Center Pivot Irrigation

Objectives:

1. Increase understanding of irrigation efficiency, losses, and distribution uniformity associated with center pivot irrigation.
2. Increase understanding and application of best management practices to improve efficiency and uniformity of center pivot irrigation.

Key Points:

1. Low pressure center pivot and linear sprinkler irrigation systems are more water efficient and energy efficient than high pressure systems.
2. Low pressure systems include Low Energy Precision Application (LEPA), Low Elevation Spray Application (LESA), Mid-Elevation Spray Application (MESA), and Low Pressure In-Canopy (LPIC) systems. LEPA is an irrigation and field management package.
3. Crop-specific water requirements, soil texture, field topography, water quantity and quality, and other factors should be considered in selecting a sprinkler irrigation system.
4. Sprinkler systems are well-suited to automation, and they offer potential to apply fairly precise irrigation amounts (light, frequent irrigations to less frequent heavy applications) as needed by the crop or for other field activities (such as chemigation applications).
5. Sprinkler nozzle packages should be inspected periodically and updated as needed.
6. Management and maintenance are key to good results with any pressurized sprinkler system.

Assess your knowledge:

1. What are the normal pressure ranges for a high pressure center pivot and a low pressure center pivot?
2. Why are low pressure center pivot irrigation systems considered more efficient than high pressure systems?
3. Center pivot irrigation systems are available with two different types of drive systems. What are they? What are the advantages and limitations of each?
4. On a typical commercially available center pivot system, how is the desired irrigation application depth achieved? (How do you control the depth of application?)
5. What is the role of furrow diking in sprinkler (or LEPA) irrigation management?
6. When is a chemigation check valve required on an irrigation system? What is the purpose of the chemigation check valve?
7. If an irrigation system has a capacity to deliver 3 gpm/acre, how many inches per week can be applied to the field?

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-SprayÔ 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.

Some other considerations:

In sloping fields, pressure regulators may be warranted to improve irrigation distribution uniformity in the field. This reduces occurrence of “wet spots” and “dry spots” in the field. Good distribution uniformity is also essential to effective chemigation/fertigation.

In many semi-arid areas, including the Texas Southern High Plains, pre-season irrigation or excess early season irrigation is used to provide moisture from crop establishment and to fill soil moisture storage capacity to augment often deficit irrigation during peak crop water use periods. Pre-season irrigation water losses through evaporation and deep percolation can be quite high. Hence it is important for growers to understand how much water their soil root zone will hold, taking into account effective root zone depth and soil moisture storage capacity per foot of soil. Applying more water than the soil can hold can result in deep percolation losses or runoff; starting irrigation too early increases opportunity for evaporation losses. These risks need to be balanced with irrigation system capacity issues.

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.

Suggestions for realizing the benefits of advanced irrigation technology:

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.

Irrigation Management

Crop water requirements are crop-specific, and they vary with weather and growth stage.  Water management is especially important for critical periods in crop development.  Apply knowledge of the root zone to optimize irrigation management; take into account the crop’s effective rooting depth, the soil moisture storage capacity, and field-specific conditions (shallow soils, caliche layers, etc.). In irrigation scheduling, consider using soil moisture monitoring, evapotranspiration information, and/or plant indicators to fine-tune water applications to meet crop needs.

References

• Center Pivot Workbook (B-6162)
• Utilizing Center Pivot Sprinkler Irrigation Systems to Maximize Water Savings