed not to exceed the surface storage volume.
Low Elevation Spray Application or LESA and Mid-elevation Spray Application or MESA: These describe similar irrigation application systems that embody the LEPA technology but do not meet one or more of the criteria to be called LEPA. These systems are designed to operate either on a center-pivot or a lateral-move sprinkler machine. Typically LESA systems are one to two feet above the ground while MESA systems can vary from five to 10 feet above the ground.
Low pressure systems offer cost savings due to reduced energy requirements as compared with high pressure systems. They also facilitate increased irrigation application efficiency, due to decreased evaporation losses during application. Considering high energy costs and in many areas limited water capacities, high irrigation efficiency can help to lower overall pumping costs, or at least optimize crop yield/quality return relative to water and energy inputs.
LEPA irrigation applies water directly to the soil surface through drag hoses (primarily) or through “bubbler” type applicators, (such as the LEPA mode of Senninger Irrigation Inc. Quad-Spray® products.) Notably LEPA involves more than just the hardware through which water is applied. It involves farming in a circular pattern (for center pivot irrigation systems) or straight rows (for linear irrigation systems). It also includes use of furrow dikes and/or residue management to hold water in place until it can infiltrate into the soil.
LEPA irrigation generally is applied to alternate furrows; reducing overall wetted surface area, and hence reducing evaporation losses immediately following an irrigation application. Because relatively large amount of water is applied to a relatively small surface area, there is risk of runoff losses from LEPA, especially on clay soils and/or sloping ground. Furrow dikes and circular planting patterns help reduce the runoff risk. Still, LEPA is not universally applicable; some slopes are just too steep for effective application of LEPA irrigation.
Low pressure spray systems – LESA, MESA and LPIC – offer more flexibility in row orientation, and they may be easier for some growers to manage, especially on clay soils or sloping fields. Objectives with these systems include applying water at low elevation (generally 1-2 feet from the soil surface for LESA; often 5 – 10 feet for MESA) to reduce evaporation losses from water droplets (especially important in windy conditions); applying water at a rate not exceeding the soil’s infiltration capacity (preventing runoff); and selecting a nozzle package that provides good distribution uniformity and appropriate droplet size and wetting pattern.
New Irrigation Systems (Center Pivot and Linear Irrigation Systems)
Start with a good design. Work with a qualified designer (Certified Irrigation Designer or licensed Professional Engineer). Design for realistic well capacities; be realistic, not optimistic. Consider whether the water delivery is likely to decrease during the season. Compare “apples to apples” on designs; a cheaper package may not be better. Things to look for in a design include adequate pressure/vacuum relief; flexibility to accommodate crop rotations and well capacity fluctuations as needed; ease of maintenance; and appropriately sized underground pipelines (consider friction losses, especially in longer pipeline runs). Consider whether pressure regulators are needed; they are more likely to be justified in sloping fields. Install the system correctly, and follow design specifications.
Older systems: Considerations
Periodically evaluate the irrigation system to determine if it is performing according to design specifications. Consider wear and maintenance requirements on electrical, mechanical, and hydraulic components; replace worn parts, and upgrade as needed.
Consider whether the sprinkler should be re-nozzled. Has there been a significant drop in well capacity? Has the nozzle package “drifted” over time? (Broken or lost nozzles may be “temporarily” replaced with the wrong size nozzle. Over time these quick fixes can lead to poor distribution uniformity.) Are pressure regulators or nozzles functioning properly? Replace them as needed.
Calibrate the pivot system and conduct a distribution uniformity test periodically to ensure the correct application rates are applied, and that applications are uniform over the field. These are especially important for chemigation applications. Pressure gauges and flow meters can simplify pivot evaluation and trouble-shooting.
Some thoughts on LEPA vs. LESA:
Properly managed, LEPA is potentially more water-efficient than LESA. Both systems – PROPERLY MANAGED – can be very efficient. LEPA allows for alternate furrow irrigation – there are alternate dry “traffic” furrows that are more accessible for timely field applications. By limiting field operation traffic to the dry furrows, infiltration capacity of soil in the “wet” irrigated furrows is maintained. LEPA allows for irrigation without foliar wetting. For some crops this can offer reduced foliar disease risk. If water quality (salinity) is an issue, LEPA can reduce salt damage to foliage.
In very coarse soils, there sometimes may be insufficient lateral soil water movement from alternate furrow LEPA applications. This is mainly a concern for seed germination, shallow rooted crops and peanuts that require a moist zone near the soil surface for pegging and pod development. Spray irrigation (LESA and MESA) wet the soil surface more uniformly than LEPA. It is possible to apply LESA for crop germination / establishment, then convert to LEPA to take advantage of the higher irrigation application efficiency in season, and convert back to spray applications for chemigation or for uniform wetting of the shallow root zone as needed.